Symposium SB02—From Hydrogel Fundamentals to Novel Applications via Additive Manufacturing

Our lab has developed hydrogel coatings for electrospun vascular grafts that resist platelet adhesion and promote endothelial cell adhesion for acute and sustained thromboresistance.¹ To improve homogeneity of the coating, we developed a diffusion-mediated redox polymerization method to generate conformal hydrogel coatings with improved thickness control.² Reduced thickness of the hydrogel coating resulted in reduced damage due to forceps gripping; however, these coatings still displayed damage due to twisting and torqueing associated with implantation that exposed the underlying thrombogenic substrate.³ In this study, we developed a new hydrogel chemistry based on interpenetrating networks of polyether urethane diacrylamide (PEUDAm) and poly(N-acryloyl glycinamide) (pNAGA) with improved damage resistance. Investigation of the redox hydrogel coating methodology was then used to identify parameters that enabled fabrication of hydrogel-mesh vascular grafts with requisite laminate strength.
First, the reducing agent, iron gluconate (IG), was adsorbed onto the electrospun mesh substrate, then the coated substrate was immersed in an aqueous macromer solution with the initiator, ammonium persulfate (APS). IG diffusion from the mesh substrate into the APS solution results in the formation of persulfate radicals and crosslinking of the PEUDAm hydrogel. The hydrogel coating thickness was controlled with incubation time. The effect of solution viscosity and rate of crosslinking on hydrogel coating laminate strength was studied by systematically tuning molecular weight and APS concentration to identify parameters for suitable coating application. A second network was introduced by swelling in NAGA and photopolymerization with the total hydrogel coating thickness set by the first PEUDAm hydrogel coating.

Our studies identified key parameters necessary to apply these coatings using redox diffusion-mediated crosslinking with improved control of hydrogel thickness. Reducing initiator concentration was necessary to slow cure rate to promote better penetration of higher molecular weight macromers and prevent swelling-mediated delamination; however, reduction of initiator concentration also required additional consideration of gelation time to achieve sufficient crosslink density. The resulting hydrogel coatings were resistant to damage from suturing and torqueing of coatings under 180° twisting with hemostat gripping. We hypothesized that the increased molecular weight between crosslinking points facilitated greater elongations and the additional hydrogen bonding motifs dissipated fracture energy to increase the resistance to hydrogel coating damage with handling. These results demonstrate the improved damage resistance of PEUDAm-NAGA hydrogels and the potential of this hydrogel design for coating vascular grafts.

References:(1) Browning, M. B., et al. Acta Biomaterialia 2012, 8 (3), 1010-21; (2) Wancura, M., et al. J. Mater. Chem. B 2020, 8 (19), 4289-4298. (3) Post, A., et al. Acta Biomaterialia 2018, 69, 313-322;

The surfaces of host cells and pathogens are covered with short polymers of sugars known as glycans attached to the proteins and lipids in the biological membranes. There are two predominant types of glycans - N and O glycans. N-glycans have a highly conserved structure, constitute greater than 50% of the glycosylation type in secreted proteins. O-glycans have a more varied structure and are present in the mucins of host cells. Among the five sugars typically present in N- and O- glycans, four are common to both. The goal is to determine if there are clear rules for how the interfacial phenomena on biotic and abiotic surface varies with the sugar composition of the glycans present, particularly their terminal sugar profile. Terminal sugars on N- and O- glycans were systematically removed and the changes in aggregation and adhesion patterns were monitored. Rules of adhesion and aggregation between different combinations of terminal glycan sugars were verified in multiple systems. Mannore sugars consistently showed short-range 'Velcro-like' adhesions with other mannose sugars, whereas SA residues showed long-range 'slime-like' adhesions with other terminal SA sugars in the presence of salt. The adhesion patterns were not sensitive to the structure of sugar presentation - they existed irrespective of whether the sugars were present in abiotic monolayers or in N-glycans or in O-glycans. In addition to these sugar-sugar self-adhesions, cross-adhesions were observed only between specific pairs of sugars. N-glycans abound on the surfaces of viruses such as HIV, Ebola, Dengue, SARs, and Influenza. O- glycans abound on mucins present in the mucus gel within respiratory, gastric, and vaginal cavities. Rules for which sugars on these glycans will self- and cross-adhere would enable predicting when pathegens will self-aggregate, when mucins will adhere to pathogens, how glycosylated abiotic surfaces will behave in host environment, and how changes in glycosylation that occur with age, race, and chronic conditions will alter the interfacial phenomena on biotic and abiotic surfaces.

Resilin-like polypeptides (RLPs) are intrinsically disordered proteins (IDPs) that may undergo liquid-liquid phase separation (LLPS). This IDP property is involved in the control of the formation and dissolution of membraneless organelles inside cells through interactions between the IDP amino acids or interactions with polyanions such as RNA. Polyphosphate is a polyanion that is capable of promoting the LLPS of resilin-based IDPs, emulating the behavior observed between other IDPs and RNA in living cells. By regulating the concentration and localization of polyphosphate deposited over acrylamide functional RLP solutions, transition temperature gradients are generated, allowing the creation of different types and sizes of microstructures upon crosslinking of the resulting emulsions. These studies help us to take advantage of a phenomenon naturally occurring in cells and utilize it for making RLP-based hydrogels with controlled microstructures with potential applications in fields such as tissue engineering and drug delivery.

Shape transformation in response to changes in environmental conditions is a prevalent function observed in living systems, from the blooming of flowers to muscle actuation. Current shape-changing synthetic materials that mimic this behavior typically respond to non-specific stimuli such as pH, light, or temperature. Nevertheless, these stimuli are not specific to the material and may affect various components of the surrounding environment. New materials are needed that respond mechanically in a programmed manner by detecting specific biochemical cues. In this work, we develop 3D-printable engineered living materials (ELMs) capable of responding to very specific and pre-determined biochemical stimuli. We synthesize shape-changing ELMs from living probiotics embedded in cellulose nanocrystal (CNC)/polyacrylamide hydrogels. The effects of CNC at concentrations between 5-14 wt% and 11-22 wt% were studied to evaluate ELM volume expansion in active (with yeast) and passive (cell-free) composites respectively. Using direct ink write printing, we spatially patterned cellular proliferation within a monolith to induce complex shape transformations. By encapsulating genetically engineered probiotics (Saccharomyces boulardii) that proliferate only in the presence of specific biochemical cues, ELM capsules were designed that deliver a model drug. To create these drug delivery devices, structures composed of cell-free and yeast-embedding materials were printed to obtain hollow capsules containing a hydrophobic model drug. In this device, the ELM portion, containing an engineered S. boulardii strain, only changes in volume in the presence of a particular amino acid, which leads to expansion of one portion of the capsule. This expansion ruptures the device and releases 11 % ± 3 % of the total model drug to the surrounding environment within 48 h of growth. The use of genetically modified probiotics enables devices that respond to subtle biological cues found in the surrounding environment. Our study may enable new opportunities to develop drug-delivery devices for the diagnosis and treatment of gastrointestinal disorders.

We report the design and properties of a new class of self-reinforced homocomposite hydrogels (HHGs) comprised of interpenetrating networks of multiscale hierarchy. They are made possible by the introduction of novel soft matter called soft dendritic colloids (“dendricolloids”). The dendricolloids are fabricated by a process of the eddy-guided polymer precipitation in turbulently sheared liquid medium. They have hierarchical morphology similar to molecular-scale polymer dendrimers, but two orders of magnitude larger in scale [Nature Mater. 18:1315, 2019]. The branched dendricolloids have very large excluded volume, and are surrounded by a corona of polymer nanofibers, which are highly adhesive due to van der Waals forces. Dendricolloids made of alginate can serve as reinforcement networks of molecular alginate, forming jointly homocomposite hydrogels [Nature Comm. 12:2834, 2021]. The reinforcement of the molecular gel with the nanofibrillar colloidal network of the same biopolymer results in a remarkable increase in the mechanical properties of the homocomposite hydrogels. The HHG’s show >3× larger storage modulus and >4× larger Young's modulus than either constitutive network at the same concentration. The synergistically reinforced colloidal-molecular HHGs open numerous opportunities for formulation of biocompatible gels with robust structure-property relationships. They can be used in 3D printing with Homocomposite Thixotropic Paste (HTP), a method that was originally developed in our group for 3D printing with ultrasoft silicone HTP [Adv. Mater. 29:1701554, 2017]. The homocomposite hydrogels enable the making of thixotropic pastes for precise and efficient 3D printing via extrusion. We will demonstrate how the ratio of the HHG precursors facilitates precise control of the yield stress and timed rate of self-reinforcement. The resulting hydrogel architectures have excellent mechanical properties and potential for broad range of biomedical and soft robotics applications.

Natural materials are programmed hierarchically to form homo- and heterogeneous three-dimensional (3D) architectures, yielding intelligent systems that can complete complex tasks. Designing artificial materials that mimic and even improve upon natural systems requires the precise spatiotemporal control of molecular entities' chemical structures, assemblies, homo- and heterogeneous placements, and macroscale 3D architectures. My research aims to develop next-generation responsive polymers by integrating cyclodextrin (CD)-based polypseudorotaxanes with the direct ink writing technique. The understanding and control of the molecular-to-macroscopic hierarchical assembly within in a pre-designed 3D geometry enables the fabrication of responsive polyrotaxanes to complete complex tasks.
In this presentation, I will introduce our development of a CD-based sidechain polypseudorotaxane system and the synthesis of 3D-printable hydrogels with spatiotemporally programmable shape morphing and dynamic mechanical properties. We demonstrated a de novo molecular design to synthesize sidechain polypseudorotaxanes with tunable nano-to-macroscale properties for multi-material 3D printing. These sidechain polypseudorotaxanes were formed by copolymers with PEG sidechains and α-CDs. When α-CDs fully covered the PEG sidechains, the rod-like polypseudorotaxane sidechains acted like rigid collagen helices at the nanoscale to aggregate in water, forming crystalline bundles at the mesoscale. We revealed that longer copolymer chains formed disordered polypseudorotaxane crystalline aggregates through systematic variations of the copolymer chain lengths, which exhibited physically crosslinked 3D networks with good 3D printability. Shorter copolymer chains favored ordered lamella crystalline aggregates. Upon α-CD threading and dethreading, copolymers with varied PEG grafting densities showed different stiffness. When these sidechain polypseudorotaxane networks were covalently crosslinked, they were transformed into α-CD-responsive mechanically adaptive hydrogels, featuring more than 100 times elastic moduli change and multi-stage shape persistence. By incorporating a slide-ring crosslinker in the hydrogel, we achieved large tensile strain and significant elastic-to-plastic property variation upon α-CD (de)threading. These discoveries enabled us to integrate two α-CD-responsive hydrogels through multi-material 3D printing and fabricate a sea cucumber mimic possessing multi-stage stiffness change and shape morphing. Our demonstration of designing a macroscale multi-responsive system directly from simple molecular entities showcased an integrated nano-to-macroscale design.

References
[1] Q. Lin, X. Hou, and C. Ke, Angew. Chem. Int. Ed. 2017, 56, 4452-4457.
[2] Q. Lin, L. Li, M. Tang, X. Hou, and C. Ke, J. Mater. Chem. C, 2018, 6, 11956-11960.
[3] L. Li, Q. Lin, M. Tang, E. Tsai, and C. Ke, Angew. Chem. Int. Ed. 2021, 60, 10186-10193.

3D printing helps to provide further control on electrode design. By using 3D printing to tune the geometry, porosity and dimensions of the structure mass transport to functional microbes or sites in the structure can be improved. By embedding living microbes directly into the ink, we can create 3D printed structures to help harness the activity of microbes and use them in functional devices. Here we demonstrate the incorporation of the living bacteria Shewanella Oneidensis MR-1 (S. Oneidensis MR-1) directly into an ink used for creating 3D printed structures. S. Oneidensis MR-1 has been commonly used in microbial fuel cells (MFC). By incorporating S. Oneidensis MR-1 into a sodium alginate-cellulose ink we showed that S. Oneidensis MR-1 survives the 3D printing process through the prominent degradation of methyl orange azo dye from the surrounding solution in the presence of the living 3D printed structure. The viability was further confirmed using confocal microscopy. By incorporating carbon black into this ink we further demonstrate the direct printing of a living MFC anode. Electrochemical measurements showed there was good charge transfer between the S. Oneidensis MR-1 and the electrode surface. To our knowledge, this is the first report on implementing 3D printed bacteria structure as a living electrode for an MFC system. The capability of printing living and functional 3D bacterial structure could open up new possibilities in design and fabrication of microbial devices as well as fundamental research on the interaction between different bacterial strains, electrode materials, and surrounding environments.

Traditional polymeric hydrogels are often static in nature with nanoscale pores, which can limit cell infiltration, spreading, and extracellular matrix deposition. Granular hydrogels overcome these limitations, due to their macroporous structure [1]. Composed of micro-scale building blocks (microgels), granular hydrogels can undergo a glass transition into a jammed state, giving rise to macroscale elasticity. The resulting voids between microgels form a three-dimensional interconnected network of pores. Recent studies have exploited granular hydrogels to meet the complex demands of cell culture [2] and regenerative medicine [3,4]. Despite the wide interest in granular hydrogels, the mechanism by which the properties of the building blocks control the rheological behavior of these systems remains largely unexplored and is essential to ensure robust translation.

In this work, we relate the emergence of granular hydrogel mechanics to building block properties and provide in depth understanding of the dynamic rheology of soft and jammed granular systems. Thiol–ene based microgels were formed leveraging microfluidic templating for precise synthesis of the building blocks. The microgels were then jammed to produce macroscopic materials that behave like an elastic solid at low strain, but also exhibit shear-thinning and self-healing properties. Oscillatory shear rheometry showed that the mechanical properties of granular hydrogels can be tuned over several orders of magnitude through the control of microgel stiffness and size. To unveil the emergence of the macroscale behavior of granular hydrogels, we computationally generated jammed hard sphere packings and simulated the three-dimensional soft microgel scaffolds. We leveraged contact mechanics principles to establish predictive descriptions of the mechanical response. The model revealed that the extent of contact deformation is proportional to the size and inversely proportional to the stiffness of the microgels, which dictates the linear viscoelastic plateau moduli and zero shear viscosity [5]. With our predictions of the contact landscape of the building blocks, we further explained the differences in the non-linear rheology of granular hydrogels. The results expand our ability to tailor the material properties of granular hydrogels toward a range of biomedical applications and enable advanced engineering of granular hydrogels of complex architecture.

We are translating our study on the relationship between building block properties and the macroscopic response into rational design of granular hydrogels for the development of in vitro models and the engineering of regenerative biomaterials. We present additional results on the effects of controlled microgel environments that enable cell spreading, proliferation, and differentiation.

[1] Bhattacharjee, T., et al. ACS Biomater. Sci. Eng. 2016, 2, 10, 1787-1795.
[2] Caldwell A.S., et al. Biomaterials. 2020, 232, 119725.
[3] Shin, M., et al. Adv. Sci. 2019, 1901229.
[4] Griffin, D., et al. Nature Mater. 2015, 14, 737-744.
[5] Emiroglu, D.B., et al. in preparation.

Recently there has been an increase in demand for soft and biocompatible electronic devices capable of withstanding large stretch. Conductive hydrogels present a promising class of soft materials for the emerging applications due to their ability to realize charge transport across wet and swollen polymer networks, while preserving the desired mechanical and chemical features. As opposed to electron transfer in traditional electrical conductors, the charge transport across a polymer swollen with an electrolyte solution is achieved through ion migration – such materials are therefore known as ionotronic. The applications of ionically conductive hydrogels are vast, from healthcare and soft robotics to sensors. Moreover, the ability to successfully predict ion migration in soft and wet materials opens the door for implementation of biocompatible hydrogels as an interface between living tissue and external devices.

Hydration level and water diffusion through the polymer network have a significant impact on both the conductive and mechanical behavior of hydrogels. In addition to water molecules and mobile ionic species, fixed charge groups might be tethered onto the polymeric backbone. As a consequence of variation in pH, the ions might interact with the fixed groups through association/dissociation chemical reactions, thus changing the overall electrolyte strength. This significant environmental responsiveness of hydrogels identifies them as active materials with highly tunable properties, and therefore front-runners for implementation in soft electric circuits and devices.

Ionotronic materials and systems are characterized primarily through DC measurements, electrochemical impedance spectroscopy (EIS), and thermo-mechanical testing. EIS consists of applying a time-varying voltage across the material while measuring the corresponding current, and provides insights into the underlying electrochemical mechanisms driving the ion transport. Despite the rapid increase in the number of potential applications and experimental procedures, there is a lack of overarching design tools, hindering the further advance of ionotronic technology. The main challenge in developing reliable simulation capabilities is the complex and coupled electro-chemo-mechanical behavior of ionically conductive hydrogels.

In this work, we propose a multiphysics framework involving the coupled effects of ion transport, electric fields, network hydration, and large deformation. In addition, the developed constitutive model takes into account the difference in conductivity at various hydration levels, along with association/dissociation chemical reactions triggered by pH variations. The model also provides capabilities for computing current generated by the time-varying voltage, and prediction of EIS results based on material microstructure and environmental conditions. We numerically solve this multiphysics system by employing the finite element method. Such an approach will enable quantitative evaluation of material design and ionotronic devices. The accuracy of the developed model and numerical method is ensured through validation against the EIS data. The usefulness of the developed multiphysics model is showcased by simulating the representative ion transport problems and the operation of a soft ionotronic device.

Spiders possess fascinating multi-feats with spiderweb for maximizing adhesion force. Here, we explore an approach to behavioral characteristics of cyclosa japonicas inspired ionic spider webs (ISWs), which is capturable, self-powered sensible and vibrationally dust-cleanable, all of which the principles are solely based on electrostaticity in a single platform [1]. It was made with ionically conducting organogels capsulated with dielectric elastomers as a strand shape. Its thinness and transmittance led ISWs could be camouflageable. Plasma etching and (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane treatment on dielectric elastomer readily boosted cleanability and it even prevented the evaporation of organogel, which led increase of durability of ionic spider web. Its high stretchability and translucency led it endure applied strain upto 50 % to capture targets larger than ISWs, which cannot be realized by rigid one, and realized it can be camouflageable, respectively. The ISW robustly captured a wide range of materials including a leaf, acryl, glass, and aluminum of which weight is 50 times heavier than its own weight. The capturability was ensured with its additional feats including electrostatic sensing ability and resonance cleanability.

References
[1] Lee, Y., Song, W. J., Jung, Y., Yoo, H., Kim, M. Y., Kim, H. Y., & Sun, J. Y. (2020). Ionic spiderwebs. Science Robotics, 5(44).

Selective depletion of overproduced nitric oxide (NO) with nanoscavengers is a promising approach for treating rheumatoid arthritis (RA), preventing both oxidative/nitrosative stress and the upregulation of immune cells. However, its practical applications are limited owing to the minimum time interval between intra-articular injections and unwanted off-target NO depletion. At an appropriate level, NO plays a crucial role in modulating various biological functions. To address this issue, we herein report the rational design of an injectable in situ micellar hybrid NO-scavenging and sequential drug-releasing (M-NO) gel platform for the combinatorial treatment of RA by incorporating a “clickable” NO-cleavable cross-linker (DA-NOCCL). This network is held together with micellar structures to achieve a self-healing capability for visco-supplementation and on-demand dual drug (both hydrophilic and hydrophobic)-releasing properties, depending on the NO concentration. Moreover, consecutive NO-scavenging action reduces pro-inflammatory cytokine levels in LPS-stimulated macrophage cell lines in vitro. Finally, the intra-articularly injected M-NO gel with anti-inflammatory dexamethasone significantly alleviated the symptoms of RA, with negligible toxicity, in animal models.
This proposed concept and the platform with favorable results showed promising application potential for the combinatorial treatment of various NO-related diseases, delivering various hydrophobic and hydrophilic drugs in a single platform with a consecutive NO-scavenging action.

Biopolymer networks comprise an essential and substantial proportion of 3D bioprinting materials because of their abundance and role in living tissues. While biopolymers serve to strengthen tissues in vivo, biopolymer networks in vitro are notoriously soft and weak; bioprinted structures made from biopolymer networks exhibit the same limitations. To stiffen and strengthen printable in vitro biopolymer networks, we developed a biomaterial made from collagen-1 in which the polymer mesh-space is filled with micron-scale microgel particles made from polyethylene glycol (PEG). We find that this combination of material components synergistically enhances the system material properties. For example, by filling in the mesh space of the polymer networks with low volume fraction microgels, the elastic modulus can increase by more than an order of magnitude. By packing the microgels into the polymer network at volume fractions approaching jamming, the material takes on a shape-memory property, where the material will spontaneously lock-in deformations at intermediate strain levels; reversal of these strains requires a stress-reversal and results in the material resulting to its original deformation state. This new class of materials opens up wide possibilities in its capacity as a 3D bioprinting material and has potential applications in fabrication of model tissues.

Surface wetting prevents surface fogging on transparent materials by facilitating filmwise condensation with specific chemistry but suffers from material and geometry selectivity. Extreme environments associated with high humidity and mechanical loading further limit their anti-fogging persistence. Here, we report a stretchable anti-fogging tape (SAT) that can be applied to diverse transparent materials with varied curvatures for persistent fogging prevention. The SAT consists of three synergistically combined transparent layers: i) a stretchable and tough layer with large elastic recovery, ii) an endurant anti-fogging layer insensitive to ambient humidity, and iii) a robustly and reversibly adhesive layer. The SAT maintains high total transmittance (>90%) and low diffuse transmittance (<5%) in high-humidity environments, under various modes of mechanical deformations, and over a prolonged lifetime (193 days tested so far). Two applications are demonstrated, including the SAT-adhered eyeglasses and goggles for clear fog-free vision, and the SAT-adhered condensation cover for efficient solar-powered freshwater production.

Soft materials have enabled diverse modern technologies, but their practical deployment is usually limited by their mechanical failures. Fracture and fatigue of polymer networks are two important causes of mechanical failures of soft materials. A soft material fails by fracture when a monotonic load reaches its fracture toughness or fails by fatigue when a cyclic load reaches its fatigue threshold. Fracture toughness is usually much larger than fatigue threshold for randomly crosslinked polymer networks (e.g., elastomers and gels). This work systematically studies the fracture and fatigue of ideal polymer networks with controlled densities of dangling-chain defects. We show that the fracture toughness and fatigue threshold of an ideal polymer network almost without defects are the same. After introducing a low density of dangling-chain defects into the ideal polymer network, its fracture toughness and fatigue threshold still maintain approximately the same. The fracture toughness of the ideal polymer network is also independent of the loading rate. We further use the recently developed defect network model to explain the fatigue threshold (i.e., intrinsic fracture energy) of ideal polymer networks with controlled densities of dangling-chain defects.

Solar desalination system has been considered as an effective method to address the global water scarcity, which thanks to its eco-friendliness and sustainability. However, the heat loss of evaporation surface to water reservoir, fouling due to the formation of salt crystals on evaporation surfaces and water channels, and the difficulty of continuous brine supply without external power are the major obstacles in the practical use of the solar desalination. Herein, through the separation of water pumping and evaporation part, a novel solar desalination system showed no heat loss during evaporation and had an antifouling property. Moreover, using thermosensitive hydrogel composites, a solar desalination system could work by diurnal temperature variation. An irreversible water flow was achieved by using composites of thermosensitive poly(N-isopropylacrylamide) (PNIPAm) hydrogel and hydrophobic polydimethylsiloxane (PDMS), which have fast deswelling kinetics responsive to temperature above 32 °C (LCST). The composites were made by directional melt crystallization (DMC) of solvent. By the DMC method, a porous PNIPAm having aligned and continuous three-dimensional pores was prepared. The structure enables a combination of two different materials. It can be achieved by the infiltration of PDMS to the aligned pores of PNIPAm. Anisotropic composites have PNIPAm/PDMS phase at the bottom of the composites, which has a faster deswelling rate than pure PNIPAm phase. As temperature rises above the LCST of PNIPAm, water can be released from the top surface of the PNIPAm, and this water pumping could be repeated by the diurnal temperature variance. Average pump efficiency of our composites is 88%. Between the pumping composite and the water reservoir, porous polyethylene glycol (PEG) was placed for the water channel. PEG can absorb brine from the water reservoir and diffuse brine into the entire phases of PNIPAm/PDMS composite. Water from the pumped brine evaporates on the surface of reduced graphene oxide (rGO) coated cellulose paper, which showed broad range of light wavelength. When brine is released, the rGO coated cellulose paper float, and salt accumulation is formed at the bottom of the paper. This crystallized salt is dissolved in the next pumped water. Our three-dimensional solar desalination system was operated stably in several cyclic tests in one-sun condition. Diurnal operating strategy makes the solar evaporator eco-friendly, and the antifouling properties increase the sustainability of solar desalination.

Electroconductive biocompatible polymers have been of interest for use in a wide range of biomedical applications such as biosensors, regenerative therapeutics and protheses. This surge of curiosity has sparked the development of soft functional materials to meet these demands. In the field of neural regenerative medicine, we have demonstrated that resident brain neural precursor cells are electrosensitive cells that respond to electric field application by proliferating, differentiating and migrating in a rapid and directed fashion. Harnessing these properties can potentially be used to facilitate neural regeneration following injury or disease. The design of current electrodes, typically made of conductive metals, are stiff and result in additional tissue injury as a result of implantation. The mechanical mismatch of these materials to the soft and wet physiological conditions that they would be subjected to in their use in the brain, presents significant limitations of this technology as a potential therapeutic. Recently, there has been interest in using biologically derived polymers for their relative abundance and consequently, economic feasibility. Sodium alginate is an attractive example of such and can be used to form hydrogels whose soft and flexible nature as well as functionality in aqueous environments presents a potential solution to this issue of mechanical mismatch. We have fabricated a novel conductive and biocompatible hydrogel that more closely mimics the modulus of the brain as a solution to this problem. By designing a hydrogel based system comprised of alginate and conductive polymer, poly(3,4-ethylenedioxythiophene (PEDOT), we propose an innovative new material for use in the development of novel neural regenerative therapeutics. These alginate/PEDOT blends exhibit sheer thinning behaviour at both ambient and body temperatures as well as exhibit satisfactory conductivity for their intended therapeutic application.

Inspired by the work of Stupp and colleagues on the supramolecular exchange of oppositely charged molecules, we used two complementary peptides derived from ASCPYD inflammasomes. The addition of the positive and negatively charged complements in water produced heterotypic hydrogels dependent on ratio and the use of N-terminal acetyl or naphthalene capping groups. We found that mixing one naphthalene capped positively charged peptide at 0.5wt% with an acetyl capped negatively charged complement created hydrogels across all ratios of 0.5, 1, 2, 3, and 4 times the amount of positively charged peptide. The addition of the peptides of the oppositely charged peptides in solution produces heterotypic hydrogels. Rheology measurement shows that ratios of the complementary peptides affect the viscoelasticity of the resulted hydrogel. Circular dichroism indicates that the addition of the complementary peptides results in electrostatic interactions that modulate self-assembly. Lastly, transmission electron microscopy reveals that the ratio of the complementary peptides controls the morphology of the heterotypic peptide assemblies. This work illustrates a rational, biomimetic approach that uses the structural information from the protein data base (PDB) for developing heterotypic peptide materials via self-assembly.

Much work has been done on the use of heating to trigger reactions via the temperature-dependent removal of a barrier or constraint separating reagents. An example of this method is the use of wax microbeads to encapsulate reagents which are only liberated to react with each other when the wax is melted by heating. Far less work, however, has been done on the use of cooling to achieve a similar goal. Numerous applications, such as those involving components or materials susceptible to persistent low temperatures and cases in which energy for heating is not available would benefit from this inverse approach. Hence, in this study we explore whether LCST polymers can be reliably used as thermoresponsive constraints that allow reagents to react only upon cooling. We aim to achieve this by loading reagents into separate adjacent blocks of thermoresponsive polymer gels- in this case, 400 cP methylcellulose- and showing that said reagents can only react with each other after the temperature of the polymer falls below its LCST. Additionally, to demonstrate generalizability and the innate robustness of the concept, we explore situations involving radically different reaction types: one reaction based on a colorimetric enzyme assay (using a colorless HRP solution to oxidize a colorless TMB substrate, resulting in a soluble blue reaction product), and another based on a change in pH (using basic colorless NaOH solution to change colorless slightly acidic phenolphthalein solution from colorless to pink). Finally, to explore the tunability of the process, we alter the LCST of the polymer and observe the resulting change in the extent of cooling necessary to allow a chemical reaction to occur. This change in LCST is achieved by adding different quantities of NaCl to methylcellulose and is corroborated by rheology.

At elevated temperatures (above the LCST), the reagents remain sequestered in separate gels and no reaction occurs. We have demonstrated that this “OFF” state is stable for extended periods of time. When the system is allowed to cool, the gels liquify and flow into each other, allowing mixing of the embedded reagents (“ON” state). By quantitatively assessing the progress of the stated color change reactions over time, we determine that these reactions are triggered reproducibly at different temperatures based on the polymer’s tuned LCST (determined by the NaCl content of the methylcellulose), as well as the proximity of the reagent-laden gels and viscosity of the liquified materials after cooling. With no NaCl present, reactions are triggered 18 minutes (the time required for the gels to liquify and flow into each other) after cooling below 50 degrees Celsius. With 1.5% and 2% NaCl, reactions are triggered 18 minutes after cooling below 36 and 30 degrees Celsius, respectively. In all three cases, the same geometry is used (to maintain the same gel proximity) and the viscosity of liquified materials is similar. We therefore conclude that thermoresponsive polymers can, in fact, act as reliable temperature-sensitive constraints that allow reagents to react only upon cooling. We successfully demonstrate that this method can be applied to a variety of reactions and that the temperature required for triggering a reaction can be predictably and reproducibly tuned.

Hydrogels possess outstanding biocompatibility and swelling properties and thus find extensive applications in fields such as biomedical devices and soft robotics. Here, a high elastic modulus hydrogel with improved strength and toughness is synthesized. The hydrogel can be stretched many times its initial dimensions, where the material returns to its original shape in a reversible manner. Glucose solution is used as the chemical stimulus and the hydrogel swells several times its initial size when immersed in glucose solution and shrinks when it removed from the solution. This swelling and deswelling behavior of the hydrogel can be used to perform useful work. The swelling behavior of the hydrogel as a function of the amount of glucose in the solution is studied. Rheology of hydrogel material is investigated so that the material has desired properties for 3 D printing. Mechanical and viscoelastic properties of hydrogel before and after swelling are tested. The swelling kinetics are analyzed, so that swelling and deswelling behavior can be precisely controlled.

About 20% of the world’s total energy consumption spent to overcome friction. Friction is also present in biological systems such as in hips and joints, and correlated with joint diseases such as osteoarthritis. Due to the molecular complexity of the biological systems, the mechanism of lubrication is still not clear. Apart from our efforts to find the major components responsible for the low friction in biological systems, we also try to exploit nature’s solution for lubricating interfaces such as cartilage by mimicking nature’s strategies of boundary lubrication, which lead to an extreme reduction of friction in aqueous environments. In my talk, I will present several examples of bio-inspired lubrication by polymers and in polymer networks. We carried out systematic investigations from the molecular to the macroscopic level, demonstrating excellent lubrication by polymer assemblies (1) and hydrogel composites (2), attributed to the hydration lubrication mechanism acting at highly-hydrated boundary layers.

1) Angayarkanni et al., (2019) Langmuir. 35, 48, p. 15469-15480.
2) Lin et al., (2020) Science. 370, 6514, p. 335-338.

Traumatic joint injuries can initiate early cartilage degeneration in the presence of elevated levels of inflammatory cytokines, leading to post-traumatic osteoarthritis (PTOA). There are currently no disease modifying drugs for osteoarthritis, and a major challenge is the ability to achieve sustained levels of potential therapeutics inside a target tissue, with no side effects, after intra-articular delivery. We have developed a human in vitro microphysiological system (MPS) for studies of drug discovery for OA/PTOA as well as drug delivery into cartilage. Human knee osteochondral plugs subjected to a single impact mechanical injury are co-cultured with joint capsule synovium. Cytokines secreted mainly from the synovium emulate the inflammatory component of the response to joint injury. Combination therapeutics (e.g., glucocorticoids, growth factors) are tested which can inhibit matrix degradation and cell death in cartilage. Experiments involving this human knee MPS were recently launched on SpaceX-21 to the International Space Station to study the effects of microgravity and radiation on disease-like progression and the effects of selected therapeutics. Parallel in vitro and animal studies are aimed at approaches to targeted tissue drug delivery. For these studies, electrostatic interactions involving cationic nanoparticles to which drugs are functionalized are found to augment transport into and through the cartilage tissue. Thus, the transport properties of the negatively charged gel-like extracellular matrix of cartilage, even during initial degradative stages of the disease model, can provide benefit to enhance drug delivery.

There are approximately 900,000 people in the US suffering from damage to the articular cartilage, with the knee being most commonly affected. Articular cartilage lacks a vasculature and has a limited ability to heal. A variety of surgical treatments have been developed to repair cartilage lesions. Current strategies for cartilage repair include microfracture, autologous chondrocyte implantation (ACI) and osteochondral allograft transfer (OAT). These strategies suffer from high failure rates (25-50% at 10 years), long rehabilitation times (more than 12 months) and decreasing efficacy in patients older than 40-50 years. Focal joint resurfacing with traditional orthopedic materials is being explored as an alternative strategy, but due to their high stiffness and coefficient of friction relative to cartilage, these implants may ultimately contribute to joint degeneration through abnormal stress and wear. A focal joint resurfacing method that is widely available, allows immediate weight bearing, has short recovery times and has low long-term failure rates remains an unmet need.
Our strategy to address this need is to (1) develop a material that mimics the properties of cartilage and (2) attach this material to a titanium base to enable integration with bone. We developed the first hydrogel to achieve the strength and modulus of cartilage in both tension and compression. This hydrogel also exhibits cartilage-equivalent tensile fatigue at 100,000 cycles. The hydrogel was created by infiltrating a PVA-PAMPS double-network hydrogel into a bacterial cellulose (BC) nanofiber network. The BC provides tensile strength in a manner analogous to collagen in cartilage. The PAMPS provides a fixed negative charge and osmotic restoring force similar to the role of aggrecan in cartilage. The hydrogel is not toxic, has the same aggregate modulus and permeability as cartilage, has a coefficient of friction 45% lower than cartilage, and is 4.4 times more wear-resistance than a PVA hydrogel. These properties make the BC-PVA-PAMPS hydrogel an excellent candidate material for the replacement of damaged cartilage.
Current strategies for adhering hydrogel to a surface are 10 times weaker than the shear strength with which cartilage is attached to the bone. The osteochondral junction is characterized by collagen nanofibers anchoring cartilage to bone. We sought to mimic this strategy by bonding freeze-dried BC to porous titanium with a hydroxyapatite-forming cement. The cement penetrates about 10 microns into the bacterial cellulose, forming a nanofiber-reinforced zone of adhesion. The PVA-PAMPS hydrogel is then infiltrated into the bonded bacterial cellulose. This strategy achieved a shear strength of attachment three times greater than the state of the art. Such improved strategies for attaching hydrogels to a titanium surface with sufficient strength to allow for weight-bearing can enable the creation of hydrogel-capped titanium implants for cartilage repair.

Metal-organic frameworks, called MOFs, are crystalline porous materials constructed by repeating metal clusters and organic linkers thus having permanent porosity. The structure and characters of MOF libraries have been widely reported and some MOFs have been applied to the wound healing process by using the porosity to release drugs, gases, and metal ions promoting the wound healing process. However, our research focused on removing substances from living organisms, and these studies have not been reported before.
In the wound healing process, the important mediators that lead to inflammation and scar formation are nitric oxide, cytokines, and reactive oxygen species (ROS). However, the overproduction of these inflammatory agents causes abnormal inflammation and excessive scarring, so it is challenging to control adequate amounts of mediators to complete wound healing with barely scarred.
In this study, MOF was introduced to suppress the excess nitric oxide, cytokines, and ROS, that based on their MOF’s own structure such as micropores, surface, and catalytic activities. we systemically devised and applied a MOF hybrid hydrogel (and silicone patch) that optimizes the wound healing performance by removing excess immune mediators with MOF as well as simultaneously supplying moisture and uniformly applying MOF by hydrogel matrix. After the removal of excess mediators was experimentally analyzed based on MOF chemistry, the wound healing process of MOF was analyzed through cell-derived substances, and finally applied the MOF to animal testing for a real actual wounded environment. It was confirmed that the wound recovered rapidly within 14 days after the introduction of MOF-hydrogel, which showed that the system incorporating MOF is effective in wound healing.

Network materials are encountered broadly in the engineering and natural worlds and gels are an important subset of this broad class. These materials have a molecular or fiber network as their main structural component and perform various functions of which the most prevalent is structural. The integrity of network materials is critical in many applications. Therefore, it is of interest to determine ways to control their strength and toughness. This work focuses on the relation between structural parameters of the network, the network architecture and its strength and toughness. We report the dependence of the strength on network density, crosslink density and strength, and fiber properties. We also investigate the toughness of networks without pre-existing defects and report its dependence on the same set of structural parameters. We observe and characterize the intermittent dynamics of rupture in networks and relate it to similar processes in other materials with stochastic structure. Network design rules emerge from these results. Numerical results are compared with equivalent experimental data.

Multiscale models are employed to study how the microscopic and tissue scale properties of the facet capsule ligament of the spine combine to cause anomalous collagen fiber realignment (AR). The facet capsule ligament, located between the spinal processes, is primarily made up from type I collagen. Fiber reorientation in the FCL has been linked to neck and back pain and it is therefore a critical phenomenon to understand. Anomalous realignment is experimentally observed in the FCL using quantitative polarized light imaging (QPLI). However, no experimental techniques are currently available to probe the root causes of anomalous realignment. This talk will discuss how the interplay of tissue scale collagen orientation, fiber scale instabilities, and fiber failure can cause anomalous realignment in the facet capsule ligament.

Materials adapting their shape reversibly to external stimuli have gathered increasing attention for a range of applications in medicine and robotics. In particular, photoresponsive systems have shown promise through the facile and benign nature of the trigger. However, most photoresponsive molecules, which can be used to convert light into mechanical work, are susceptible to decomposition upon conditions commonly used in preparation of polymeric systems. In this work we present new light-responsive liquid crystalline network (LCN) systems using benign Diels–Alder chemistry and employ this novel platform to compare a range of different photoswitches from the well-established azobenzene scaffold to the novel Donor-Acceptor Stenhouse adducts.

Biologics—including peptides, therapeutic proteins, enzymes, monoclonal antibodies, vaccines, and viral vectors—comprise the majority of new drugs entering the market. However, biologics are highly susceptible to thermal fluctuations during storage and transport, as aggregation, denaturation, and oxidative degradation render them ineffective. In order to guarantee their quality and safety, biologics must be maintained within a tight temperature window (2–8 °C) from manufacture to point of use. This global ‘cold chain’ infrastructure cost over 17 billion USD in 2020 and poses significant health risks as failures in the cold chain can lead to community health outbreaks, as in the case of ineffective vaccines. An emerging strategy to impart thermal stabilisation is to immobilize biologics directly within a protective matrix that can hold the molecules in place to resist conformational changes and prevent translation. One of the major challenges for these matrix-based approaches, however, is to achieve on-demand release of the biologics from the protective matrix.

To meet these challenges, we designed a safe and easy-to-use synthetic reversible hydrogel for the encapsulation and stabilisation of a broad range of biologics against thermal stress [1]. Our approach is based on the use of dynamic covalent boronic ester cross-links that enable on-demand recovery through network dissolution upon addition of sugars [2, 3]. Using these reversible hydrogels, we thermally stabilised a wide range of biologics, starting with model enzymes (β-galactosidase, leucine aminopeptidase, and alkaline phosphatase). We attributed protection to the formation of polymer networks around the biologics, which restricted their mobility and physically restrained them from interacting with each other, reducing aggregation and activity loss after exposure to heat. In addition, we showed that the gels could be used to protect extremely heat sensitive therapeutic enzymes, such as E. coli DNA gyrase and human topoisomerase I. Both enzymes could be stored within the gels for months at room temperature without significant activity loss, vastly outperforming the unencapsulated (free) enzymes.

Finally, we present our results on the thermal stabilisation of vaccines. We stabilised a protein-based vaccine, recombinant influenza A H5N1 hemagglutinin, and a live virus, adenovirus Type 5 (Ad5). Viral vectors, such as Ad5, are central to gene therapy and genome editing techniques (i.e., CRISPR-Cas9), and are becoming increasingly important for use in vaccine development (i.e., COVID-19 vaccines). Live viruses such as Ad5 present a particular challenge for thermal stabilisation because both protein conformation and nucleic acid integrity must be preserved to retain viral infectivity and immunogenicity; even simple drying of live viruses is often challenging. Gel-encapsulated Ad5 outperformed free Ad5 at all tested temperatures (4 °C to 65 °C) and retained close to 100 % activity after 4 weeks at 27 °C, while the infectivity of free Ad5 decreased by 3 logs. In total, our work demonstrates that reversible hydrogels are suitable for the thermal stabilisation of multiple classes of biologics, with little formulation work required to stabilise each type of biologic. The ability to protect biologics from thermal stress with a simple material solution offers a useful approach to mitigate the costs and risks associated with reliance on a continuous cold chain for biologic transport and storage.

[1] Marco-Dufort et al., in preparation.
[2] Marco-Dufort et al., J. Am. Chem. Soc. 2020, 142, 15371.
[3] Marco-Dufort et al., Biomacromolecules 2021, 22, 146.

Materials manufacturing strategies that use little energy, valorize waste, and result in degradable products are urgently needed. Strategies that transform abundant biomass into functional materials form one approach to new manufacturing, but the resulting products of the biomass are often dictated by the organisms. From a biology standpoint, morphogenesis of biological tissues is a “manufacturing” mode without energy-intensive processes, large carbon footprints, and toxic wastes. However, most living organisms cannot form materials that are suitable for engineering applications. Inspired by biological morphogenesis, we have proposed a manufacturing strategy by embedding living Saccharomyces cerevisiae (baker’s yeast) within a synthetic acrylic, such as poly(2-hydroxyethyl acrylate), hydrogel matrix. The resulting living composites derive functionality from both the biological activity and material properties. By culturing the living composites in media derived from bread waste, encapsulated yeast cells can proliferate resulting in a dramatic dry mass and volume increase of the whole living composite. After growth, the final composite is up to 96 wt% biomass and 590% larger in volume than the initial object. By spatial-patterning the cell viability through UV irradiation or photodynamic therapy, the living composites can also form complex user-defined relief surfaces or 3D objects during growth. The resulting living composites also have tunable mechanical properties by controlling the extent of the cell growth or the selection of polymer matrix. After growth and drying, the modulus of the composites can be tuned from 290 MPa to 856 MPa. Ultimately, the grown structures can be degraded into water-soluble polymers and yeast. The accelerated degradation of the living composites can be tuned by changing the molecular weight of polyester crosslinkers from 4 to 8 days. The proposed living composite strategy cultured from biowaste may pave the way for future ecologically friendly manufacturing of materials.

Oral drug products have become indispensable in modern medicine because of their exceptional patient compliance. However, poor bioavailability of ubiquitous low-water-soluble active pharmaceutical ingredients (APIs) and lack of efficient oral drug formulations remain as significant challenges. Nanocrystalline formulations are an attractive route to increase API solubility, but typically require abrasive mechanical milling and several processing steps to create an oral dosage form. Using the dual amphiphilic and thermoresponsive properties of methylcellulose (MC), a new thermogelling nanoemulsion and a facile thermal dripping method are developed for efficient formulation of composite particles with the MC matrix embedded with precisely controlled API nanocrystals. Moreover, a fast and tunable release performance is achieved with the combination of a fast-eroding MC matrix and fast-dissolving API nanocrystals. Using the versatile thermal processing approach, the thermogelling nanoemulsion is easily formulated into a wide variety of dosage forms (nanoparticle suspension, drug tablet, and oral thin film) in a manner that avoids nanomilling. Overall, the proposed thermogelling nanoemulsion platform not only broadens the applications of thermoresponsive nanoemulsions but also shows great promise for more efficient formulation of oral drug products with high quality and tunable fast release.

The general knowledge of many cell-based processes comes from experimentation on flat surfaces, mainly in culture wells made of either a polymer or glass. These are 2D, stiff and non-physiological environments, which are simple for experimentation, yet do not replicate all the conditions a cell experiences in vivo.
In this work a methodology to develop an alginate hydrogel based material with calcium phosphate particles suspensions is presented. This material has the potential to act as a scaffold with a complex architecture processed via additive manufacturing, and is shown in the shape of a triply periodic minimal surface. Being a known biocompatible material, and formed in a shape suitable for cell culturing, a method to capture information from this scaffold is presented.
The formed hydrogel shows elastic properties suitable to implement traction force microscopy (TFM), a computational technique to compute force fields exerted on the substrate. This is achieved using digital lensless holographic microscopy, a holographic technique that vastly reduces costs, experimental setup and permits the analysis of transparent objects with sizes in the range of a few micrometers. The calcium phosphate particles in the hydrogel are used as fiducial markers to calculate displacement in TFM. This way, a complex, more physiological environment can be achieved, and the means to recover information such as traction forces are shown to work, facilitating acquisition to offer biological insight.

Gels thermal responsivity is usually associated to a Volume Phase Transition, VPT. However, different and equally relevant for biomedical applications, responsive behaviors to temperature can be found in micro- and nanogel particles.
In this contribution we report on the morphological changes around physiological temperature of hybrid nanogel particles made by crosslinking via click chemistry an azido-derivative of hyaluronic acid, HA, and a derivative of poly(N-isopropyl acrylamide), PNiPAm, having propargyl functions at both ends, with interesting fallouts concerning targeting and drug delivery. In in vitro tests, doxorubicin loaded nanoparticles with a PNiPAAm/HA ratio of 33 % (w/w) increased its cytotoxicity on HT-29 tumor cells over the NIH 3T3 healthy fibroblasts, by a factor of two.
In this HA/PNiPAAm nanogel, the hydrophobic – hydrophylic interplay of the two crosslinked polymer components gives rise to a reshuffling of the surface composition passing a quite narrow threshold around 33 °C, taken as the VPT temperature, VPTT, of the system. In this process, the room temperature surface structure is a collection of PNiPAAm and hyaluronate patches. Approaching the VPTT, the PNiPAAm hydrophobicity promotes the interparticle aggregation. Above the VPTT, the NiPAAm residues self-segregate in the particle core, leaving the charged HA moiety on the surface. This moieties redistribution, and not the size reduction, within the microgel particle, assessed by an atomic force microscopy, AFM, study of the surface, dynamic light scattering, DLS, zeta potential analysis and small angle neutron scattering, SANS, is at the base of the recognition of tumor cells via the interaction HA/receptors of overexpressed CD44 transmembrane proteins and of the specific drug delivery mechanism.

In PNiPAAm hydrogels, drug release has been often coupled with the temperature response of the matrix. However, besides the temperature, responsiveness to other state variables, such as pressure, or to the combination of both temperature and pressure can be considered. Two microgels having PNiPAAm as the only polymer component but different architectures, obtained by precipitation polymerization, were investigated: a morphology with (i) an un-crosslinked core and a crosslinked shell of PNIPAM chains and a softer one, (ii), having crosslinked PNIPAM chains as particle core with a shell of tethered un-crosslinked PNIPAM chains to the core. Confocal microscopy was a useful tool to validate the different morphologies. Differential scanning calorimetry, DSC, and DLS were used to follow the microgels as a function of temperature, pressure and both, pressure being osmotically controlled with a high molecular weight non-interacting polymer. Under osmotic stress, soft microgels can be compressed above the VPTT, reaching extremely low hydration degrees. The microgel volume behavior as a function of pressure and temperature allowed to estimate the isothermal compressibility and thermal expansion coefficients for the different morphologies. The VPT enthalpy is scarcely affected by the network architecture, whereas the work on osmotically stressed PNiPAAm is strongly influenced by the morphology. As biological counterpart, a very low cytotoxicity was highlighted by MTT tests of all morphologies on HaCAT cell line, while it is noteworthy that the softer morphology (ii) of microgels enhances the phagocytosis by RAW 264 macrophages.

Fibrous networks are ubiquitous in biology. The mechanical properties of the biopolymer fibers, the way in which the fibers are linked into networks, and the types of cells or particles within the network mesh all affect the macroscopic mechanical response of the material. Filamentous polymer networks containing volume-conserving particles are found in the cytoskeleton and extracellular matrix of animal cells and tissues, bacterial biofilms, fungal mycelia, and other structures spanning multiple biological kingdoms. The responses of biomaterials to uniaxial and shear deformations depends critically on the volume fraction of particles within networks and on the binding of particles to the network, but current data suggest that the mechanical properties of the particles themselves have only minor effects on the response of the composite material. Controlling biopolymer network connections and incorporation of appropriately sized particles within them can lead to viscoelastic properties that mimic those of biological tissues more closely than polymer networks alone.

Cartilage matrix is composed of a dense collagen mesh within which the highly charged glycoprotein, aggrecan, is produced by chondrocytes and remains entangled in high concentrations. The inclusion and entanglement of large complexes of aggrecan molecules attached to long hyaluronic acid chains is responsible for matrix osmotic swelling that profoundly affects the behavior of the composite tissue. The equilibrium between the swelling and the mechanical constraints imposed by the collagen matrix is what provides cushioning of bones at load bearing joints. The relationship between osmotic and mechanical properties on one hand, and matrix composition and architecture on the other, is poorly understood. The composition of the tissue changes through tissue depth. Moreover, in growing cartilage, the relative composition and organization of the different components changes with time and along the bone axis and in the vicinity of chondrocytes, the pericellular matrix (PCM). Axial mapping of the mechanics is further complicated due to the need to physically section the tissue to gain access.
Here we study mouse cartilage. Even though mice are extensively used as model systems for a wide range of pathologies including arthritis, there are limited studies mapping matrix mechanics across different regions. Newborn mouse cartilage adds the complication that the tissue is densely populated with chondrocytes creating a complex 3-D composite that is continuously varying both in space and time. Nanomechanical studies, typically performed by indenting tissue sections, are subject to errors due to matrix collapse, surface roughness and loss of aggrecan from the section surface by diffusion and degradation. Furthermore, data interpretation often poses its own challenges not least because of the natural inhomogeneity of the tissue.

Here, we performed high-resolution (1um) elasticity mapping of the knee cartilage matrix of newborn mice. The indentations were performed on thin (~12um) cartilage sections that were cut parallel to the bone longitudinal axis and mildly fixed to minimize loss of aggrecan. We chose the diameter of the microspheres used as indentation probes and we applied indentations that were large enough as to minimize the effects of surface roughness and geometric inhomogeneities. To consistently analyze the data, we developed a novel method for obtaining an effective contact point on the tissue surface. Our model fitting protocol ensures that we are probing bulk matrix properties and minimizes the effects of surface damage, tissue inhomogeneity and finite sample thickness. Matrix regions were extracted by correlation with optical images. Our results show interesting variation of properties between different regions and large variations in the vicinity of chondrocytes. The latter presented an additional challenge because the septa between chondrocytes are often very narrow adding to the difficulty of interpretation. Further modeling and analysis points to continuously and strongly varying properties of the PCM. The variability of matrix elasticity across the growing cartilage will be described and possible correlations with composition will be discussed.

Many efforts have been made on developing adhesives for the marine environment that are inspired by organisms that fix themselves to underwater surfaces, like mussels. These catechol-based glues are easily oxidized and so eventually lose their adhesiveness, making them less than satisfactory for their intended purpose. Recently, we explored the possibility of developing adhesives that utilize electrostatic interaction to stick to negatively charged surfaces such as rocks, glass, and metals under the sea [1]. We built polymer chains made from two types of monomers as building blocks. One contains a positively charged “cationic” residue and the other contains an “aromatic” ring. In biosystems, adjacent cationic-aromatic amino acid sequences in proteins are known to facilitate electrostatic interactions in saline water. It has been challenging, however, to introduce such sequences in synthetic polymers due to difficulties in controlling monomer sequences. We discovered that the synthetic polymers with adjacent cationic-aromatic sequences could easily be manufactured using a highly scalable, cost-effective method through cation–π complex-aided free-radical polymerization. We found that the two residue types in polymers bonded together to form a hydrogel that stuck well to negatively charged solid surfaces in saltwater. Hydrogels composed of a variety of cationic-aromatic monomer combinations exhibited fast, strong, but reversible adhesion to the surfaces. Adhesion was largely thanks to the electrostatic interaction between the positively charged residues on the polymers and the negatively charged surfaces. But, interestingly, polymers made from these cationic -aromatic monomers without adjacent sequences weren’t nearly as adherent, indicating that adjacent aromatic residue enhances electrostatic interaction in high ionic strength environments. Based on the above result, we also developed hydrogel adhesives working in pure water [2].

References
1) Hailong Fan, Jiahui Wang, Zhen Tao, Junchao Huang, Ping Rao, Takayuki Kurokawa, Jian Ping Gong, "Adjacent Cationic-Aromatic Sequences Yield Strong Electrostatic Adhesion of Hydrogels in Seawater", Nature Communications, 10, 5127 (2019).
2) Hailong Fan, Jiahui Wang, Jian Ping Gong, "Barnacle Cement Protein-Inspired Tough Hydrogels with Robust, Long-Lasting, and Repeatable Underwater Adhesion", Advanced Functional Materials, 31(11), 2009334 (8 pages) (2021) (Web 2020)

The development of protein-based 3D printable hydrogels with required physicochemical and structural tunability is a critical challenge. One of our major focus is to design and develop novel 3D printable hydrogels that must possess both appropriate mechanics to be printable, are both shear-thinning and self-healing, and possess appropriate biocompatibility to support cellular interaction. Many widely used bioinks do not currently allow for this flexibility. By expanding the number and tunability of materials that are available for 3D printing, we aim to fabricate complex, multi-material constructs [1-7] and use of customizable, engineered scaffolds to support cell or microbiota growth with controlled porosity, elasticity, water absorption characteristics and mechanical properties. This presentation will highlight the effects of different types of nanocellulose on the hierarchical structure (nano-micro), mechanical properties (compression and shear), cellular response, and the printability of the fabricated protein/polysaccharide biomimetic hydrogels. The incorporation of fibrous polysaccharide significantly increased the toughness of the hydrogel without compromising cellular activities. The extrusion-based 3D printing of protein based hydrogels photochemically co-crosslinked between globular protein and fibrous protein has also been investigated in this work. The hierarchical structure of hybrid inks and their organizational transition during printing condition (shear) was investigated using small-/ultra-small-angle neutron scattering combined with rheology (Rheo-SANS/USANS). The properties of fabricated hydrogels was investigated using photorheology and atomic force microscopy; where, the hybrid hydrogels exhibited tuneable storage and Young’s modulus. The 3D printed hybrid hydrogels showed micropore size significantly higher than their solution casted counterparts. Moreover, the fabricated hybrid hydrogels show good mouse fibroblast cell attachment, viability and proliferation demonstrating their potential for tissue engineering applications.
References
(1) Dorishetty, P.; Balu, R.; Sreekumar, A.; de Campo, L.; Mata, J. P.; Choudhury, N. R.; Dutta, N. K. ACS Sustainable Chem. Eng. 2019, 7 (10), 9257-9271.
(2) Kundu, B.; Kurland, N. E.; Bano, S.; Patra, C.; Engel, F. B.; Yadavalli, V. K.; Kundu, S. C. Prog. Polym. Sci. 2014, 39 (2), 251-267.
(3) Whittaker, J. L.; Choudhury, N. R.; Dutta, N. K.; Zannettino, J. Mater. Chem. B 2014, 2 (37), 6259-6270.
(4) Whittaker, J. L.; Dutta, N. K.; Elvin, C. M.; Choudhury, N. R. J. Mater. Chem. B 2015, 3 (32), 6576-6579.
(5) Whittaker, J. L.; Dutta, N. K.; Zannettino, A.; Choudhury, N. R.J. Mater. Chem. B 2016, 4 (33), 5519-5533.
(6) Balu, R.; Reeder, S.; Knott, R.; Mata, J.; de Campo, L.; Dutta, N. K.; Choudhury, N. R. Langmuir 2018, 34 (31), 9238-9251.
(7) Dorishetty, P.; Balu, R.; Athukoralalage, S. S.; Greaves, T. L.; Mata, J.; de Campo, L.; Saha, N.; Zannettino, A. C. W.; Dutta, N. K.; Choudhury, N. R. ACS Sustainable Chem. Eng. 2020, 8 (6), 2375-2389.

3D printing of structures and devices with shape or property change, or 4D printing, are highly desirable in many applications such as mechanical actuators, soft robotics, and artificial muscles. Early work in 4D printing used shape memory polymers (SMPs) as the shape change mechanism, but SMPs require complicated thermo-mechanical loading and most SMPs do not have reversible shape change capability. It is still a challenge to develop simple methods to freestanding, reversible, shape changing structures and devices. Desolvation is a novel method for shape changing devices where polymers with different crosslinking densities can absorb different amount of water or solvent to create different deformation. In recent years, our group has explored using the desolvation concept as a means for 4D printing. We first found that the light intensity gradient along the thickness direction created by photoabsorbers can be used to generate bending deformation. The level of the bending deformation depends on the light irradiation dose, which can be further controlled by light intensity. Therefore, by controlling light intensity pattern, we could create different bending distributions, or origami. This concept was further used to create reversible origami-based shape transformations. We then further developed such method to print 3D structures with rapid shape transformation by carefully managing the light penetration among different layers. Finally, we applied this method to extrusion based DIW printing of inks with high short fiber glass content to create shape-changing stiff structures. The design of all these desolvation-based 4D printing methods were guided by the theoretical modeling to achieve more accurate control of shape transformation.

One major challenge facing the advancement of diffusion-weighted imaging (DWI) methods is phantom development and fabrication. Isotropic diffusion phantoms are not sufficient to validate and calibrate more advanced DWI sequences that aim to distinguish between isotropic hindered diffusion and anisotropic randomly oriented restricted diffusion. Enhancing the contrast between these descriptions of diffusion regimes is the goal of most diffusion MRI method development teams. In reality, the size of the microstructural features that exhibit this behavior in biological tissue is on the order of 1-10 μm, while the features of the most commonly used phantoms are on the order of 20-100 μm. We plan on addressing the technical challenge of fabricating a 3D phantom that contains both randomly oriented highly anisotropic and isotropic structural features. We hope to reach high aspect ratios for these features to better resemble the target brain tissue. We tested this 3D Printed MRI phantom using DWI that reproduced the size and orientation distribution of the 3D printed pores. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

Wound healing is a complex, highly organized biological and physiological process comprising of four overlapping phases: hemostasis, inflammation, re-epithelialization, and remodeling. A standard wound healing process is dependent on leukocyte recruitment and a cascade of pro- and anti- inflammatory cytokine responses. Over expression of the inflammatory cytokines such as interleukins (IL-1, IL-6) and tumor necrosis factor (TNF-α) as well as excessive recruitment of neutrophils and macrophages arrests the healing process in the chronic inflammatory phase. Once the wound is stalled in this phase, it becomes chronic or non-healing and is prone to infections. Chronic foot ulcers are one of the most common chronic wound condition arising from complication of diabetes, suffered by ~25% of diabetic patients. These confer a high-risk prognosis for lower extremity amputation to prevent the spread of infection. Current standard of care for chronic foot ulcer includes wound debridement, wound dressings for moisture balance, revascularization procedures to treat ischemia and improve blood flow, and hyperbaric oxygen therapy. Pharmacological treatments include using biological actives like topical formulations of growth factors or anti-inflammatory cytokines. The efficacy of topical formulations is hindered by their proteolytic breakdown in the hostile environment that results in therapeutically ineffective concentration and prevents their diffusion into deeper wound layers. Moreover, these approaches are labor intensive, expensive and focus on treating the symptoms of inflammation rather than the inflammation itself. Thus, it is important to develop de novo multivalent materials that can unlock the local inflammatory phase by extracting the molecular drivers of inflammation in a highly selective and concerted fashion and aid in the reparative phase. In this work, we developed a novel non-pharmacological approach to modulate chronic inflammation. A 3D-bioprinted polymer scaffold was fabricated by in-situ phosphate crosslinking of a poly (vinyl alcohol) polymer, that when applied on the wound, can modulate local inflammation by sequestering pro-inflammatory cytokines from the wound site via physicochemical and electrostatic interactions. Moreover, this polymer scaffold also contributes to the rapid wound healing by disrupting the constant recruitment of leukocytes to the wound site. We demonstrated the therapeutic efficacy of the polymer scaffold in a skin excision mouse model by applying the gel directly on the wound. The ability of the scaffold to capture the positively charged pro-inflammatory cytokines were evaluated by Luminex assay on the scaffold removed from the mice. Compared to the controls (PVA crosslinked with glycidyl methacrylate), the polymer scaffold was more effective in capturing the pro-inflammatory cytokines (IL-1α , IL-6 and TNF-α). Furthermore, flow cytometry studies showed that leukocytes were captured by the scaffold. These studies support that the polymer scaffold enhanced wound healing by trapping the excess pro-inflammatory cytokines and leukocytes in a tightly controlled manner and disrupting the chronic inflammatory cycle in wounds.

3D Bioprinting enables the fabrication of structures with living cells, biomaterials and other active ingredients (e.g. drugs and/or growth factors) strategically arranged in three dimensions. The biomaterials used are traditionally hydrogels since these provide adequate mechanical support to ensure structure integrity along with a cytocompatible environment.

The engagement of cells with the biomaterial (gel or the precursors) during and after printing determines the success or otherwise of any 3D Bioprinting strategy.

The interaction of gel or the precursors and cells determines the ability to retain viable suspensions in bioink reservoirs.

The rheological properties of the gels used determines how they behave and how they might protect cells during printing.

The modulus and porosity of gels determines the capacity of cells to interact with each other and the immediate environment.

Using several 3D bioprinting case studies aimed at meeting clinical challenges we will explore the critical properties of gels for such applications.

The clinical challenges include:
- cartilage regeneration in the knee (1,2)
- islet cell transplant ion to treat diabetes (3,4)
- skin regeneration (5,6)

With a view to developing protocols that are implementable in the clinic we will also review some of the additional requirements such gels must meet. These include the availability of a reliable and reproducible source material and compatibility with sterilisation methods traditionally used.

The cell - gel engagement is short lived and the temporal dimension is critical to a successful relationship. The role of the gel as a function of cell and tissue development time will be discussed.

References

(1) Duchi, S., Francis, S., O’Connell, C.D., Aguilar, L.M.C., Doyle, S., Yue, Z., Wallace, G.G., Choong, P.F., Di Bella, C. FLASH: Fluorescently LAbelled Sensitive Hydrogel to monitor bioscaffolds degradation during neocartilage generation, Biomaterials 2021, 264, 120383.

(2) O’Connell, C.D., Di Bella, C., Thompson, F., Augustine, C., Beirne, S., Cornock, R., Richards, C.J., Chung, J., Gambhir, S., Yue, Z., Bourke, J., Zhang, B., Taylor, A., Quigley, A., Kapsa, R., Choong, P., Wallace, G.G. Development of the Biopen: A handheld device for surgical printing of adipose stem cells at a chondral wound site, Biofabrication 2016 8, 015019.

(3) Kim, J., Hope, C.M., Gantumur, N., Perkins, G.B., Stead, S.O., Yue, Z., Liu, X., Asua, A.U., Kette, F.D., Penko, D., Drogemuller, C.J., Carroll, R.P., Barry, S.C., Wallace, G.G., Coates, P.T. Encapsulation of Human Natural and Induced Regulatory T-Cells in IL-2 and CCL1 Supplemented Alginate-GelMA Hydrogel for 3D Bioprinting, Advanced Functional Materials 2020, 30 (15), 2000544.

(4) Liu, X., Carter, S.-S.D., Renes, M.J., Kim, J., Rojas-Canales, D.M., Penko, D., Angus, C., Beirne, S., Drogemuller, C.J., Yue, Z., Coates, P.T., Wallace, G.G. Development of a co-axial 3D printing platform for biofabrication of implantable islet-containing constructs, Advanced Healthcare Materials 2019, 8 (7), 1801181.

(5) Zhou, Y., Kang, L., Yue, Z., Liu, X., Wallace, G.G. Composite Tissue Adhesive Containing Catechol-Modified Hyaluronic Acid and Poly-L-lysine, ACS Applied Bio Materials 2020, 3 (1), 628-638.

(6) Daikuara, L.Y., Yue, Z., Skropeta, D., Wallace, G.G. In vitro characterisation of 3D printed platelet lysate-based bioink for potential application in skin tissue engineering, Acta Biomaterialia 2021, 123, 286-297.

Biocompatible fluidic systems play a vital role in a wide range of biological applications, such as tissue engineering, biochemical sensing, cell-to-cell interaction studies, microfluidic bioreactors and so on. Hydrogels are one of the best candidates to use as a base material when fabricating the above systems due to their characteristic properties, including compliance similar to biological tissues, permeability to small molecules, biocompatibility with most cells and optical clarity under visible light. On the other hand, the swelling properties (volume change) of hydrogels is considered to be a negative factor in flow channel design, and therefore non/less-swellable hydrogels tend to be used to fabricate hydrogel-based fluidic systems. By leveraging the swelling properties of hydrogels, we can turn this disadvantage into a unique, new method to create fluidic devices from various functional hydrogels.
In this study, we demonstrate the use of swelling-driven buckle-delamination, a physical phenomenon observed in hydrogel/solid substrate bilayer composites, for fabricating fluidic channels.^[1] By spatially controlling the interfacial bonding between a thin polyacrylamide (PAAm) gel film and a glass substrate, swelling-driven compressive stress induces the formation of channel architectures through a buckle-delamination process. Connecting flow tubes to the channels with a 3D printed connecter results in a fluidic device, enabling pressure-driven flow without leakage or rupture. Furthermore, by stacking less-swellable bulk gels on the device, we obtained a tough, permeable, and biocompatible fluidic device, which successfully demonstrates the use of both swellable and less-swellable hydrogels as base materials for developing fluidic systems. Finally, we performed cell cultures with the device and applied chemical stimulation to cells through diffusion of molecules from the fluidic channels. ^[2] This work should shed light on the design of diverse hydrogel-based fluidic systems and may be useful for applications in the fields of bioanalysis, tissue-engineering and cell biology.

[1] R. Takahashi, et al. ACS Appl. Mater. Interfaces, 2019, 11, 28267-28277.
[2] R. Takahashi, et al. Lab Chip, 2021, 21, 1307-1317.

[Introudction]
The fluctuation of biological membranes has fascinated scientists and engineers with various backgrounds. The membrane fluctuation induces essential features for life, such as cell compartmentalization, cell motility, cell division, and vesicle trafficking. The membrane fluctuation is derived from the following parameters: the thermal fluctuation of phospholipids in the membrane layer, ATP-dependent conformational changes of transmembrane proteins. Mimicking the membrane fluctuation using artificial active matters is important to understand the origin of the dynamic behavior of life.
In this context, our group has developed self-oscillating gels with living organism-like functions. The self-oscillating gel shows dynamic behavior by chemo-mechanical motion, driven by a Belousov-Zahbontinsky (BZ) reaction. The self-oscillating gel is fabricated by introducing a tris(2,2’-bipyridine) ruthenium (Ru(bpy)₃²⁺), a metal catalyst for the BZ reaction, into a thermoresponsive hydrogel network. Cyclic redox changes of Ru(bpy)₃ (Ru(bpy)₃²⁺ ↔ Ru(bpy)₃³⁺⁾ during the BZ reaction lead the gel to present periodic and autonomous swelling-deswelling oscillation without any external stimuli.¹
Here we present a self-oscillating capsule-shaped gel that shows the cell-like membrane fluctuation. Self-oscillating gels are one of the promising materials for mimicking the membrane fluctuation because the gels show the regional deformation randomly and spontaneously in various regions. We fabricated a capsule-shaped hollow structure using the self-oscillating gel, which provides a structural resemblance compared with biological cells. The capsule-shaped gel exhibits the surface fluctuation by the propagation of the BZ reaction wave similar to the shape fluctuation of the living cells.

[Materials and Method]
The capsule-shaped hollow self-oscillating gel was fabricated by four processes described below: (1) An alginate/calcium (Alg/Ca) spherical bead, which is a template of the capsule-shaped gel, was fabricated by adding sodium alginate solution to CaCl₂ solution. (2) Poly ((N-isopropylacrylamide)-co-(N-(3-aminopropyl)methacrylamide)) (poly(NIPAAm-co-NAPMAm) layer was synthesized by immersing the Alg/Ca gel into ammonium persulfate (APS) solution and monomer solution, in a sequence. (3) The Alg/Ca template was dissolved by putting the gel into a sodium citrate solution. (4) Ru(bpy)₃-NHS was conjugated to capsule-shaped hollow gel. The gel surface fluctuations by volumetric oscillation during the BZ reaction were evaluated.

[Result and Discussion]
The cross-sectional images of the gels in each fabricating process show that the capsule-shaped hollow structure was successfully formed. On a millimeter scale, the volumetric oscillation of the self-oscillating gel appears with a spreading chemical signal by the BZ reaction.² Therefore, in this study, the chemical signals of the BZ reaction run along to the surface of the entire gel, causing the surface fluctuating phenomenon like a living red blood cell. Moreover, it becomes clear that the amplitude of fluctuation increases as layer thickness grows. The thickened self-oscillating gel layer leads to improved local deformation, which brings about the more intense surface fluctuation phenomenon. The thickness deformation-induced surface fluctuation could correspond to the effect of the ATP-driven conformational change of protein, which induces the shape fluctuation in the living cells. We expect that this study can be utilized to realize the artificial cell which shows various cell-like functions.

[Reference]
1. Yoshida, R. Self-oscillating polymer gels as biomimetic and smart softmaterials. Adv. Mater. Lett. 9, 836–842 (2018).
2. Kim, Y. S., Tamate, R., Akimoto, A. M. & Yoshida, R. Recent developments in self-oscillating polymeric systems as smart materials: From polymers to bulk hydrogels. Mater. Horizons 4, 38–54 (2017).

Poly(dimethylsiloxane) (PDMS) sponges are an emergent technology, garnering much attention in recent years due to their unique mechanical properties, high surface area and hydrophobicity/oleophilicity. In this work, a hydroscopic calcium chloride hard template is used to prepare a PDMS sponge in 6 hours, faster than previously reported template removal methods. Furthermore, the unique deliquescent properties of the calcium chloride are harnessed to provide a spontaneously self-removing sacrificial template within 4 days. The resultant PDMS sponges displayed excellent mechanical robustness, exhibiting maximum tensile and compressive stress values of 110 ± 20 kPa and 2.8 ± 0.1 MPa, respectively. The PDMS sponges displayed far greater preferential absorption of toxic solvents over simulated gastric fluid.

Living hydrogels were made by embedding microalgae of the species Chlorella vulgaris in alginate-based hydrogels. It was shown through detection of chlorophyll-a content that these algal cells were able to grow as expected within the hydrogel matrix. This system was formed the basis for a Chlorella vulgaris algal (hydrogel) sensing system for the detection of the exposure to toxic compounds. For example, exposure to dichloroethane resulted in a 43% decrease in Chlorella vulgaris growth. This work demonstrates that living hydrogel materials containing Chlorella vulgaris cells can successfully act as toxicity indicators. This could suggest their suitability in applications of living hydrogels in water quality assessment.

There is a current need for the development of robust smart materials for a variety of medical, consumer, and manufacturing applications. In addition, these materials require the ability to act as 3D printable inks to undergo rapid prototyping. A UV curable gelatin-epoxy amine double network hydrogel with water content of 80% can be simply produced by a one-pot synthesis. This synthesis method can be performed by methacrylating the individual networks by either a one-step process (simultaneously) or a two-step process (independently). It was found that the two-step process is more effective in producing a mechanically robust material, capable of exhibiting a compressive stress at failure of 2.5 ±0.2 MPa. Significantly, a new 3D printing method has been developed to allow the material to be cured post printing resulting in no significant loss in mechanical strength compared to gels prepared by casting. This method uses the entanglement of gelatin (below 37°C) to give support to the individual printed layers of the structure eliminating the need for UV curing after each layer is printed. Post printing, the structure can be UV cured to increase the mechanical strength of the material to that of the hydrogels prepared by casting. It is envisage that this new printing method in conjunction with new hydrogel chemistries can be utilised to fabricated new materials for applications ranging from biomedical devices to soft robotics.

In this presentation the 3D printing of living hydrogels based on alginate-gelatin gels embedded green algae Chlorella Vulgaris is presented. It is shown that alginate-gelatin gels crosslinked with 0.05M CaCl2 displayed excellent mechanical properties whilst still allowing for suitable algal cell growth. The latter was determined using dissolved oxygen ratio.

The rheological flow behaviour of these hydrogels systems was investigated to assess their potential for extrusion printing. Our results indicated that a combination involving 4% (w/w) alginate and 4% (w/w) gelatin resulted in a mechanically robust 3D structure that maintained algal cell viability. 3D printed living hydrogel with excellent mechanical stability offer new possibilities to connect the inorganic and biological worlds.

There are several techniques used in the manufacturing of bone implants such as 3D printing. Rapid prototyping/3D printing is an important process in the development of implants used in bone fixation surgeries, its importance lies within the ability to customize and create patient specific systems to increase the efficiency of bone fixation surgeries. The main aim of this study was to develop and test new (and softer) materials for the purpose of surgincal staples using 3D printing. Our results indicate that the mechanical characteristics of staples is suitable to enable their use in low impact locations within the body (such as upper extremities). This study demonstrated that biocompatible polymers are a viable alternative for metal alloys in the use of surgical staples.

The biochemistry of cartilage has been extensively studied in the last couple of decades. The major proteoglycan component of catilage extracellular matrix is the high molecular weight (1x10⁶ < M < 3x10⁶) bottlebrush shaped aggrecan. In the presence of hyaluronic acid (HA) and link protein, aggrecan self-assembles into a supramolecular structure with as many as 100 aggrecan molecules attached as side chains on a filament of HA. This supramolecular array yields a hydrated, viscous gel that provides the osmotic properties necessary for the cartilage to resist deswelling under compressive load. In cartilage these complexes are interspersed in the collagen matrix.
Imaging techniques (electron microscopy, atomic force microscopy, etc) have revealed the size and structural pattern of large distinct aggrecan-HA acid complexes. Such methods yield detailed information on the morphology within a selected area of the sample. However, the interactions that govern the morphology and dynamics of aggrecan-HA assemblies in solution remained poorly understood. Owing to the complexity of the aggrecan-HA system the determination of the relationship between its structure and function requires a range of experimental approaches combining both physical and biochemical techniques.
We investigate the spatial organization both in solutions of aggrecan and aggrecan-HA complexes over a length scale range between 1 and 500 nm. The static properties are studied by small angle neutron scattering (SANS) and static light scattering (SLS), while the dynamics are probed by dynamic light scattering (DLS). The length scale range probed by scattering methods detects changes in the architecture on the supramolecular scale. Changes at the molecular level are detected by virtue of the average effect that they exert on the larger scale structures. Better understanding of the organization of aggrecan assemblies and their effect on cartilage mechanical properties is essential to advance our knowledge of arthritiis and develop treatment strategies.

Background:
In some colloidal suspensions, viscosity increases (shear thickening) or decreases (shear shining) when shear stress is applied. The shear thickening material is a candidate for wide range applications such as damper systems in machinery, and such phenomena can cause a flow to be obstructed. However, these viscosity changes occur in a dynamic process in which shear is applied, and the viscosity returns to its original state when the shear stress is removed. Therefore, it has been difficult to measure the dynamic microstructural changes that lead to the shear thickening phenomena. In this study, we performed simultaneous measurements of solution viscosity and small-angle X-ray scattering (SAXS) using an X-ray transmission solution cell. By observing the structural changes of colloidal particle clusters under shear stress in real time, we investigated the aggregation process of colloidal particles responsible for the phenomenon.

Experiment:
The silica colloid sample was prepared by dispersing a silica sol (Snowtex ST-S, Nissan Chemical) with a particle size of 8 nm in a 0.8 wt% PEG solution [1]. The viscosity and SAXS were measured simultaneously using a rheometer (ONRH-1, Ohna Tech Inc.) in combination with a polycarbonate X-ray transmission Couette cell. The obtained spectra were divided into perpendicular and parallel components with respect to the direction of the cylinder rotation, and it was plotted against the scattering vector Q. The experiments were performed at the Sumitomo Electric Beamline BL16 at the Kyushu Synchrotron Light Source Center and Spring-8 BL40XU.

Results:
The viscosity measurement results of the silica colloidal solution showing shear thickening properties. This property was observed at a shear rate of 10 [1/s], where the viscosity increased rapidly. SAXS spectra were acquired while rotating at a constant speed at the shear rate before and after the viscosity change, and the spectra in direction of perpendicular and parallel to the rotation were compared.
In both directions, a characteristic peak at about 3.4 [1/nm] was observed, which is attributed to a particle cluster spacing and reflects an isotropic cluster dispersion. The anisotropic shift of the peaks were observed after viscosity increase. The spacing of the parallel to the shear direction decreased, while that of perpendicular increased, suggesting that a compressive force was applied to the cluster in the direction of rotation. These results suggest that the application of shear stress caused an aggregating cluster and an increasing viscosity [2]. These results demonstrated an effectiveness of the viscosity-SAXS measurement method.

Acknowledgments:
This work was supported by Innovative Science and Technology Initiative for Security Grant Number JPJ004596,ATLA, Japan.

[1] Kamibayashi, M., Ogura, H. & Otsubo, Y. J. Colloid Interface Sci. 321, 294–301 (2008).
[2] Cheng, X., McCoy, J. H., Israelachvili, J. N. & Cohen, I. Science 333, 1276–9 (2011).

Tough and stretchable hydrogels are highly studied in recent years, due to its promising potential applications in various field such as for actuators, wearable electronic devices, and soft robotics. To have a hydrogel with improved mechanical property, its viscoelastic dissipation needs to be enhanced, which is difficult to be obtained in a hydrogel containing of covalent bonds only. To overcome this problem, in this study, physical interaction in form of ionic bond and micelles incorporations are used. For the micelles, Pluronic F-127 micelles was chosen, other than for mechanical enhancement, it also has the capability to have a quick viscosity sift depending on temperature. This capability is generated owing to its structure, a triblock copolymer which consists of hydrophobic poly (propylene oxide) (PPO) core block and hydrophilic poly (ethylene oxide) (PEO) side blocks. At elevated temperature, the hydrophobic core block dehydrates, which leads to micelles formation and increasing its viscosity. The controllable viscosity is highly useful for 3D printing, as it is important to prevent the spread of the ink upon deposition to a substrate. The Pluronic F-127 was also synthesized to have methacrylate group to allow photo-polymerization after printing, to have immediate permanent shape. With this method, hydrogels with complex shapes in various toughness and stretchability could be achieved, and it opens the possibility for further applications.

High aspect ratio pillar structure was suitable for implementing various functional surfaces due to the easy shape transform. The high aspect ratio pillars composed of elastomers or rigid polymers were unstable and easily clustered by external forces such as capillary forces. This clustering property was used in various field. However, easy clustering property of the high aspect ratio pillars posed an obstacle to apply functional surfaces because special treatment was essential for high aspect ratio pillars to return to their original state from clustering state. Therefore, we reported reasonable criteria to control the clustering phenomena in high aspect ratio pillar arrays. We adjusted two main factors that were the crosslinker density of thermal-responsive hydrogels and the temperature of the surrounding environment to control the elastic modulus, which was the most important characteristic for clustering property of high aspect ratio pillars. Experimentally, the clustering behavior of the high aspect ratio pillars was respectively analyzed by changing the mechanical property according to the water content and temperature. In addition, theoretical analysis supported the criteria for controlling condition with clustering and de-clustering phenomena of the high aspect ratio pillars.

Recent advances in bioremediation techniques in geotechnical engineering has pioneered the replacement of pore water in soil by biopolymers to improve the physical and hydraulic properties of soils. In determining the type of biopolymer additive that will not reduce the soil strength, several factors need to be considered. Ideally, the biopolymer-based pore fluid needs to be self-assembling and can recover after deformation. Recent advances in geotechnical engineering have shown hydrogels as a soil additive to improve soil stability. Hydrogels, particularly with clay additives, can help mitigate soil erosion, reduce soil swelling, and optimize soil near building foundations. While experiments have successfully demonstrated the effect of hydrogels on soil strength, these studies depend on trial and error to identify the suitable biopolymer and type and concentration of clay fillers.

This study analyzes hydrogel systems consisting of two types of polymer chains and clay nanoplatelets dispersed in an explicit solvent and assesses the effect of clay filler structure, filler amount, and polymer-polymer attraction on the mechanical and structural properties of the hydrogel. To simulate the formation and behavior of the hydrogel, the systems were studied using coarse-grained molecular dynamics. A series of simulations were run using various combinations and concentrations of clays and the different polymers. We analyzed the effect of including interactions between the clay and the polymers and polymer – polymer interactions. Mechanical properties were measured via viscosity and stress autocorrelation (SAC) calculations, while the radial distribution function (RDF), was used to measure degree of phase separation or random mixing of polymers.

Our results show that by changing the polymer-polymer interaction we observed morphologies ranging interpenetrating networks to single polymer networks. Higher percentage of clay fillers and larger values of attraction between the polymers allows us to tailor the viscosity of the hydrogel pore fluid. High shear rates lowered viscosity values, leading to the pore fluids being shear thinning in nature. Overall, our results show that by choosing the hydrogel components and the nanofiller, the properties of the pore fluid can be controlled. These results will allow better design and selection of the biopolymer-based pore fluids for various biogeotechnology applications.

All-optical switching, or dynamically changing optical properties by light stimulation, has received much attention because of its capability of non-contact controlling. Volume phase transition of temperature-responsive hydrogel causes change of optical transmittance, and metal nanoparticles embedded in the hydrogel can enhance the transmittance change by light stimulation, owning to photo-thermal conversion via the localized surface plasmon resonance (LSPR). In this study, we propose a method to control spatially-localized optical transmittance of temperature-responsive hydrogel by external light stimulation, by the use of the metal microstructures fabricated with multi-photon photoreduction (MPR) method. MPR is the reduction of metal ions induced by ultrashort laser pulses which enables direct writing of metal microstructures composed of the nanoparticles. Grating microstructures with different line spacing were fabricated inside poly(N-isopropylacrylamide) (PNIPAm) hydrogel containing gold ions by computer-aided femtosecond-laser scanning. Since the gold microstructures formed in the hydrogel showed an absorbance peak around 520 nm, 520 nm laser diode (maximum laser power: 50 mW) was used as the light source of stimulation. As a result, decrease in the transmittance (more than 66 %) was observed at laser powers higher than 7 mW for the light stimulation, when hydrogel with line spacing of 5 µm was used. The threshold of the laser power that required to decrease the transmittance became higher as hydrogel with wider line spacing was used, demonstrating transmission property can be easily changed with the same materials and the fabrication method. Our results indicate the potential to fabricate all-optical switching devices with hydrogel.

Ionic conductive hydrogels can be used in many sensing and actuators applications, including chemical detection, artificial skin, and biosensors. We report herein the development of a discriminative sensor array used to detect heavy metals commonly found dissolved in waters in aquatic ecosystems. Each sensing unit is comprised by 4 interdigitated electrodes on which poly(ethylene glycol):poly(ethylene glycol) diacrylate (PEG:PEG-DA) films were formed by UVA (370 nm) curing. After crosslinking, the hydrogels were washed and functionalized by drop casting solutions of 3 different organic semiconductors: poly(3-hexathiophene) (P3HT), poly(o-methoxy anyline) (POMA) and a poly(vinyl carbazole):2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PVK:PBD) blend. The resulting sensor array was formed by 3 electrodes with organic semiconductor functionalized hydrogels, in addition to a “blank” hydrogel film, used as reference. Water solutions of iron (Fe), manganese (Mn) and molybdenum (Mo), at concentrations varying from 20 ppm up to 200 ppm were used to demonstrate the discriminative capability for heavy metal detection by using the global electrode response extracted from electrical impedance/capacitance measurements. The frequency-dependent response of the electrodes was analyzed by considering a generalized Warburg impedance model to extract the electrical response parameters. The electrical characteristics of the functionalized hydrogels were compared to the changes in the morphological, thermal and physio-chemical properties of the films, analyzed via scanning electron microscopy, thermogravimetry analysis, FTIR and Raman spectroscopy.

The phase behavior of materials ranging from gases to polymer solutions can often be understood semi-quantitatively in terms of their first few virial coefficients that quantify intermolecular interactions in the limit of low molecular concentration. The second (A₂) and third (A₃) virial coefficients, or their equivalent chi-interaction parameters in a polymer solution context, have correspondingly been adopted as fundamental measures of intermolecular interaction having fundamental significance for materials classification, design and characterization. It is often implicit in this type of description of phase behavior that the interactions between the molecules can be described by pairwise decomposable interactions and it is not clear if patterns of phase behavior applies to complex liquids, such as ionic and polyelectrolyte solutions, polyelectrolyte gels, etc. in which many-body interactions associated with polymer and ion solvation, and hydrogen bonding interactions of the water molecules, and the associated “hydrophobic effect” governing solvation of charged molecules or uncharged moieties in water, might lead to entirely different patters of phase behavior. Along this lines, simulations of simple hydrophobic solutes (modeled as Lennard-Jones molecules or simple molecules such as methane or noble gases) in water at ambient temperatures have indicated that while the interaction of two particles may be predominantly repulsive, corresponding to a positive second virial, the interactions between many particles can become attractive, corresponding to higher virials being negative, and this can lead to phase separation.This pattern of phase behavior is exhibited experimentally by all the polyelectrolyte gels that we study. We thus observe clear evidence of many-body interactions on the solution properties of polyelectrolyte gels.

Microscopies based on focused electron probes allow the cell biologist to image the 3D ultrastructure of eukaryotic cells and tissues extending over large volumes, thus providing new insight into the quantitative relationship between cellular architecture and function of organelles. Here we compare three such techniques: (i) electron tomography in conjunction with axial bright-field scanning transmission electron microscopy (BF-STEM), (ii) serial block face scanning electron microscopy (SB-SEM), and (iii) focused ion beam scanning electron microscopy (FIB-SEM). The advantages and limitations of each approach are illustrated by their application to determining the 3D ultrastructure of human blood platelets, and insulin-secreting beta cells of mouse pancreatic islets of Langerhans. In comparing the three methods, we consider specimen geometry, specimen preparation of stained polymer embedded cells and tissues, beam damage and automated segmentation methods. Many features of the organelles contained in blood platelets can be visualized using all three approaches, but STEM tomography offers an almost isotropic resolution of ~ 3 nm, compared with ~5 nm in FIB-SEM, and an anisotropic resolution of ~5 nm in the block-face plane and ~30 nm perpendicular to the block-face in SB-SEM. In this regard, we demonstrate that STEM tomography is advantageous for visualizing the platelet canalicular system consisting of interconnected membranous cisternae. In contrast, SB-SEM enables visualization of large numbers of complete platelets, each of which extends ~2 µm in minimum dimension, whereas an electron tomogram acquired in the BF-STEM can typically only visualize about half of a platelet volume due to a rapid non-linear loss of signal in specimens of thickness greater than ~1.5 µm. Although cellular and tissue volumes acquired by FIB-SEM are typically smaller than those acquired by SB-SEM, the improved spatial resolution perpendicular to the block-face in FIB-SEM facilitates more accurate 3D segmentation of the ultrastructure.

Biofabrication is a specific area within the field of tissue engineering which takes advantage of rapid manufacturing (RM) techniques to generate 3D structures which mimic the natural extracellular matrix (ECM). A popular material in this respect is gelatin, as it is a cost-effective collagen derivative, which is the major constituent of the natural ECM. The material is characterized by an upper critical solution temperature making the material soluble at physiological conditions. To tackle this problem, the present work focusses on different gelatin functionalization strategies which enable covalent stabilization of 3D gelatin structures [1, 2].
In a second part, synthetic acrylate-endcapped, urethane-based precursors (AUP) based on polyethers and polyesters, will be discussed with exceptional crosslinking behaviour and CAD-CAM mimicry compared to conventional materials [3]. Within this synthetic material class, also insight will be provided on the shape memory properties of polyester-based AUPs (two filed patents). Both chain growth and step growth polymerization mechanisms (see figure) along with their mechanical properties and processability potential will also be addressed.
Several polymer processing techniques will be covered including conventional 3D printing using the Bioscaffolder 3.1 and two-photon polymerization [4, 5]. A number of biomedical applications will be tackled including adipose tissue engineering [6, 7], vascularization [8], ocular applications [9], etc. In a final part, attention will be paid to the valorization of our biomaterial platform technology through the launch of our spin-off company Xpect-INX. The results show that chemistry is a valuable tool to tailor the properties of (bio)polymers towards processing while preserving the material biocompatibility.

1. J. Van Hoorick et al., Macromolecular Rapid Communications, 2018, 39, 1800181.
2. J. Van Hoorick et al., Biomacromolecules 2017, 18, 3260-3272.
3. A. Houben et al., WO 2017/005613.
4. J. Van Hoorick et al., Biofabrication, 2020, accepted.
5. A. Arslan et al., Materials Today, 2020, accepted.
6. L. Tytgat et al., Acta Biomaterialia, 2019, accepted.
7. L. Van Damme et al., Biomaterials Science, 2020, submitted.
8. L. Tytgat et al., Macromolecular Bioscience 2020 accepted.
9. J. Van Hoorick et al., Advanced Healthcare Materials 2020 accepted.

Cartilage is a load-bearing tissue that derives its stiffness from a fibrous network, mainly consisting of collagen type II, which is pre-stressed by the osmotic swelling pressure of negatively charged proteoglycan assemblies imbedded in the collagen matrix¹. Changes in the relative amounts of the main constituents will affect the macroscopic properties of cartilage. A decrease in tissue water content that might occur in some disease states could alter these properties and lead to impaired joint function. For example, previous findings indicate that osteoarthritic cartilage contains more water than healthy cartilage and exhibits poorer load-bearing properties.¹
We intend to determine how the interactions of the major biochemical constituents of articular cartilage influence its osmotic and mechanical behavior. One of the basic challenges of existing biomimetic cartilage models and modeling is that the pre-stress is largely ignored. However, in cartilage pre-stress exists even in the absence of external loading and plays an important role in defining the load-bearing properties of this tissue¹. Recently, we developed a pre-stressed biomimetic composite gel system consisting of a relatively stiff poly(vinyl alcohol) (PVA) matrix which encloses weakly cross-linked poly(acrylic acid) microgel particles. This model allows us to explore how changes in composition affect the swelling and mechanical behavior. There are three major determinants of the biomechanical behavior of articular cartilage. The first is the osmotic swelling pressure that reflects the interactions between the polymeric components of the tissue and the ions. The second is the elastic resistance of the cartilage matrix itself, and the third is the fluid-solid interaction that can affect hydration. It is demonstrated that our model satisfactorily describes the osmotic and swelling properties of healthy and osteoarthritic cartilage.²

1. Basser, P. J., Schneiderman, R., Bank, A., Wachtel, E. & Maroudas, A. Mechanical properties of the collagen network in human articular cartilage as measured by osmotic stress technique. Archives of Biochemistry and Biophysics 351, 207–219 (1998).
2. Horkay, F., Basser, P.J. Composite Hydrogel Model of Cartilage Predicts Its Load-Bearing Ability. Scientific Reports 10, 8103 (2020).

Nature designs a vast library of soft materials in living bodies to fulfil many unique and specific functions, including strong mechanics, high actuation and sensitivity, better biocompatibility, and fast self-healing property. Sure nature soft materials offer a rich reservoir for rational design and engineering of synthetic soft materials (polymers and hydrogels). Herein, we uniquely integrate theoretical models, molecular simulations, and biophysical experiments to conduct fundamental and applied research at the interface of computational structural biology, biophysics, biomaterials, and nanotechnology, with the goals of better understanding of biophysicochemical interactions at the biological interfaces and rationally engineering biomolecules for practical applications in biomaterials and medicine. In this talk, I will show different general strategy to design soft living materials with extraordinary properties. Guided by our computational theory, I will demonstrate the design of synthetic polymers and peptides for antifouling coatings, smart hydrogels, and amyloid inhibitors. The interplays between theory, experiments and design will be emphasized throughout the talk.

Genipin is a naturally occurring small molecule that has been used extensively as a protein crosslinker for engineering application-specific biomedical materials. Genipin crosslinked proteins have been used for applications ranging from tissue engineering scaffolds to drug delivery matrices. Here, we will discuss the use of genipin as an enabling additive to improve the mechanical properties and processability of both collagen-based networks for medical devices and fibrin-based networks for use as a strategy for reinforcement after embolization. In the first part of the talk, we will describe structure-processing-property relationships in genipin/collagen hybrids. The composition and processing history of these precursor materials can be controlled to produce materials ranging from rigid bioplastics to compliant hydrogel-based networks. Applications as structural materials in biomedical devices and bioelectronics will be discussed. In the second part of the talk, we will describe the use of genipin as a crosslinker to improve the robustness of fibrin networks in the context of embolization devices. Specifically, we will describe genipin-eluting polymer fibers as an approach to improve the efficacy of embolizing intracranial aneurysms. Diffusion-reaction kinetics of genipin released from polymeric fibers within fibrin hydrogels using an in vitro aneurysm flow model will be described. Taken together, this talk will elaborate on the use of genipin as an enabling component of protein-based hydrogels as biomedical materials.

When two or more nanoscale components assemble, they create new, advanced, and multifunctional composite systems. The assembly systems reveal advanced property/function that is not possible to achieve by individual types of nanoparticles alone. A wide range of various assemblies can be prepared such as self-assembly, templated assembly, directed assembly, etc. Considering the chemical nature of polymeric materials which are soft and flexible, it is more challenging and interesting to create the engineering architectures of nanoscale assemblies in hierarchical order. In general, the engineering of polymer nanoparticles with controlled properties is very promising for various applications. Polymer nanoparticles’ interior is mainly made up of the cross linked network. On the other side, the surface of polymer nanoparticles can be tailored with soft, flexible, and responsive molecules and macromolecules as potential support for the controlled particulate assemblies. Molecular surfactants and polyelectrolytes as interfacial agents improve the stability of the nanoparticles whereas a swellable and soft shell like cross linked polymeric layer at the interface can significantly enhance the uptake of guest nano constituents during their interactions to form the assembly architectures. Moreover, layer by layer surface functionalization holds the ability to provide a high variability in assembly architectures of different interfacial properties. Based on these aspects, various assembly architectures of polymer nanoparticles of tunable size, shapes, morphology, and tailored interfaces together with controllable interfacial interactions are presented here. The microfluidic supported platform has been used for the synthesis of constituent polymer nanoparticles of various structural and interfacial properties, and their assemblies are prepared in batch or flow conditions.

In recent years, there has been a continuous endeavor to develop synthetic shape-morphing materials of different architectures and scales and to decipher the underlying principles that govern their behavior and functionality. Here, we present 2D highly ordered shape-morphing networks of interconnected mesoscale fibers, constructed of a thermoresponsive copolymer. The networks are fabricated using the jet writing approach, which enables a high spatial precision over fiber deposition, a well-defined long-range order, and high control over the fiber diameter.
When immersed in a hot aqueous environment, the networks maintain their ordered as-spun morphology. However, upon cooling below, the fibers swell and the networks shape-morph, exhibiting two distinct morphing behaviors that are determined by the diameter of the diameter and the mesh density. In the case of thick fibers or dense networks, the networks swell while preserving their original ordered morphology and expands homogeneously. In the case of thin fibers and sparse networks, upon swelling the fibers exhibit significant in-plane buckling, and the network loses its ordered morphology. However, regardless of the morphing mode, the networks exhibit memory and return to their originally ordered, as spun, configuration upon heating. Simulations of the morphing of the network using a mesh-of-springs model indicated that for mesoscale hydrogel fibers, the change in the mechanical properties during the swelling process can induce a transition between the two morphing modes as a function of the fiber diameter.
The morphing behavior exhibited for networks of mesoscale morphing fibers differs significantly from bulk systems and macroscale networks of similar compositions, and hence demonstrates a novel structure-and-function relationship, which was not exhibited in other 2D morphing systems. Such meshes may be used in novel applications, ranging from artificial micro-muscles to shape-morphing optical devices, and may provide insights into the morphing mechanisms of complex biological filament networks such as actin-myosin and microtubules networks.

Stimuli-responsive hydrogels forms an interesting class of crosslinked hydrophilic polymeric materials, capable of absorbing large amount of water without dissolving. Their physical properties can be easily tuned to different external stimulus such as heat, light, pressure, pH, ionic strength, etc., by careful selection of functional groups and proper molecular design. This has in turn rendered their tremendous potential use for a wide array of applications such as biosensing, actuation, smart electronics, sensing and as osmotic agents.

Our research group has demonstrated previously use of thermally responsive hydrogels as smart regenerable draw agents for quasi-continuous forward osmosis (FO) where the swelling pressure of hydrogel drives the water permeation across the semi-permeable membrane [1, 2]. Hence, it can easily overcome reverse solute diffusion, one of the major drawbacks using conventional draw solutes and provide direct water recovery by heating the hydrogel above its volume phase transition temperature. High water recovery using N-isopropylacrylamide based hydrogels has gained special attention as its hydrophilicity can be tuned at physiological temperature i.e., 32 °C. On heating above this temperature, the change in the polymer solvent interaction results in conformational change of the polymer resulting in water release. However, its slow response and low absorption capacity has left them obscure to be used as draw agents.

To overcome the low absorption capacity and enhance the water release kinetics semi-IPN architecture of the hydrogel was previously proposed by our research group. By dispersing a hydrophilic polyelectrolyte, like poly sodium acrylate (PSA), within the PNIPAM network not only the water absorption capacity of the hydrogel can be improved but also fast water release was also observed. Yet only low water flux was achievable using the above system when used for FO. Although semi-IPN hydrogel is widely studied, yet there is no systematic study done regarding retention of polyelectrolyte within the gel network. Based on our recent work we have found that a number of factors play a critical role in defining the morphology of the semi-IPN hydrogel and also for enhanced retention of PSA in PNIPAM network[3]. Using a series of model PNIPAM/PSA semi-IPN hydrogel we not only determined the optimum synthesis conditions for higher retention of PSA within the PNIPAM network but also decoupled the swelling/deswelling kinetics from the measured swelling ratio. Using a lower polymerization temperature, more dense and homogenous structure is formed resulting in faster water uptake whereas higher polymerization temperature results in more porous fibrillar like structure with faster water release kinetics. By optimising the polymerization temperature, PSA concentration and Mw we were able to synthesize PNIPAM/PSA semi-IPN hydrogel with swelling ratio as high as 260 g/g with fast swelling kinetics. The ongoing work in this direction entails on tailoring compositional and morphological properties for enhancing interaction between hydrogel and membrane surface to improve the water flux and permeation.
References:
1. Cai, Y., et al., Exploration of using thermally responsive polyionic liquid hydrogels as draw agents in forward osmosis. RSC Advances, 2015. 5(118): p. 97143-97150.
2. Cai, Y., et al., Towards temperature driven forward osmosis desalination using Semi-IPN hydrogels as reversible draw agents. Water Research, 2013. 47(11): p. 3773-3781.
3. Gupta, N., et al., Design Rationale for Stimuli-Responsive, Semi-interpenetrating Polymer Network Hydrogels–A Quantitative Approach. Macromolecular Rapid Communications, 2020. n/a(n/a): p. 2000199.

Polymeric microfibers have been utilized as porous scaffolds for various types of three-dimensional (3-D) cell cultures but there still exist practical issues such as limited bioactivity, unfavorable mechanical stiffness, poor dimension controllability, etc. In this research, we developed water-soluble polymer(e.g., polyvinyl alcohol, hyaluronic acid and gelatin)-based negatively-charged hydrogel microfibers with enhanced bioactivity and adjustable mechanical properties. In order to increase dimensional controllability of hydrogel microfibers, we homogenized these microfibers and precipitated them by adding cationic polyelectrolyte, leading to the entangled hydrogel microfibrillar network with microscale porosity. Furthermore, because those fibers were made by combination with natural polymers, the hydrogel microfibrillar network was degradable by enzyme treatment (>95% by hydrolase such as collagenase or hyaluronidase). Also following the degree of crosslinking, the degradability and size of entangled hydrogel microfibrillar network were well modulated by controlling their composition and fiber diameters. Finally, we demonstrated that these colloidal hydrogel microfibers mixed with cells could be patterned by using a conventional 3D printer, suggesting that resultant hydrogel microfiber scaffolds are suitable for various types of 3-D cell cultures.

Ionogels are compelling materials for energy storage devices, ionotronics, and actuators due to their excellent ionic conductivity, thermal and electrochemical stability and nonvolatility. However, most existing ionogels suffer from low strength and toughness. Here, we report a simple one-step method to achieve ultra-tough and stretchable ionogels by randomly copolymerizing two common monomers in ionic liquid. Copolymerization leads to a single covalent network with controllable polymer- and solvent-rich phases that form in situ due to the phase behavior of the polymer in ionic liquid. The polymer-rich phase forms hydrogen bonds that dissipate energy and thereby toughen the ionogel during extension, while the solvent-rich phase remains elastic to enable large strain. The copolymer ionogels composed of acrylamide and acrylic acid exhibit extraordinary mechanical properties, including fracture strength (12.6 MPa), fracture energy (~24 kJ m-2), and Young’s modulus (46.5 MPa), setting new records among reported ionogels. The tough ionogels are highly stretchable (~600% strain) and possess good self-recovery, as well as excellent self-healing and shape-memory properties. This concept extends to other monomers and ionic liquids, which offers a promising and general way to tune microstructure in situ during one-step polymerization that solves the longstanding mechanical challenges in ionogels.