Symposium SM08 : Smart Hydrogels and Living Materials

The development of methods for interfacing high performance functional devices with biology could impact regenerative medicine, smart prosthetics, and human-machine interfaces. Indeed, the ability to three-dimensionally interweave biological and functional materials could enable the creation of devices possessing unique geometries, properties, and functionalities. Yet, most high quality functional materials are two dimensional, hard and brittle, and require high crystallization temperatures for maximal performance. These properties render the corresponding devices incompatible with biology, which is three-dimensional, soft, stretchable, and temperature sensitive. We overcome these dichotomies by: 1) using 3D printing and scanning for customized, interwoven, anatomically accurate device architectures; 2) employing nanotechnology as an enabling route for overcoming mechanical discrepancies while retaining high performance; and 3) 3D printing a range of soft and nanoscale materials to enable the integration of a diverse palette of high quality functional nanomaterials with biology. 3D printing is a multi-scale platform, allowing for the incorporation of functional nanoscale inks, the printing of microscale features, and ultimately the creation of macroscale devices. This three-dimensional blending of functional materials and ‘living’ platforms may enable next-generation 3D printed devices.

3D Bio-Printing uses cells and biomaterials as building blocks to fabricate personalized 3D structures or functional in vitro biological models. The technology has been widely applied to regenerative medicine, disease study and drug discovery. This presentation will report our recent research on printing cells for construction of micro-organ chips and for building in vitro 3D tumor models. An overview of advances of 3D Bio-Printing will be given. Enabling methods for cell printing will be described. Examples for 3D Printing of tissue engineering model, drug metabolism model and disease model will be reported, along with results of printing parameters on cell viability and 3D tumor structural formation, characterization of cell morphologies, proliferations, protein expressions and chemoresistances. Comparison of biological data derived from 3D printed models with 2D planar petri-dishes models will be conducted. Discussions on challenges and opportunities of 3D Bio-Printing will also be presented.

To our knowledge, we are the first to demonstrate the multiphoton fabrication of alginate hydrogel structures. The physiochemical properties of hydrogels are well suited to facilitate three-dimensional (3D) cell culture as they mimic the characteristics of a native cellular microenvironment. Alginate is one of the most prevalent biopolymers used for hydrogels due to its biocompatibility, reversible ionic gelation, and natural abundancy. Multiphoton fabrication allows for structures to be fabricated with sub-micron feature size, and therefore on the scale of single cells, while under conditions which living cells can tolerate. Combining the high resolution of multiphoton fabrication with the desirable properties of alginate enables control of the geometrical and mechanical properties of artificial cell microenvironments.
Alginate was functionalized with methacrylate groups in order to make it compatible with multiphoton polymerization. The methacrylation of the alginate was achieved using sodium alginate and 2-aminoethyl methacrylate (AEMA) in combination with the coupling agent 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT) and 4-methylmorpholine (NMM). The degree of methacrylation could be controlled by varying the amount of reactive compounds and can be used to tune the mechanical properties of the hydrogel. The methacrylated alginate was crosslinked in the presence of a water soluble photoinitiator, P2CK, developed for efficient multiphoton polymerization and biocompatibility. One to two weight percent of methacrylated alginate in water enabled the fabrication of microstructures with a minimum feature size of approximately 200 nm, characterized by scanning electron microscopy. Microstructures with the dimensions 50μm x 50μm x 10μm were fabricated at a laser scanning speed of 20 mm/s, printing one structure in 1-2 min, which is in one of the fastest speeds reported for multiphoton fabrication of hydrogels. We demonstrated that the microstructured alginate was stable under culture conditions with the gram-negative bacteria Pseudomonas aeruginosa over a period of 24 hours at 37°C. The viability of the bacteria cultured in the presence of the microstructures was shown by imaging the fluorescent marker that the bacteria were genetically modified to produce constitutively.
Here we report a method of alginate methacrylation that enables the multiphoton fabrication of biocompatible alginate hydrogels with sub-micron resolution. We predict this approach will enable the engineering of artificial cell microenvironments for a range of cell types and applications.

The lightweight construction, robust mechanical properties, and novel hardening mechanisms of invertebrate biting and piercing structures highlight their potential as models for advanced polymer systems. The mechanical properties of these structures often surpass those of synthetic polymers and can be comparable to mineralized composite tissues found in higher organisms. In addition, the natural structures also demonstrate mechanical gradients tuned for specific functions. Spatial and reconfigurable control of material mechanical properties would have many applications from shock-resistant electronic platforms to smart, responsive bandages and sutures. We have demonstrated the ability to express and purify large scale quantities of the protein Nvjp-1 (identified as the primary protein in marine worm jaw structures) and to process the purified protein into macro-scale hydrogels. Through a hierarchical sequence of ion treatments, involving exposure to divalent zinc cations, the mechanical properties of Nvjp-1 hydrogels can be modulated over a >100 fold range. Under certain conditions Nvjp-1 hydrogel sclerotization is very specific for zinc cations. However, it was observed that other transition metal cations (such as Cu²⁺, Ni²⁺ and Co²⁺) could induce a significant increase in elastic modulus if the hydrogel pH was modulated after metal binding. As with zinc-induced sclerotization, these changes are completely reversible and can be modulated over many cycles. Thus, modulation of polymer parameters through the incorporation of transition metal cations will potentially enable structures and materials with graded and reconfigurable properties.

Biological systems emerge through interactions between similar components. Individual components at a given hierarchy interact with one another, as well as with the ambient environment, to arrive at the next hierarchy with increased complexity and functionality. The precise nature of these interactions is yet to be resolved. We will discuss two examples where long range mechanical interactions between cells through extra cellular matrix or a compliant substrate lead to unusual collective behavior. First example involves primary cardiomyocytes cultured on soft gel substrate apart from each other. Here, contraction of any cell results in a stretch in the neighboring cells. This influences the beating of the neighbors as long as the stretch exceeds a threshold. They then behave as coupled oscillators. Stretch sensitive ion channels mediate this coupling. Over time, these cells beat in synchrony and the entire cell cluster beats as a single oscillator. The second example involves compaction of ECM by sparsely populated muscle cells (C2C12). Here, each cell remodels the matrix around itself. If the cells are far apart, their remodeled domains do not overlap, and they do not interact. If they are within a critical distance apart when their remodeled zones overlap, they interact and approach each other compacting the matrix. The critical gap appears to be around 100 µm. The cell-cell interaction lead to muscle formation in differentiation media. The interaction fails if the cell-cell gap is more than 100 µm, and gel compaction does not occur.

We have investigated the mechanical and fracture properties of dual crosslink hydrogels, i.e. gels containing two types of crosslinks: permanent covalent crosslinks and dynamic crosslinks. Although there are clear similarities in linear rheology with a well-defined single relaxation time, the non-linear behavior and fracture properties of the two different gel systems display significant differences when tested at different applied strain rates. The first system has been previously reported¹ and is composed of polyvinyl alcohol chains chemically crosslinked with glutaraldehyde and physically crosslinked with borate ions while the second system is a random copolymer of polyacrylamide and vinyl-imidazole chemically crosslinked with methyl bisacrylamide and physically crosslinked with metal ions. Both systems show a strong dependence of the fracture properties on the applied strain rate. However for the PVOH/Borax system the material becomes increasingly brittle with increasing strain rate while for the imidazole system the material becomes tougher with increasing strain rate. More interestingly we show that even if the fracture energy is reported as a function of a reduced strain rate (product of strain rate and rheological relaxation time) their behavior remains markedly different at high strain rates and we will discuss these differences in terms of a recently proposed model². We also obtain the very interesting result that, at low strain rate, the dual crosslink gels have a much larger extensibility than the simply chemically crosslinked gels even in a strain rate regime where the reduced strain rate << 1 and the physical crosslinks have very little influence on the stress-strain curve.


1. Mayumi, K.; Guo, J.; Narita, T.; Hui, C. Y.; Creton, C., Fracture of dual crosslink gels with permanent and transient crosslinks. Extreme Mechanics Letters 2016, 6, 52-59.
2. Long, R.; Mayumi, K.; Creton, C.; Narita, T.; Hui, C.-Y., Time Dependent Behavior of a Dual Cross-Link Self-Healing Gel: Theory and Experiments. Macromolecules 2014, 47 (20), 7243-7250.

We aim to develop composite materials that combine the attractive mechanical properties of gels with new functionality, with a focus on optical properties. Inspired by processes in living organisms, we create micro-structure in gels by driving phase separation of the solvent. The resulting structure is highly uniform, and the characteristic length scale can be controlled by tuning the network mechanical properties. The resulting composites are highly stretchable and can demonstrate strain-dependent optical properties.

Machine technology frequently puts magnetic or electrostatic repulsive forces to practical use, as in maglev trains, vehicle suspensions or non-contact bearings. In contrast, materials design overwhelmingly focuses on attractive interactions, such as in the many advanced polymer-based composites, where inorganic fillers interact with a polymer matrix to improve mechanical properties. However, articular cartilage strikingly illustrates how electrostatic repulsion can be harnessed to achieve unparalleled functional efficiency: it permits virtually frictionless mechanical motion within joints, even under high compression. Here we describe a composite hydrogel with anisotropic mechanical properties dominated by electrostatic repulsion between negatively charged unilamellar titanate nanosheets embedded within it. Crucial to the behaviour of this hydrogel is the serendipitous discovery of cofacial nanosheet alignment in aqueous colloidal dispersions subjected to a strong magnetic field, which maximizes electrostatic repulsion and thereby induces a quasi-crystalline structural ordering over macroscopic length scales and with uniformly large face-to-face nanosheet separation. We fix this transiently induced structural order by transforming the dispersion into a hydrogel using light-triggered in situ vinyl polymerization. The resultant hydrogel, containing charged inorganic structures that align cofacially in a magnetic flux, deforms easily under shear forces applied parallel to the embedded nanosheets yet resists compressive forces applied orthogonally. This electrostatically anisotropic structure allowed us to realize unidirectional motions in response to heat or light or even autonomously. We envision that this strategy, inspired by articular cartilage, will open up new possibilities for developing soft materials with anomalous functions.

References
[1] Q. Wang et al., T. Aida, Nature 2010, 463, 339–343.
[2] M. Liu et al., Nature Commun. 2013, 4, 2029.
[3] M. Liu et al., Nature 2015, 517, 68–72.
[4] Y.-S. Kim et al., Nature Mat. 2015, 14, 1002–1007.
[5] Y.-S. Kim, R. Yoshida et al., to be published.

Based on a linear poroelastic formulation, we present an asymptotic solution of the crack tip fields for steady-state crack growth in polymer gels. A linear finite element method is developed for numerical analysis. A strip with a semi-infinite crack under plane-strain condition is studied in details. The crack-tip fields by the numerical analysis agree with the asymptotic solution with a crack-tip stress intensity factor, which varies slightly with the crack velocity due to the poroelastic effect and generally smaller than the stress intensity factor predicted by linear elasticity. The size of the poroelastic crack-tip field is characterized by a diffusion length scale that depends on the crack velocity. For relatively fast crack growth, the diffusion length is small compared to the strip thickness, and the crack-tip field transitions to the elastic K-field at a distance proportional to the diffusion length. In this case, the energy release rate by a modified J-integral decreases with increasing crack velocity. For relatively slow crack growth, the diffusion length is comparable to or greater than the strip thickness, and the crack-tip field is confined by the strip thickness and transitions to a one-dimensional diffusion zone ahead of the crack tip. In this case, the energy release rate increases with increasing crack velocity. These results suggest that, if the intrinsic fracture toughness of the gel is independent of the crack velocity, the apparent fracture toughness including the energy dissipation due to solvent diffusion would be generally greater than the intrinsic toughness and depends non-monotonically on the crack velocity or the strip thickness.

Active networks are omnipresent in nature, ranging from the molecular level (with polymeric networks powered by molecular motors), micron scale (with cell and microbial colonies) and macroscopic level (with swarms of insects and larvae). Owing to their transient bond dynamics and intrinsic energy input, these networks display a rich class of behaviors, including elasticity, viscous flow, self-healing and growth. Although classical theories in rheology and mechanics have enabled us to characterize these materials, there is still a gap in our understanding on how the individual properties, i.e. the mechanics of the building blocks and their interconnections, affect the emerging response of the network.
In this presentation, we will discuss an alternative way to think about these networks from a statistical point of view. More specifically, a network will be seen as a collection of individual building blocks connected by elastic chains that can associate and dissociate over time. From the knowledge of these individual chains (elasticity, transient attachment, and detachment events), we will construct a statistical description of the population and derive an evolution equation of their distribution based on applied deformation and their local interactions. Upon appropriate averaging operations, we will then show that these distributions can be used to determine important macroscopic measures such as stress, energy storage, and dissipation in the network.
Using this approach, we will then discuss the mechanics of fire-ant aggregations, whose swarming behavior has shown impressive dynamics that culminates with the aggregation’s capacity to self-heal and adapt to the environment. In this context, we will show how the physical characteristics and behavior of single ants lead to the elasticity, rheological properties, and activity of the aggregation. Numerical simulations of the aggregation’s response in diverse situations will be presented and compared to experimental observations and measurements.

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 (i.e. hydrogels). Synthetic polymer hydrogels as soft-wet materials, consisting of three-dimensional cross-linked networks and a large amount of water (50–90%), possess many unique properties such as swelling/deswelling, stimuli-responsiveness, shock absorption, and low sliding friction, making them as potential excellent biomimetics for substitution of soft living materials. However, conventional hydrogels often suffer from weak mechanical properties, which greatly limit their extensive uses for many other applications. In this talk, we will present different design strategies to prepare tough and multifunctional hydrogels with unconventional polymer network architectures and extraordinary properties. Guided by our design principle, we will demonstrate different hydrogels with high mechanical properties, self-healing, actuation, mechanoresponse, antifouling, and/or wound healing to mimic cartilages, artificial muscles, and mussel-inspired glues. In parallel, molecular simulations will be presented to given atomic-details of structure-properties relationship. Finally, several unique aspects for future development of tough hydrogels will be suggested.

Tissues in living organisms are subject to dynamic environments orchestrated by multifaceted biochemical reactions and mechanical motions. As such, tissues undergo gradual or sudden remodeling which accompanies changes of mechanical properties, chemical properties, microstructure, and geometry. These transient behaviors of tissues influence biomolecular transports and cellular phenotypic activities. To this end, we have been interested in interrogating the extent to which transient properties of tissues modulate health, disease progression, and therapeutic efficacy of drug or cells. In these efforts, we have been developing hydrogel systems with controlled bioactivity and deformability. We have been also evolving these hydrogel systems to let them undergo shape and structural changes in response to external stimuli. My talk will highlight some significant findings we made by using hydrogel systems that can self-transform and regulate the molecular transports and cellular phenotypes for enhanced treatments of tissue defects.

Stimuli responsive materials have attracted considerable interest during the past years due to their potential use in sensor and actuator applications. They are designed to transform small external stimuli e.g. temperature and humidity changes into a significant response. While a large number of alloys or synthetic polymers are well-established at this point, we explore the potential of the biomaterial gelatin to respond to temperature and humidity for possible use as biological active control elements or as switchable scaffolds.
To tailor the stimuli responsiveness of gelatin, it is crosslinked by high energy electron irradiation which is nontoxic and thus enables biomedical applications. Thereby, a temperature dependent shape memory is introduced which can be utilized to develop a temperature-responding system.
Furthermore, we will show that electron irradiated gelatin has a high potential as a biocompatible and stimuli responsive demonstrator responding to humidity. By adaption of environmental parameters such as irradiation dose, gel concentration, pH-value and salt concentration, the response of the responsive element can be precisely tuned.

Nanomaterials that can be assembled with a high degree of control over their physical features are of interest for use in a variety of biological and therapeutic applications. Biotemplated nanomaterials have demonstrated great promise in this regard, owing to their ability to overcome low solubility, off-target toxicity, and poor biodegradability. There remains an important unmet need: trackable nanomaterials that are also highly biocompatible. Ideally, these would also be readily synthesized in a controllable manner.
Here, we report self-assembled quantum dot DNA hydrogels (QDH) that exhibit both size and spectral tunability. We successfully incorporate DNA-templated quantum dots with high quantum yield, long-term photostability, and low cytotoxicity into a hydrogel network in a single step. By leveraging DNA-guided interactions, we introduce multifunctionality for a variety of applications, including enzyme-responsive drug delivery and cell-specific targeting. We report that QDH delivery of doxorubicin, an anticancer drug, therapeutic efficacy in in vivo increases potency 9-fold against cancer cells. This approach also demonstrated high biocompatibility, trackability, and breastcancer xenograft mice. This work paves the way for the development of newtunable biotemplated nanomaterials with multiple synergistic functionalities for biomedical applications.

Temperature and biomolecular responsive hydrogels offer the possibility for the creation of smart devices that respond autonomously to physiological or environmental cues. However, it can be challenging to achieve the same level of control that can be obtained with electrically wired or pneumatic devices. In this talk, the design, fabrication and characterization of temperature and DNA responsive of soft shape change devices will be discussed including those that display programmability, multi-state and complex shape change. Gels are micropatterned using multilayer photolithography or 3D printing and swelling or collapse of gels drives shape change. Experiments are guided by mechanics and this synergy enables a high level of design. In addition to the intellectual elements, potential applications of these shape change devices will be highlighted.

Gel is composed of cross-linked polymer network and solvent molecules. Gels have broad application in many engineering fields such as drug delivery, tissue scaffold, soft robots and so on. Mechanical characterization of soft gels has been challenging. Recently there is a growing interest in using indentation techniques on gels because of the practical easiness. While relaxation indentation has been developed in characterizing the poroelastic properties of gels, dynamic indentation has been found to provide more accurate measurements in small scale, in which the gel is under oscillatory loading. In this study, we use the characteristic phase lag between the applied indentation displacement and the force on the indenter due to the energy dissipation from solvent flow in the gel to characterize the poroelasticity of gels. We will show that the phase lag degree is a function of two parameters, Poisson’s ratio and normalized angular frequency. The solutions are derived for several shapes of indenters. The maximum value of the phase lag over a spectrum of actuation frequencies can be used to characterize the Poisson’s ratio of the gel, and the characteristic frequency corresponding to the maximum phase lag can be used to characterize its diffusivity.

The stiffness of cells and the extracellular matrix is governed by fibrillar hydrogels. Already at low concentrations, these soft gels form porous network that become rapidly stiffer as soon as they are strained.
Some years ago, we found a unique hydrogel that closely mimics the architecture and the linear and nonlinear mechanical properties of cytoskeletal and extracellular matrix materials.¹ The synthetic nature of the material allowed us to tune the (mechanical) properties by changing straightforward parameters, like polymer length, concentration and environmental conditions.² Despite their broadly distributed nanoscale architecture, the mechanical properties of the gels are well described by models for semi-flexible networks,³ even in the limits of network formation.⁴
The mechanics of the cytoskeleton and the extracellular matrix, however, are not determined by a single component, but are the result of the composite of various fibrous proteins. Here, we will discuss how the presence of flexible, semi-flexible and stiff components change the mechanical properties of a semi-flexible hydrogel⁵ and how we can use these hybrids to make extremely responsive materials, for instance a material that increases its stiffness 5000% when heated only 1 °C.

References:
1. P.H.J. Kouwer, et al. Nature 493, 651 (2013).
2. (a) M. Jaspers, M. Dennison, M.F.J. Mabesoone, F.C. Mackintosh, A.E. Rowan, P.H.J. Kouwer, Nat. Commun. 5, 5808 (2014); (b) M. Jaspers, A.E. Rowan, P.H.J. Kouwer, Adv. Funct. Mater. 25, 6503 (2015); (c) M. Jaspers, A.C.H. Pape, I.K. Voets, A.E. Rowan, G. Portale, P.H.J. Kouwer, Biomacromolecules 17, 2642 (2016).
3. (a) F.C. Mackintosh, J. Kas, P.A. Janmey, Phys. Rev. Lett. 75, 4425 (1995); (b) M.L. Gardel, J.H. Shin, F.C. Mackintosh, L. Mahadevan, N. Matsuda, D.A. Weitz, Science 304, 1301 (2004); (c) C. Storm, J.J. Pastore, F.C. Mackintosh, T.C. Lubensky, P.A. Janmey, Nature 435, 191 (2005).
4. M. Dennison, M. Jaspers, P.H.J. Kouwer, C. Storm, A.E. Rowan, F.C. Mackintosh, Soft Matter 12, 6995 (2016).
5. M. Jaspers, S.L. Vaessen, P. Van Schayik, D. Voerman, A.E. Rowan, P.H.J. Kouwer, Nat. Commun. 8, 15478 (2017).

Hydrogels have shown tremendous potential in providing initial mechanical support to encapsulated cells during the process of tissue regeneration. Specifically, active hydrogels that respond to biological stimuli are found to be highly effective in enabling tissue growth by relaxation, degradation, etc. Once cells are encapsulated, the hydrogel can be designed to degrade and allow the transport of large extra-cellular matrix that matures into newly regenerated cartilage tissue. One of the toughest challenges here is programming hydrogel degradation for optimum growth conditions. For instance, hydrogel degradation that is too fast can result in complete loss of mechanical integrity whereas if it is too slow, it can deter growth. Understanding the physics that drive the processes of degradation and growth is, therefore, crucial in developing models that will help transition degradable hydrogels from the lab to the clinics. Supported by experiments and models, a key finding is that successful tissue growth occurs when there is a smooth transfer of mechanical properties from hydrogel to the new tissue. This can be achieved by: (a) matrix deposition with localized degradation and, (b) ensuring overall structural connectivity of the composite gel and neo-tissue. Localized degradation is possible with smart hydrogels whose bonds are designed to be sensitive to enzymes released from cells. This restricts transport of the extra-cellular matrix to the immediate vicinity of the cells where the hydrogel has degraded. The spatio-temporal behavior of hydrogel degradation and matrix deposition depend on the hydrogel properties and, often complex, cell behavior. Therefore, we have developed scaling laws that quantify these processes and potentially help tuning degradation. This microscale behavior is then transformed to predict the macroscopic evolution of mechanical properties based on the mechanics of composites. To ensure structural connectivity, we show that the presence of spatially well-connected and dense cell clusters is ideal. This is because the cell clusters create internal percolating tissue networks as the hydrogel degrades locally. The contribution of this work is an important step towards developing reliable design tools that help tune smart bio-responsive hydrogels for cartilage regeneration.

Phototropism is well developed in nature as a self-adapting functionality for many living organisms that rely on solar energy to survive. Plant such as the sunflower can turn its head following the sun by producing auxin that promotes stem elongation on the non-illuminated side of the stem. Thus the elongated shaded side of the stem pushes the stem to bend toward the sun. It takes millions of years for the plants to develop such a smart and self-adaptive functionality to harness the solar energy more efficiently. Today, mankind is facing unprecedented challenges including energy shortage, global warming, air pollutions, etc., due to the over burning of the fossil fuel to meet the ever increasing energy demand of the economic growth. Research societies are seeking solutions from the nature. However, there has no such a functionality being developed in synthetic materials to detect and track the light fully autonomously. Photomechano-responsive material systems have been attracting more and more attention throughout recent years, owing to their unique potentials to achieve smart and self-regulating photonic devices. Among various mechanisms that provide the functionality, photo-thermo-mechanical energy conversion has been considered as the one of the most promising routes to address the challenge. In this work, we report a soft material system that can self-adaptively track the incident energy source from arbitrary directions in the three-dimensional space around the material. Uniquely this soft material is physically symmetric in geometry and composition throughout the entire thermal-responsive hydrogel body with evenly distributed Au Nanoparticles as the photonic absorbers for specific frequency of incident light. Such soft photo-tracker can achieve fast and omni-directional real-time tracking in all direction covering 360 degrees of azimuthal plane, within 10s of seconds at a remarkably high tracking accuracy > 99.8%. The dynamic photo-thermo-mechanical process of the spontaneous tracking has been systematically characterized experimentally and elucidated by hydrogel mechanics modeling. With the micron-scale photo-tracker array, we further present an omni-directional solar vapor generator exhibiting extraordinary water vapor generation efficiency at all different incident angles constantly as high as at the 90 degrees normal incidence, indicating a significant compensation of energy loss caused by the angular incidence by the tracking functionality. The presented material system has broad applications that require maximizing the energy input, for enhanced solar harvesting, energetic emissive signal detecting and tracking, energy-efficient smart windows, and other self-regulative optic devices.

Soft and stretchable materials play an important role in the emerging fields of soft robotics, wearables, and stretchable electronics. Hydrogels are compelling materials for this application space because they are soft, chemically tunable, biocompatible, and ionically conductive. As such, hydrogels have been used for skin mountable sensors, such as electrocardiogram (ECG) electrodes, and show promise in emerging devices as stretchable transparent electrodes. Ultimately, these types of devices interface the hydrogel with rigid metallic electrodes to connect with electronic circuitry. Here, we show it is possible to interface hydrogel with liquid metal (eutectic gallium indium, EGaIn) electrodes to create completely soft and deformable contacts. EGaIn is noted for its low viscosity, low toxicity, and negligible volatility. The alloy can be patterned into non-spherical 2D and 3D shapes due to the presence of an ultra-thin oxide skin that forms on its surface. Because it is a liquid, the metal is extremely soft. As a case study, we interfaced EgaIn with hydrogels and tested it as soft electrodes for ECG. These electrodes require low impedance at biomedically relevant frequencies (1-50 Hz). Potentiostatic electrochemical impedance spectroscopy measurements show that capacitive effects at the hydrogel-EGaIn interface dominate the impedance at these low frequencies, yet can be reduced by utilizing embedded acidic and basic hydrogels that remove the native oxide skin. Increasing the ionic strength of the hydrogel also helps lower the impedance of the metal-hydrogel electrodes. The resulting devices have signal-to-noise ratios that exceed commercial ECG electrodes. The softness of these hydrogels can be modified without compromising the electrical properties to create truly soft electrodes. In addition to creating low impedance electrodes, it is possible to manipulate the interface between liquid metals and gels to create completely soft memory devices and soft diodes, or to utilize the electrodes to actuate hydrogel. Combining liquid metal with hydrogels represents a potential strategy of creating soft electrodes and other electronic componentns for bioelectronic applications, e-skins, and next-generation soft robotics.

Our lab at MIT has been interested in how the 1D and 2D electronic structures of carbon nanotubes and graphene respectively can be utilized to advance new concepts in molecular detection. We introduce CoPhMoRe or corona phase molecular recognition1 as a method of discovering synthetic antibodies, or nanotube-templated recognition sites from a heteropolymer library. We show that certain synthetic heteropolymers, once constrained onto a single-walled carbon nanotube by chemical adsorption, also form a new corona phase that exhibits highly selective recognition for specific molecules. To prove the generality of this phenomenon, we report three examples of heteropolymers–nanotube recognition complexes for riboflavin, L-thyroxine and estradiol. The platform opens new opportunities to create synthetic recognition sites for molecular detection. We have also extended this molecular recognition technique to neurotransmitters, producing the first fluorescent sensor for dopamine. Another area of advancement in biosensor development is the use of near infrared fluorescent carbon nanotube sensors for in-vivo detection2. Here, we show that PEG-ligated d(AAAT)7 DNA wrapped SWNT are selective for nitric oxide, a vasodilator of blood vessels, and can be tail vein injected into mice and localized within the viable mouse liver. We use an SJL mouse model to study liver inflammation in vivo using the spatially and spectrally resolved nIR signature of the localized SWNT sensors. Lastly, we discuss graphene as an interfacial optical biosensor, showing that it possesses two pKa values in alkaline and basic ranges. We use this response to measure dopamine in real time, spatially resolved at the interface with living PC12 cells which efflux dopamine, indicating graphene’s promise as an interfacial sensor in biology.
Zhang, JQ et. al. Molecular recognition using corona phase complexes made of synthetic polymers adsorbed on carbon nanotubes. Nature Nanotechnology, 8, 12, 2013, 959-968
Iverson, NM, et. al. In vivo biosensing via tissue-localizable near-infrared-fluorescent single-walled carbon nanotubes. Nature Nanotechnology, 8, 11, 2013, 873-880

Powered by recent advances in electronics and materials technologies, more and more devices start to be used around human body in highly intimate manner than ever. However, conventional engineering materials in such devices suffer from stark dissimilarities in mechanical and chemical properties to biological tissues. In light of the challenge of bridging artificial systems and human body, hydrogel-based human-machine interfaces or so-called “hydrogel machines” have emerged in a wide range of applications including tissue engineering, ingestible devices, smart wound dressing, neural interfaces, and soft robotic implants to name a few. However, more extensive use of hydrogels in the next generation hydrogel machines has faced a significant challenge from the weak and unstable interfacial integration between hydrogels and various engineering and biological materials. Notably, the recent development of tough bonding of hydrogels has enabled unprecedented level of robust hydrogel-based hybrid materials, which address the challenge and shed light toward the new class of hydrogel machines. Here, we will discuss tough bonding of hydrogels for a wide class of materials, with emphasis on mechanistic principles behind tough bonding of hydrogels. We will further highlight our recent tough bonding based advances in novel hydrogel machines including hydrogel coatings for medical devices, epidermal living sensors, hydrogel soft actuators and robots, and hydrogel-based hybrid neural probes.

Highly toxic chemical warfare agents (CWAs) such as sarin and related compounds possess potential threats at trace concentration, however, detection as low as Acute Exposure Guideline Levels (AEGLs) tier 1 (6.9 × 10^-09 mg/cm³ for 10 min) remains a challenge. Extensive efforts have been made to develop signal amplification technique for early detection of biological and chemical species. We previously reported that chemical gradients in hydrogel films can concentrate molecule at least 40-fold (by 24 h) by biasing diffusion. While we believe the concept of gradient-directed transport is attractive, and enhances the sensor response, however the slow transport velocity limits the applicability for real time detection. In this work, we propose a novel method in which a target molecule is fragmented by a catalyst into small parts and a specific small fragmented specie is directionally transported via imbedded chemical gradient in gel to a small-sized sensor (integrated in the gel). Since the small fragmented specie diffuses considerably faster than the original molecule, this method should accelerate the low velocity of the molecular transportation by at least two-orders of magnitude. For a proof of concept measurement, we will demostrate that an aerosol deposited sarin simulant, diisopropyl fluorophosphates (DFP) absorbs in a hydrogel and subsequently hydrolyzes upon contact with imbedded enzyme, diisopropyl fluorophosphatase (DFPase), producing F^-. The F^- is then concentrated via a built-in ionic chemical gradient to a miniature electrochemical sensor, thus giving 30-fold amplified respond within 10 minutes. This method allows us to detect airborne sarin as low as tier-1 AEGLs and opens a new detection platform for other potentially dangerous chemical and biological agents at very low concentration before they have done too much harm.

Gels are composed of crosslinked polymer network and solvent molecules. When the main chain network is incorporated with functional groups that can undergo photo-chemical reaction upon light irradiation, the gel becomes light-responsive. Under irradiation, the photosensitive groups may undergo photo-ionization process and generate charges that are attached to the main chain or diffuse into the solvent. The newly generated ions disturb the osmotic balance of the gel medium. As a result, water molecules and mobile ions will be driven into or out of the network to compensate the osmotic imbalance, which will eventually lead to macroscopic swelling or shrinking of the gel. In this work, we develop a rigorous nonequilibrium thermodynamic framework to study the coupled photo-chemo-electro-mechanical responses of the photo-ionizable gels. We first discuss the mathematical descriptions of the light propagation and photo-induced chemical reactions inside the gel, as well as the equations governing the kinetics of the photo-chemical reactions. We then explore the consequences of the fundamental laws of thermodynamics in deriving the governing equations of the photo-ionizable gels. The continuous light irradiation drives the gel system towards a new thermodynamic stationary state that is away from equilibrium and is accompanied by energy dissipation. Next, we focus on the photo stationary state of the gel and explore the consequences of the continuous irradiation on the mechanical response of the gel in both optically thin and optically thick configurations. In the optically thin cases, we quantitatively compare the theoretical prediction with experimental data available in the literature. In one example, we show that the model can quantitatively capture the photo-tunable volume-phase transition of the Poly(N-isopropylacrylamide) (PNIPAM) gel grafted with photo-responsive triphenylmethane leucocyanide groups. In another example, we show that the model can quantitatively study the effect of salt concentration and pH value of the external solution on the photo-induced swelling of the polyacrylamide gels incorporated with triphenylmethane leucohydroxide groups. Finally, for the optically thick gels, we develop a finite element code to study their inhomogeneous deformations due to the light attenuation. This work will be of great importance for precise control and optimal design of photo-ionizable gels in future applications.

In recent decades, unprecedented amounts of electronics are being integrated with human body as wearable, implantable and edible devices for monitoring, sensing and responding. One main challenge relates to the high stiffness of the typical electrical components used in these devices when compared with the surrounding compliant soft biological tissues. To accommodate the modulus mismatch between electrical components and biological tissues, synthetic soft materials are widely adopted to achieve flexibility and stretchability. Particularly, hydrogels (i.e. polymer networks infiltrated with water) not only mechanically soft and flexible but also accommodating transportation and reaction of species for versatile functional responses are one of the most ideal soft materials for bioelectronics. However, existing hydrogel-based electronics mostly suffer from the limitation of mechanical robustness owing to the weak nature of common synthetic hydrogels and poor interfacial performance between hydrogels and other surrounding materials. Therefore, the understandings of the fracture process in soft materials and relevant interfaces are imperative to the design of mechanically robust bioelectronics and devices, which enables long-term biocompatibility and functionality (e.g. electrical stability).

Although various soft tough materials have been developed in recent decades, it is still not well understood how the intrinsic fracture energy of soft materials (i.e. the energy required to rupture a layer of polymer chains in front of the crack) and the mechanical dissipation in process zones around crack cooperate synergistically to give rise to high toughness of soft materials. Here, we report a theoretical scaling and continuum model that quantitatively account for the synergistic contributions of intrinsic fracture energies and dissipations to the total fracture energies of soft materials. Based on the model, we further calculate a toughening diagram that can guide the design of new soft materials.

In addition, we systematically study the formation, transition and interaction of mechanical instabilities in soft elastic interfacial layers under tension. Through combined experimental, numerical and theoretical analysis, we find that the mode of instability is determined by both geometry and mechanical properties of the layer through two non-dimensional parameters: layer thickness over its lateral dimension and elastocapillary length over the defect size. A phase diagram is calculated to quantitatively predict the occurrence of any mode of instability. Systematically understanding of the formation and interactions of various mechanical instabilities in elastic layers under tension can provide a guideline for the design of robust adhesives by rationally harnessing the desired mode of instabilities while suppressing the other modes.

Liquid crystal polymer networks have emerged as promising smart materials where reversible shape change can be hardcoded into the material microstructure. However, liquid crystal polymer networks are typically hydrophobic and only respond to stimuli that would be incompatible with biological environments, such as high temperatures and organic solvents. Chromonic liquid crystals are a type of lyotropic liquid crystal which exhibits ordering in aqueous environments. Recently, we have shown that by orienting and then crosslinking chromonic monomers, hydrogels with anisotropy on the molecular level can be synthesized. Specifically, we will discuss using photopatterned surfaces to direct the self-assembly of an aqueous solution of a liquid crystalline, perylene diimide-based dimethacrylate. By copolymerizing this dimethacrylate with common functional comonomers, liquid crystal gels can be synthesized that undergo reversible shape change in response to temperature and pH. This shape change is anisotropic, as dictated by the orientation of the liquid crystal phase during crosslinking. By rationally programming the order of the gel, we demonstrate a range of hydrogels capable of autonomous shape change from dynamic coatings to free-standing actuators. Finally, we will discuss using the intrinsic semiconducting nature of perylene diimide to create multifunctional sensing, actuating gels.

Soft and tough materials are critical for engineering applications in medical devices, stretchable and wearable electronics, and soft robotics. Toughness in synthetic materials is mostly accomplished by increasing energy dissipation near the crack tip with various techniques from mesoscale approaches like particle-filled composites to molecular scale techniques including hybrid and double network gels and polymers. However, bio-materials exhibit extreme toughness by combining multi-scale energy dissipation with the ability to deflect and blunt an advancing crack tip. Here, we demonstrate a synthetic material architecture that also exhibits multi-modal toughening, where by embedding a suspension of micron sized and highly deformable liquid metal (LM) droplets inside a soft elastomer, the fracture energy dramatically increases by up to 50x (from 250 ± 50 J/m² to 11,900 ± 2,600 J/m²) over an unfilled polymer. For some LM-embedded elastomer (LMEE) compositions, the toughness is measured to be as high as 33,500 ± 4,300 J/m², which far exceeds the highest value previously reported for a soft elastic material. This extreme toughening is achieved by means of (i) increasing energy dissipation, (ii) adaptive crack movement, and (iii) effective elimination of the crack tip. Such properties arise from the deformability and dynamic rearrangement of the LM inclusions during loading, providing a new mechanism to not only prevent crack initiation, but also resist the propagation of existing tears for ultra tough, highly functional soft materials.

Introducing methods for instant strong bonding between hydrogels and antagonistic materials – from soft to hard – allows us to demonstrate elastic, yet tough biomimetic devices and machines with a high level of complexity [1]. Tough hydrogels strongly attach, within seconds, to plastics, elastomers, leather, bone and metals reaching unprecedented interfacial toughness exceeding 2000 J/m². Our approach is applicable in rapid prototyping and in delicate environments inaccessible for extended curing and cross-linking. The combination of ionic hydrogels with antagonistic materials such as elastomers, polymers and metals allows to create soft electronics and hybrid machines. We demonstrate stretchable batteries for self-powered soft devices, adaptive lenses, and autonomous electronic skin for triggered drug delivery. We further introduce a new family of biodegradable hydrogels that are able to self-heal and are resistant to dehydration. Soft machines and robots – built from hydrogels with tuned mechanical properties – are designed to be operated in ambient conditions and degrade after use. Besides progressing stand-alone soft machines, our advances in the synthesis of biodegradable hydrogels bring bionic soft robots a step closer to nature.



[1] D. Wirthl, R. Pichler, M. Drack, G. Kettlgruber, R. Moser, R. Gerstmayr, F. Hartmann, E. Bradt, R. Kaltseis, C. M. Siket, S. E. Schausberger, S. Hild, S. Bauer, M. Kaltenbrunner, "Instant tough bonding of hydrogels for soft machines and electronics”, Science Advances, 3(6), e1700053 (2017).

Genetically engineered protein copolymers, which can combine different unique peptide sequences derived from natural protein materials, offer great opportunities for making advanced materials with tailored mechanical properties and self-assembling propensities. Here we report for the first time biosynthesis and self-assembly of a recombinant resilin−silk (RS) copolymer consisting of repeating units of silk and resilin blocks. The copolymer in aqueous solution self-assembled into diverse types of nanostructures in processes that are temperature-dependent, and the assembled nanoparticles further form nano- to microscale fibers in a time-dependent manner at body temperature, whereas such fibers were not formed upon incubation of the copolymer at either low or high temperatures. In contrast, a resilin-like polypeptide without the silk blocks exhibited a typical thermoresponsive dual-phase transition behavior and was incapable of selfassembling into fibers. More interestingly, the microscale fibers self-assembled from a moderately concentrated RS solution (20 wt %) could interact to give a self-supporting, semitransparent hydrogel with elastic modulus at approximately 195 Pa. Furthermore, photo-cross-linking of either freshly prepared or annealed RS copolymer led to the formation of stiff hydrogels and the material mechanical property was superior upon annealing of the RS solution for a longer time up to 4 h, with elastic modulus ranging from 2.9 to 7.0 kPa. These results not only shed light on the fundamental hierarchical assembly mechanism of a new family of genetically engineered RS copolymer but also suggest future opportunities for these thermoresponsive polymers in fabrication of hydrogel materials with tunable mechanical properties for diverse applications.

In this study, we introduced an evaporation-driven method to fabricate Janus droplets based on phase-separation mechanism of aqueous two-phase system (ATPS). Specifically, phase transition of ATPS were driven by changes in the concentration of solutes at a constant temperature. Without any additional step, two-immiscible phases were induced to form at room temperature by evaporation of water. Taking advantage of this property of ATPS, we firstly generated single-phase droplets and then converted them into Janus ones by evaporation. Consequently, the single-phase droplets separated into two immiscible phases with clear interfaces.
In a typical experiment, a flow-focusing device was used to produce the homogeneous emulsion drops. An aqueous solution containing PEG and dextran was injected into the flow-focusing device as the dispersed phase, while FC-40 with 5% PFPE-PEG-PFPE surfactant was injected as the dispersed phase. The resultant droplets were single-phase initially due to the low concentration of PEG and dextran. Afterwards, we collected the emulsions on a hydrophobic plate and exposed them to air. Water inside emulsions evaporated through FC-40, resulting in the increase of the concentration of PEG and dextran. Consequently, the homogeneous droplets separated into two immiscible phases, and Janus droplets formed. The size of the Janus droplets were tuned from 4.4μm to 103μm, by adjusting the height of microfluidic device and the flow rate ratio of inner phase to outer phase. Volume ratio between different compartments was changed from 0.38 to 1.43 by altering the initial concentration ratio of PEG/dextran from 0.1 to 4.5.

This ATPS-based approach enabled the activity of encapsulated bio-ingredients to be preserved at a relatively high level, since phase separation was triggered under mild conditions by evaporating water. To further confirm the biocompatibility of the proposed method, we introduced horseradish peroxidase (HRP) and cells into the inner phase respectively. After droplet generation and phase separation, 71% of the relative activity of HRP was preserved, and 82.8% of the cells were still alive inside the Janus droplets. In comparison, the relative activity of HRP dropped to 51% and all the cells became dead with conventional organic-solvent-based phase-separation.

In conclusion, we successfully fabricated Janus droplets by the ATPS-based phase-separation method. The resultant Janus droplets were fabricated with clear interfaces and highly controlled volume ratio of the two compartments. The activity of encapsulated bio-gradients was highly preserved. Consequently, the proposed method potentially extended the applications of Janus droplets for encapsulating delicate biomolecules.

(This abstract is based on our published paper: H. Yuan, Q. Ma, Y. Song, Matthew Y. H. Tang, Y. K. Chan, H. C. Shum, Phase separation-induced formation of Janus droplets based on aqueous two-phase systems, Macromol. Chem. Phys., 2017, 218, 1600422)

Hydrogels have been extensively studied for biomedical applications such as tissue engineering and drug delivery. The physicochemical, biomechanical and biological properties of hydrogels based on various synthetic or natural polymers have been characterized, and numerous recipes to optimize these properties have been established. However, relatively little research has been conducted in relation to the optimization of their optical properties. Here, I will talk about the recent development of hydrogel-based optical devices with emphasis on light-guiding waveguides. Several wearable, implantable, biodegradable, and cell-encapsulating devices will be presented, along with proof-of-concept demonstrations of their medical applications, solving the limitations of conventional optical devices and conventional approaches.

Implantable microdevices often have static components rather than moving parts and exhibit limited biocompatibility. I will discuss two different sets of work in our lab to address these limitations, and our initial demonstrations of these technologies in mice for drug delivery. First, we have developed a platform called implantable microelectromechanical systems (iMEMS), which enables development of biocompatible implantable microdevices with a wide range of intricate moving components that can be wirelessly controlled on demand, in a manner that solves issues of device powering and biocompatibility (Science Robotics, 2017). The method can produce features in biocompatible materials down to tens of micrometers in scale, with intricate and composite patterns in each layer. By exploiting the unique mechanical properties of hydrogels, we developed a “locking mechanism” for precise actuation and movement of freely moving parts, which can provide functions such as valves, manifolds, rotors, pumps, and delivery of payloads. Hydrogel components could be tuned within a wide range of mechanical and diffusive properties and can be controlled after implantation without a sustained power supply. In a mousemodel of osteosarcoma, triggering of release of doxorubicin from the device over 10 days showed high treatment efficacy and low toxicity, at 1/10 of the standard systemic chemotherapy dose. In a second platform called iTAG (implantable thermally actuated gel), we showed how focused ultrasound can trigger functions in an implanted capsule to trigger the release of compounds. We constructed a capsule containing a co-polymer gel (NiPAAm-co-AAm) that contracts above body temperature (i.e. at 45 °C) to release compounds through an opening. This gel-containing capsule is biocompatible and free of toxic electronic or battery components. An ultrasound hardware, with a focused ultrasound (FUS) transducer and a co-axial A-mode imaging transducer, was used to image the capsule (to monitor in real time its position, temperature, and effectiveness of dose delivery), as well as to trigger a rapid local rise in temperature, contraction of gel, and release of compounds in vitro and in vivo. The combination of this gel-based capsule and compact ultrasound hardware can serve as a platform for triggering local release of compounds, including potentially in deep tissue, to achieve tailored personalized therapy.

Silk (SF)-based hydrogels have widely developed for various biomedical applications including tissue engineering, drug delivery, ect. However, current methods for SF gelation showed significant limitations such as use of nonphysiological conditions, lack of reversible crosslinking, and difficulties in controlling gelation time. In this study, we used dynamic metal-ligand coordination chemistry approach to develop SF-based hydrogel based on SF microfibers (mSF) and a polysaccharide binder under physiological conditions. The presented SF-based hydrogel showed self-healing and shear-thinning properties that give the advantages for the filling of irregularly shaped tissue defects without gel fragmentation. We used biomineralization approach to generate calcium phosphate-coated mSF (CaP@mSF). The hydrogel could be formed by simply mixed the polysaccharide binder and CaP@mSF basing on the reversible cross-linkages between bisphosphonate ligands on the backbone of the binder and the CaP on the mineralized mSF. Robust dually crosslinked (DC) hydrogel was obtained by photopolymerization of acrylamide groups of the binder. The DC SF-based hydrogel not only supported mesenchymal stem cell proliferation in vitro but also accelerated bone regeneration in rat cranial critical size defect model even without any additional growth factors delivered. Therefore, the developed self-healing and photopolymerizable SF-based hydrogel have significant potential as injectable bone regeneration scaffolds with the advantages of fit-to-shape molding.

A green and cheap silicone-based elastomer has been developed.^1,2 Through the simple mixing-in of biodiesel-originating glycerol into commercially available polydimethylsiloxane (PDMS) pre-polymer, a glycerol-in-silicone emulsion was produced. This counterintuitively stable mixture became a basis for obtaining elastomeric composites with uniformly distributed glycerol droplets. Various compositions, containing from 0 to 140 parts of glycerol per 100 parts of PDMS rubber by weight, were prepared and investigated in terms of mechanical properties as well as optical and scanning electron microscopy. The materials were proven additionally to exhibit a strong affinity to water, which was investigated by simple water absorption tests. Incorporating glycerol into PDMS decreased the Young’s modulus of the composites yet the ultimate strain of the elastomer was not compromised, even in the presence of very high loadings. The conducted experiments highlight the great potential of this new type of elastomer and reveal some possible applications especially in biomedical industry where controlled and tunable drug delivery is one of the requirements. This hybrid material was also adopted to produce glycerol-silicone elastomeric foams with adjustable densities, morphologies and mechanical properties creating a new platform for drug delivery devices.

References:
[1] P. Mazurek, S. Hvilsted, A.L. Skov, Polymer, 2016, 87, 1-7.
[2] P. Mazurek, L. Yu, R. Gerhard, W. Wirges, A.L. Skov, J. Appl. Polym. Sci., 2016, 133, 1-8.

Hydrogels are useful biomaterials whose mechanical and transport properties may be tuned via choice of polymer and cross-link density. We have utilized an inverse colloidal crystalline array of sacrificial polystyrene spheres of 140µm diameter followed by photopolymerization of a hydrogel network in the interstices of the poragen array to generate a three-dimensional tissue scaffold. Choice of poragen diameter and annealing conditions allows control of the scaffold cavity size as well as the open channels connecting adjacent cavities. When poly(ethylene glycol)-diacrylate is used as the macromonomer, it is necessary to functionalize the surface of the resulting PEG scaffold because of the well-known resistance of PEG chains to protein and cell adhesion. Through chemical coupling of collagen I we were able to create microenvironments in which human umbilical vein endothelial cells as well as freshly isolated fetal total liver cells demonstrated good attachment and growth. We have demonstrated the formation of a 3D interconnected architecture, sustaining liver-specific functions (e.g., albumin secretions, cytochrome P450 activity and viral infectivity) over a very long term. Recently, we have advanced this system to fabricate very thin hydrogel layers that allow better oxygen and nutrient diffusion while maintaining the 3D interconnected structure. This has allowed us to culture human adult primary hepatocytes, the gold standard for liver studies. These cells are highly oxygen-sensitive and have intrinsically weak cell-cell and cell-extracellular matrix interactions. The new scaffold resulted in a dramatic change in cell morphology and function, increasing the prospect of constructing highly functional adult hepatocyte-based 3D liver tissue. In addition, we have used biodegradable materials, e.g., GelMA or fibrin, in place of PEG for a transplantation study on mice to show proof-of-concept for clinical use. A highly organized biodegradable scaffold was successfully developed even with the very soft hydrogel. We are currently using 3D printing of encapsulated endothelial cells to generate vascularized liver tissue. This approach eventually may lead to a readily transplantable organ or a long-term sustainable authentic in vitro human liver model that could be used for drug screening and disease modeling.

While human tissues are mostly soft, wet and bioactive; machines are commonly hard, dry and biologically inert. Bridging human-machine interfaces is of imminent importance in addressing grand societal challenges in healthcare, security, sustainability and joy of living; but extremely challenging, due to the fundamentally contradictory properties of human and machine. At MIT SAMs Lab, we use nanoengineered bioactive hydrogels to bridge human-machine interfaces. On one side, bioactive hydrogels with similar physiological properties as tissues can naturally integrate with human body, maintaining long-term biocompatibility and bioactivity. On the other side, the hydrogels embedded with electronic, optical and mechanical components can effectively interface with external machines for functions such as sensing and stimulation. In the talk, I will discuss our recent works on the design and fabrication of hydrogel-based biorobots and their biomedical applications. In particular, I will demonstrate two unconventional actuation mechanisms for hydrogel robots, hydraulic and magnetic actuations, to achieve high-speed, high-force and multifunctionalities. We will further discuss potential applications of hydrogel robots in biomedicine such as drug delivery, stimulation of various organs, and operations in gastrointestinal tract.

Hydrogels are formed through crosslinked polymer chains within an aqueous environment. The properties and functions are pre-adapted in the thermosetting network structures for a given sample and normally not allow for post-modulation on-demand. The attraction of the capability to post-modulate material property is not only the waste reduction but also the adaptivity and flexibility in applications. In this presentation, I will introduce a class of novel crystal/polymer hybrid hydrogels developed in our lab. In these hydrogels, the crystal of organic small molecules grows along polymer chains to form hierarchical dendric architectures throughout the hydrogel to dominate its shape and mechanical properties. The nucleation and growth of the crystal in the hydrogels is fully modulated by force stimuli and thus permit unprecedentedly control over the shape, mechanical property, anisotropy, and the adhesivity of a given sample after its gelation.

Most of the drugs disseminate sooner after deploying, which cause the drug level to rise rapidly and diminish earlier, whereas targeted drug delivery allows maximizing the efficiency of the drug by releasing it slowly based on the physiological condition. An injectable hydrogel has advantages over conventional drug delivery with nanoparticle/polymer composites for the controlled release of a drug because the hydrogel is formed after the injection and hold the drug for a longer period. A hydrogel is a three-dimensional network in aqueous media and is sensitive to external stimuli such as temperature and pH consisting of different hydrophobic/hydrophilic/pH sensitive blocks. Low critical solution temperature (LCST), a temperature of which they form a gel state from a solution state, can be tuned by altering the molecular structure. Atomic simulations help to study the structure-performance relationship of drug-delivery systems (DDS) considering the interaction between components, which is focused in this study. First of all, a molecular model of the three-dimensional network of a chitosan hydrogel including water is constructed and thermal cycles are applied with NPT (isothermal-isobaric) ensembles to find the critical temperatures. Subsequently, three different drug molecules are incorporated into the model separately and the distributions are observed as the distributions will affect the drug discharge. Compatibility between the drug molecules and the carrier is predicted by the Flory-Huggins miscibility parameters. Afterward, a protein associated with a cancer is added to the supercell and the diffusion of the drug is observed by analyzing the trajectories of the drug and protein molecules from the simulations performed with canonical ensembles.

Low-density lipoproteins (LDL) make up the majority of the cholesterol found in the body, with hypercholesterolemia being a major cause of atherosclerosis. LDL receptors mediate endocytosis of LDL, diminishing the level of LDL in the blood plasma to homeostasis. After internalization, the ligand dissociates and the receptor folds back and recycles onto the cell surface, making itself available to bind to more LDL molecules. This activity is in part modulated by proprotein convertase subtilisin/kexin type 9 (PCSK9), an inhibitory enzyme that binds with the EGF-A domain of the LDL receptor and prevents the conformational change of the receptor-ligand complex. This causes the natural intracellular degradation of the receptor, preventing recycling onto the cell surface. As a result, the cholesterol level increases in the blood stream causing hypercholesterolemia. Globally, about one third of heart diseases is caused by increased cholesterol level. Therefore, there is a need for a therapeutic approach that can inhibit PCSK9. Several peptides have already been identified that mimic the EFG-A domain of the LDL receptor and bind with PCSK9, inhibiting its action and allowing the receptor to perform its normal function. However, the delivery of a small peptide is difficult without a proper vehicle, since small peptides are rapidly cleared in the body. Our approach is to develop an injectable peptide-based hydrogel delivery system that incorporates a peptide inhibitor of PCSK9. This self-assembling peptide system is thixotropic and forms a hydrogel with high epitope presentation of the PCSK9 inhibitor. The result is a technology that has better targetability and persistence and can activate more receptors. The self-assembling peptide hydrogels were tested for their biocompatibility and immunogenicity through in vitro cell culture and in vivo subcutaneous injection in mice, respectively. The cholesterol lowering effect of the hydrogels was evaluated in an in vivo mouse model where they were fed a high cholesterol diet for 3 months, with monthly dosing of hydrogel and weekly bleeds to detect cholesterol level and IgG response. The goal of our work is to create a library of clinically relevant LDL lowering peptide hydrogel drugs that can be tailored for (i) decreased dosing, (ii) increased compliance, (iii) decreased immunogenicity compared to the standard-of-care monoclonal antibody Evolucimab® that would ensure long term utility of the therapeutic, and (iv) lower cost compared to the standard-of-care. Our rational design of the therapeutic platform may usher a new generation of LDL-lowering drugs on the market.

We report the fabrication of living soft matter made as a result of the symbiotic relationship of two unicellular microorganisms.¹ The material is composed of bacterial cellulose produced in situ by Acetobacter (Acetobacter aceti NCIMB 8132) in the presence of photosynthetic microalgae (Chlamydomonas reinhardtii cc-124), which integrates into a symbiotic consortium and gets embedded in the produced cellulose composite. The same concept of growing living materials can be applied to other symbiotic microorganism pairs similar to the combination of algae and fungi in lichens, which is widespread in Nature. We demonstrate the in situ growth and immobilisation of the C. reinhardtii cells in the bacterial cellulose matrix produced by the simultaneous growth of acetobacter. The effect of the growth media composition on the produced living materials was investigated. The microstructure and the morphology of the produced living biomaterials were dependent on the shape of the growth culture container and media stirring conditions, which control the access to oxygen. As the photosynthetic C. reinhardtii cells remain viable and produce oxygen as they spontaneously integrate into the matrix of the bacterial cellulose generated by the acetobacter, such living materials have the potential for various applications in bio-hydrogen generation from the immobilised microalgae.² The proposed approach for building living soft matter can provide new ways of immobilising other commercially important microorganisms in a bacterial cellulose matrix as a result of symbiosis with acetobacter without the use of synthetic binding agents and in turn increase their production efficiency.

Reference
1. A. A. K. Das, J. Bovill, M. Ayesh, S. D. Stoyanov and V. N. Paunov, Journal of Materials Chemistry B, 2016, 4, 3685-3694.
2. A. A. K. Das, M. M. N. Esfahani, O. D. Velev, N. Pamme and V. N. Paunov, Journal of Materials Chemistry A, 2015, 3, 20698-20707.

While stem cells and their progeny have significant therapeutic promise, the difficulty and cost of expanding a large number of high-quality stem cells remains a significant barrier to widespread clinical use. Recently, 3D hydrogels have been proposed as in vitro culture platforms for the expansion of stem cell populations to overcome the space limitations of 2D culture and to mimic critical aspects of the native stem cell niche. Here we explore the material properties required to maintain the stemness of neural progenitor cells (NPCs). Using three different material platforms, we demonstrate that NPCs must be able to adaptively remodel the 3D material in order to maintain their stemness during proliferation. Mechanistically, material adaptation allows NPCs to establish cell-cell contacts that initiate beta-catenin signaling that drives expression of stem cell genes. Material adaptation could be achieved either through use of on-demand proteolytic degradation by cell-expressed enzymes or through use of reversible crosslinks that can be remodeled over time to alter the polymeric network structure. Interestingly, we also present data demonstrating that 3D matrix stiffness does not correlate with the maintenance of NPC stemness over a broad range of matrix mechanical properties (E~0.5-50 kPa). It is well-established that matrix stiffness modulates stemness in strongly contractile stem cells, including mesenchymal stem cells and muscle satellite cells, but the impact of stiffness on stemness maintenance in less contractile stem cells such as NPCs is not well known. These results highlight that different material requirements may exist for the expansion of different stem cell types. Our results have identified matrix remodeling as a previously unknown requirement for maintenance of NPC stemness in 3D hydrogels and suggest that adaptable biomaterials will be useful for expansion of therapeutically relevant numbers of NPCs.

Supramolecular biomaterials exploit rationally-designed non-covalent interactions to enable innovative approaches to drug formulation and delivery. For example, supramolecular interactions can be used to dynamically cross-linking polymer networks, yielding shear-thinning and self-healing hydrogels that allow for minimally invasive implantation in vivo though direct injection or catheter delivery to tissues. Herein, we discuss the preparation and application of shear-thinning, injectable hydrogels driven by non-covalent interactions between modified biopolymers (BPs) and biodegradable nanoparticles (NPs) comprised of poly(ethylene glycol)-block-poly(lactic acid) (PEG-b-PLA). Owing to the non-covalent interactions between PEG-b-PLA NPs and BPs, the hydrogels flow under applied stress and their mechanical properties recover completely within seconds when the stress is relaxed, demonstrating the shear-thinning and injectable nature of the material. The hierarchical construction of these biphasic hydrogels allows for multiple therapeutic compounds to be entrapped simultaneously and delivered with identical release profiles, regardless of their chemical make-up, over user-defined timeframes ranging from days to months. These materials enable novel approaches to immunotherapy, which rely on precise release of complex mixtures of compounds, as well as long-term treatment strategies for a variety of disease targets. Overall, this presentation will demonstrate the utility of a supramolecular approach to the design of biomaterials affording unique opportunities in the formulation and controlled release of therapeutics.

Takamasa Sakai
E-mail: [email protected]
Department of Bioengineering, Graduate School of Engineering, The University of Tokyo

Hydrogels are water-filled materials with characteristics similar to those of biological soft tissues and have broad application as biomaterials. However, the safety of hydrogels is difficult to guarantee throughout the full life cycle in vivo due to the inevitability of degradation-induced swelling. Indeed, the swelling pressure exerted by conventional hydrogels is sufficiently large to cause severe adverse reactions to surrounding tissues. In this respect, we report here a new class of in situ forming hydrogels that guarantee safety throughout the full life cycle. The oligo-TetraPEG hydrogel has an extremely high water content and a polymer content of only 4.0 g/L, resulting in an exceptionally low swelling pressure. The oligo-TetraPEG hydrogel was successfully injected and gelled inside a living body within a clinically relevant manipulation time and little cytotoxicity. The oligo-TetraPEG hydrogel exhibited an appropriate stiffness, similar to that of soft tissue, with extremely low swelling pressure thus being compatible with living tissues. The oligo-TetraPEG hydrogel functioned as an artificial vitreous body for over a year without any adverse effects, and was effective for treating retinal detachment as an intravitreous tamponade material． The present concept provides new guidance for the safety of biomaterials in vivo.

We report a strong enhancement in the antimicrobial action of berberine and chlorhexidine encapsulated into polyacrylic acid-based nanogels followed by further surface functionalisation with a cationic polyelectrolyte (PDAC) [1]. Due to the highly developed surface area, the nanogel carrier amplifies the contact of berberine and chlorhexidine with microbial cells and increases its antimicrobial efficiency. We show that such cationic nanogel carriers of berberine can adhere directly to the cell membranes and maintain a very high concentration of berberine directly on the cell surface. We demonstrated that the antimicrobial action of the PDAC coated nanogel loaded with berberine and chlorhexidine on E. coli, yeast, and C. reinhardtii is much higher than that of the equivalent solution of both free berberine or free chlorhexidine due to the electrostatic adhesion between the positively charged nanogel particles and the cell membranes. Our results also showed a marked increase in their antimicrobial action at shorter incubation times compared to the non-coated nanogel particles loaded with the same antimicrobial agent under identical conditions. We attribute this boost in the antimicrobial effect of these cationic nanocarriers to their accumulation on the cell membranes which sustains a high concentration of released berberine or chlorhexidine causing cell death within much shorter incubation times. This study can provide a blueprint for boosting the action of other cationic antimicrobial agents by encapsulating them into nanogel carriers functionalised with a cationic surface layer. This nanotechnology-based approach could lead to the development of more effective wound dressings, disinfecting agents, antimicrobial surfaces, and antiseptic and antialgal/antibiofouling formulations.


[1] M.J. Al-Awady, G.M. Greenway, V.N. Paunov, J. Mater. Chem. B, 2017, 5, 7885-7897.

Pancreatic islet transplantation is a promising approach of providing insulin in type 1 diabetes. One strategy to protect islets from the host immune system is encapsulation within a porous biocompatible alginate membrane. This encapsulation provides mechanical support to the cells and allows selective diffusion of oxygen, nutrients and insulin while blocking immunoglobulins. These hydrogels form by diffusion of calcium ions into the polymer network and therefore they are highly sensitive to environmental changes and fluctuations in temperature. We investigated the effects of gel concentration, crosslinking time and ambient conditions on material permeability, volume, and rigidity, all of which may change the immunoisolating characteristics of alginate. To measure diffusion coefficient as a method to capture structural changes we studied the diffusion of fluorescently tagged dextrans of different molecular weight into the midplane of alginate microcapsules, the diffusion coefficient is then calculated by fitting observed fluorescence dynamics to the mathematical solution of 1-D diffusion into a sphere. These measurements were performed after incubation in different conditions as well as after an in vivo experiment in six immunocompetent mice for seven days. Additionally, the changes in gel volume after incubation at different temperatures and environmental conditions as well as changes in compression modulus of alginate gels during crosslinking were investigated. Our result show that increase of polymer concentration and crosslinking time leads to a decrease in volume and increase in compression modulus. Furthermore, we found that samples crosslinked and placed in physiological environment, experience an increase in volume. As expected, these volume changes affect diffusion rates of fluorescent dextrans, where volume expansion is correlated with higher calculated diffusion coefficient. This observation is critical to islet protection since higher permeability due to the expansion in vivo may lead to increased permeability to immunoglobulins. Capsules from the in vivo study showed similar volume expansion and increased permeability, indicating our in vitro assay is a good predictor of volume change in vivo.

It is estimated that 2 million patients suffer each year from antibiotic-resistant infections in the U.S. At least 23,000 die as a result of the infections according to the CDC. A bio-orthogonal chemistry-based strategy to address this problem will be presented. The strategy is termed 'catch and release’ and it involves an inverse-electron demand Diels-Alder (IEDDA) reaction between tetrazine and trans-cyclooctene (TCO). A reloadable biocompatible hydrogel, modified with tetrazine is injected in the vicinity of an infected site. Prodrugs with attenuated activity and minimal side effects, containing a releasable TCO moiety are systemically injected. When the prodrug and the hydrogel come in contact, the bio-orthogonal agents react with each other through IEDDA reaction ‘catching’ the payload. Finally, the resulting intermediate isomerizes spontaneously releasing the active antibiotic from the hydrogel to perform its therapeutic function locally. In vitro data will be presented to show that the tetrazine-modified hydrogel is stable under simulated physiological conditions and capable of activating multiple doses of model prodrugs of vancomycin and daptomycin. Meanwhile, in vivo testing proved that the ‘catch and release’ strategy is capable of local activation of therapeutically meaningful quantities of vancomycin to treat methicillin-resistant Staphylococcus aureus. Multivalency of HMT allows for the process to be repeated with multiple doses of the systemically administered prodrugs.

Conductive and stretchable materials that match the elastic moduli of biological tissue (0.5-500 kPa) are desired for bio-potential electrodes and sensors with high signal strength and stability. However, inorganic conductors and dry conducting polymers typically have elastic moduli over 1 GPa. By contrast, hydrogels made with conducting polymers are promising soft electrode materials due to their high water content. We have developed a novel method for fabricating highly conductive hydrogels comprising two interpenetrating networks: one is a connected network of the conducting polymer PEDOT:PSS, while the second affords orthogonal control over the gel’s mechanical properties. With this method, we demonstrate conductivities up to 23 S/m, a record for stretchable PEDOT:PSS-based hydrogels. Additionally, we demonstrate that the elastic modulus of the gel can be tuned over three biologically relevant orders of magnitude without compromising stretchability (>100%) or conductivity (>10 S/m).

Pluronic F127, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer is a representative material for showing reversible sol-gel transition by temperature change. This behavior is achieved by micelle packing mechanism above critical gelation concentration. Micelle structure is obtained around 15 C and more micelles are formed as temperature increases because each blocks have different low critical solution temperature. This thermoreversible hydrogel has attractive characteristics for therapeutic agent delivery carriers due to its high water contents and similar mechanical property like the extracellular matrix. However, it has limitation for using in clinical application due to its low stability.
To overcome the critical drawback of Pluronic F127 hydrogel, the host-guest interaction was utilized to enhance packing ability of micelles. Due to strong host-guest interaction, it was possible to achieve highly improved mechanical stability. However, the viscosity of the blended solution was too high for injection due to existing strong host-guest interaction at injection condition (at 4 C). Thus, the system was still hard to deliver therapeutic proteins and cells.
To maintain long-term stability of hydrogel and improve injection ability, multi-guest molecules were conjugated at the end of Pluronic F127 for strengthening the micelle packing while reducing the number of polymers needed. Because of increased host-guest complex at a reduced concentration, critical gelation concentration of blended solution decreased comparing with conventional Pluronic F127 hydrogel and mono guest conjugated F127 hydrogel system. As a result, the viscosity of the injectable condition largely decreased comparing with the conventional method and the high stability was achieved in the physiological condition. In addition, this host-guest interaction based gel system enabled affinity based protein release. Host molecule modified protein showed sustained protein release profile in this system. Consequently, multi-guest conjugated Pluronic F127 hydrogel which has overcome its limitations while maintaining existing merits is expected to be used for various biomedical application

Polysaccharide-based smart cryogels respresent a promising platform for biotechnological and biomedical applications such as immobilization of bioactive molecules and cells, drug delivery, tissue engineering, among others [1]. Polymeric cryogel is a three-dimensional cross-linked system formed via the cryogenic treatment of solutions or colloidal dispersions of the appropriate precursors [2]. Recently, dual temperature and pH sensitiveness systems based on chitosan derivatives have been studied [3]. In this work, novel thermoresponsive chitosan-g-poly(N-vinylcaprolactam) cryogels were prepared from polymeric solutions using glutaraldehyde as crosslinking agent at -20 °C. The morphology of the cryogel beads were characterized by SEM and confocal microscopy. The resultant materials showed interconnected regular macroporous structure. The pore size of the cryogels is around of 10 µm. The study of equilibrium swelling shows an abrupt decrement in water absorption ability around of 34 °C. The reversible thermal responsiveness for cryogel beads was analyzed by variation of swelling in pure water (pH 6) to stepwise periodic changes in temperature between 5 and 45 °C. Below transition temperature, copolymer chains in the cryogel are hydrated, while as temperature increases to 45 °C, the macromolecular chains are slightly contracted, and water is ejected from the cryogel beads. The capacity of drug delivery carrier of the system was evaluated using citral (3,7-dimethyl-2,6-octadienal) as a model molecule, varying pH and temperature of the medium. These chitosan derivatives cryogels exhibited loading ability, and the release of citral was pH- and temperature-dependent.

References.
[1] Komarova, G. A. et al. Intelligent Gels and Cryogels with Entrapped Emulsions. Langmuir 24, 4467–4469 (2008)
[2] Mattiasson, B. Cryogels for Biotechnological Applications. in Polymeric Cryogels (ed. Okay, O.) 245–281 (Springer Intnl Pub, 2014)
[3] Argüelles-Monal, et al. Chitosan-Based Thermosensitive Materials. in Biological Activities and Application of Marine Polysaccharides (ed. Emad Shalaby) 279-302 (InTech, 2017)

Gelatin hydrogel is a kind of natural polymer from hydrolysis of collagen, having great biocompatibility and biodegradability for biomedical research and food industry. Moreover, gelatin hydrogel has thermo-responsive characteristics making this hydrogel to be easily processed into a variety of shapes for a lot of applications, even electronic devices based on solution-process. Recently, hydrogel based electronics has been researched owing to interesting properties, because it is operated by interaction of ions attributing to low driving voltage. Especially, organic electrochemical transistor (OECT) is totally different from conventional field-effect devices in aspect of direct injection of ions into the active layer, usually PEDOT:PSS (Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate). In this research, we suggest gelatin hydrogel as electrolyte and demonstrate OECT based on a sheet of gelatin. We modulate electrical characteristics of the OECT respect to pH condition of gelatin hydrogel from acid to base, and analyze its characteristics based on electrochemical theory, Nernst Equation. Moreover, we extend the gelatin based OECT to electrochemical logic circuits, for example, NOT, NOR and NAND gate.

Transforming nanoscopic dynamic molecular motions into the macroscopic scale in a predictable manner is of great interest for the advancement of smart materials. When these dynamic molecular systems are integrated with 3D printing technology, i.e. the extrusion-based direct ink writing, their micro- and macroscale properties are unleashed cooperatively as a result of the controlled assembly and complex 3D geometry. By (1) synchronizing the molecular motions, (2) hierarchically controlling materials’ nano- and mesoscale super-structures, and (3) fabricating their macroscale features into complex three-dimensional (3D) , actuators with dynamic motions have been developed successfully.¹ Herein, we report the design and synthesis of 3D printable polypseudorotaxane hydrogels (PRHs), which are composed of α-cyclodextrins (α-CDs) and polymer backbones. The hydrogen-bonding interactions between the CDs and formed crystalline domains allow PRHs to possess appropriate shear-thinning and self-healing properties to facilitate their 3D printability. The PRHs are fabricated into woodpile-lattice cubes, and photo-crosslinked to afford polyrotaxane monoliths (PMs). Through solvent exchange and pH switching induced hydrogen-bonding deformation/formation, the CD rings on the polymer axles switch between the shuttling and stationary states. These PMs are capable of lifting objects vertically against gravity, thus converting the chemical energy input into mechanical work. Our work demonstrates a general approach to control the macroscopic motions through molecular motions in response to environmental stimuli, which is another example of 4D printing.
Reference:
1. Q. Lin, X. Hou, and C. Ke, Angew. Chem. Int. Ed., 2017, 56, 4452.

Quenched polyampholytes provide a novel class of tough hydrogel that has self-healing ability, strong adhesion, and mechanical flexibility. Understanding the structure of polymer chains in the hydrogel and the phase behavior of water therein has broad impact on various applications, such as lubrication, adhesion, and electrical conductivity, as well as the hydrogel’s low temperature properties. In this paper, the structure of polymer chains of hydrogels made of a model charge-balanced polyampholyte, a random copolymer of poly(4-vinylbenzenesulfonate-co-[3-(methacryloylamino) propyl] trimethylammonium chloride), was investigated by small- and wide-angle x-ray scattering (SAXS and WAXS). The SAXS results suggested a networked globule structure in the charge-balanced polyampholyte hydrogels prevented freezing of water in the hydrogel, while the evidence of non-frozen water at low temperatures, such as –45 °C was monitored by solid-state ²H NMR. Correspondingly, we observed high ionic conductivity at low temperatures using electrochemical impedance spectroscopy (EIS). Interestingly, multiple freezing-thawing cycles did not impact the phase behavior of water in the hydrogel. We also found evidence that the crosslinked network structure of the polyampholyte chains disrupts the crystalline growth of ice, resulting in ‘slush-like’ ice formation.
Utilizing the scientific investigations, a flexible and self-healing supercapacitor with high energy density in low temperature operation was fabricated using a polyampholyte hydrogel electrolyte. The electrode material was a biochar (produced from the low-temperature pyrolysis of biological wastes) bound by self-assembled reduced graphene oxide. At the room temperature, the fabricated supercapacitor showed high energy density of 30 Wh/kg with 90% capacitance retention after 5000 charge-discharge cycles at room temperature at a power density of 50 W/kg. At –30 °C, the supercapacitor exhibited an energy sensity of 10.5 Wh/kg at a power density of 500 W/kg.
We also harnessed the tunable optical property of the polyampholyte hydrogel to fabricate a smart window. Specifically, we modulated the overall hydrophilicity/phobicity of polyampholyte chains when synthesizing the random copolymer and adjusted the upper critical solution temperature (UCST) at high precision, thus achieved a fine-tuning of UCST between 15 and 65 °C. Finally, we developed a stretchable, high-contrast, optically tunable stretchable window which consists of the PA hydrogel and a printed stretchable electric heater by our own ink recipe.
In summary, we performed fundamental studies on the phase behavior of polymer chains and water molecules in quenched polyampholyte hydrogels by using synchrotron SAXS/WAXS, solid-state NMR, EIS, and DSC. We utilized the understanding in energy storage and smart window applications, both of which are unconventional for the application of tough hydrogels.

Despite the unlimited potential for intelligent materials that can change their functionalities in response to various environmental cues, significant challenges still present for precisely controlling the material formation under different length scales. We report here the design and synthesis of a nacre-mimicking, hybrid material with tunable porosity and toughness using bacteria as the patterning and self-regenerating template. Caulobacter Crescentus is a Gram-negative, oligotrophic bacterium with exceptional strong surface binding and dense biofilm forming abilities. They can survive in a nutrient poor habitat for weeks. The outmost part of their cell envelope is covered by a monolayer of 2D crystalline proteins. This so-called S-layer is self-assembled from identical protein monomers of RsaA into hexagonal arrays with many functional residues exposed in a precisely repetitive manner. Taking advantage of these unique properties of C. Crescentus and its s-layer, a hierarchically-ordered, brick-and-mortar composites is created through programmed multicellular patterning between bacterial cells and silicon microplatlets. Selected elastin like peptides (ELPs) expressed by C. Crescentus were displayed on its s-layer protein arrays and cross-linked into a hydrogel network. The porosity and toughness of the hybrid material are reversibly controlled by temperature induced phase transition of ELP domains.

Stimuli-response membranes have been widely explored to mitigate fouling on the membrane surface to achieve long-term performance of water purification. Herein, we demonstrate a facile, one-step coating of photo-mobile materials on membrane surface to impart self-cleaning property upon exposure to ultraviolet (UV) light. Specifically, we successfully deposited 4,4’-azodianiline (AZO) on the surface of ultrafiltration (UF) membranes by co-depositing with a bio-glue of polydopamine (PDA). In the presence of UV light, the AZO undergoes photoisomerization from trans-AZO to cis-AZO while increasing surface hydrophilicity and shrinking the volume. When the UV light is switched off, the cis-AZO transits reversibly to the trans configuration. The photo-mobile behavior and the resulting volume reversible change contributes to the self-cleaning properties. The effect of the coating layer on the surface hydrophilicity and pure water permeance is systematically evaluated. The photoresponsive properties of AZO in DMF solutions and thin films with PDA is characterized using UV-Vis absorption spectroscopy. The self-cleaning behavior of the modified membranes is demonstrated in treating water containing bovine serum albumin (BSA) as a model foulant. For example, the self-cleaning of polysulfone UF membranes increases water flux by 200% when treating 1 g/L BSA aqueous solution.

Recently, due to the unique features of DNA, such as sequence programmability, distinct molecular recognition, and precise self-assembly, the smart DNA hydrogel has attracted tremendous research interests.^1-3 DNA hydrogels are generally either fully self-assembled from oligonucleotides or cross-linked from oligonucleotide-polymer hybrids. The fabrication methods are generally costly and complicated, which limits their practical applications. Here we report a method to automatically produce hydrogels by rolling circle amplifications (RCAs).⁴ In this method, the functional cross-linking sites are introduced to the polymeric DNA chains on the meantime of RCA amplifications; and the properties of the hydrogel can be tuned in a very facile way through template design. First we developed a RCA hydrogel with horseradish-peroxidase-like catalytic capability. The catalytic hydrogel exhibits highly improved stability at elevated temperatures or during a long-term storage. Integrated with glucose oxidase, the complex hydrogel can be applied to the sensitive and reliable detection of glucose.⁵ We also attempted to realize stimuli-responsive hydrogels on the basis of the i-motif structures. Through carefully studies, the keys to fabricate a stable and stimuli-responsive hydrogel are carefully identified, i.e. the choice of a moderate intermolecular interaction and the formation of hierarchical self-assemblies between DNAs and magnesium pyrophosphate (a byproduct of RCA).

Key Words：DNA; Hydrogel; rolling circle amplification
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Early identification and treatment of hepatocellular carcinoma is very important for improving the prognosis and survival rate of the patient. A theranostic agent that combines multimodal imaging with cancer treatment may be used for aunting the visualization and treatment of the cancer. Herein, we report the synthesis of a multifunctional theranostic SP94 modified polypyrrole (PPy)-BSA-ICG nanoparticles by a simple method. The multifunctional theranostic agent exhibited two mode photoacoustic and near infrared (NIR) fluorescence imaging as well as combined with photothermal therapy. This nanoparticles showed an excellent stability in physiological solutions, a higher tumor accumulation compared with unmodified nanoparticles and minimal nonspecific uptake by other organs such as liver and spleen. And most importantly, the nanoparticles could effectively inhibit the growth of the tumor through photothermal therapy with no tumor recurrence upon only single NIR laser irradiation. These results indicated that SP94 modified PPy-BSA-ICG is potentially an interesting nanotheranostic agent for imaging guided cancer therapy by overcoming the limitations of each technology and enhancing the therapeutic efficiency as well as reducing side effects.

Over 10 million people worldwide suffer from strokes each year. For the more than 42 million people around the world who have survived a stroke, their quality of life is often greatly diminished due to varying degrees of brain damage. Many of the survivors of stroke often suffer from some loss of sensory and motor function, including altered hearing and vision, abnormal gait, speech complications, and paralysis. Brain injury recovery primarily focuses on physical rehabilitation; there is currently no FDA approved neuroprotective and regenerative therapy to mute disability or to help improve original quality of life. Nerve growth factor (NGF) and brain-derived neurotropic factor (BDNF) have shown to be neuroprotective in a rat middle cerebral artery occlusion (MCAO) model. However, delivery of these growth factors to the target area requires the ability to cross the blood-brain barrier and substantial dosing to elicit any positive outcome. This issue can be overcome by an injectable delivery system that can provide a more sustained release profile. We describe a method for the development of an injectable peptide-based hydrogel delivery system that incorporates a neurogenic peptide. This drug delivery system is thixotropic (shear thinning) and biodegradable, allowing the biomaterial to flow to the target site and form a hydrogel in situ to provide sustained neurogenic peptide release. The prolonged release of the neurogenic peptide from the hydrogel can provide a neuroprotective effect following stroke and create a suitable environment for neuroregeneration. This technology platform is centered on the formation of antiparallel β-sheets, using noncovalent interactions, to allow the peptides to crosslink together to form hydrogels. Peptides can be incorporated with mimics of biological factors, presented at high epitope density during supramolecular assembly, overcoming drawbacks typical of standard growth factor delivery. The self-assembling peptide hydrogels were tested for their biocompatibility through in vitro cell culture and in vivo subcutaneous injection in mice, respectively. Their potential to provide neuroprotection and regeneration was also optimized in vitro with neurons before moving to an in vivo MCAO rat model. This proposed research aims to bridge the gap between disease management and post-stroke recovery in an attempt to restore bodily activities and improve the quality of life of stroke victims. The success of this drug delivery system may also prove efficacious for restoring perfusion and vitality to damaged brain tissue after traumatic injury.

Diabetic Retinopathy (DR) and Macular Degeneration (MD) are diseases of the posterior segment of the eye that disproportionately affect the geriatric population. These diseases lead to significant deterioration in the quality of life in affected demographic and are among the leading causes of blindness (4.8% and 8.7% of occurrence, respectively), globally. Both diseases share a common cause — aberrant neovascularization in the posterior segment that lead to occlusion, hemorrhage, and edema. Intraocular delivery of the inhibitors of vascular endothelial growth factor (VEGF) have shown promise to block the progression of the diseases, as they prevent the neovascularization in the retina and the choroid. A drawback of the current treatment is the requirement of monthly intraocular injection schedule that leads to patient discomfort and higher chances of endophthalmitis. The issue can be addressed by a less-frequent dosing schedule that would need a more sustained release profile. The development of a bifunctional hydrogel-based delivery method is described that encapsulates an anti-VEGF antibody inside an anti-angiogenic peptide scaffold. The hydrogel platform is biodegradable and thixotropic. The bimodal and sustained release of bevacizumab and the anti-angiogenic peptide from the hydrogel can inhibit abnormal neovascularization in the tissue microenvironment of the choriocapillaris, Bruch’s membrane, and the retina. The technology is based on a b-sheet-based self-assembling peptide hydrogel (SAPH) platform to which biofunctional moieties can be attached, without negatively affecting the secondary structure or viscoelastic properties. The different combinations of the peptide hydrogels with different loading of bevacizumab were tested for their biocompatibility (fibroblast cell culture), immunogenicity (mouse model), and their potential to inhibit angiogenesis in vitro (tube formation assays) and in vivo (rabbit model). The release profile of the antibody and the anti-angiogenic peptide were monitored in vitro and in vivo to test the suitability for a less-frequent dosing schedule. The proposed therapy may be an important advance toward clinically useful abrogation of aberrant neovascularization in the posterior segment that would address the root cause of DR and MD and would improve the quality of life for the patients. Modular nature of the therapy also makes it an ideal choice to be modified depending on the specific disease phenotype — for example, the sequestered drug could be changed without altering the peptide backbone to better target sub-classes of the diseases. Finally, our bimodal delivery system may prove to be useful for localized prevention of vascularization in neoplastic microenvironment, leading to translation to anti-cancer therapies in the future.

Many therapies, including surgery, chemotherapy and radiotherapy, are used to treat cancer, but most of them do not accomplish the desired purposes. For example, it is still a challenge to efficiently kill cancer cells without hurting normal tissues. Alternatively, photothermal therapy is an approach for local tumor ablation, by using nanoabsorbers to convert near-infrared (NIR) light into heat. In general, the administrated nanoparticles predominantly accumulate in tumor tissue due to the EPR effect. The retention time of nanoparticles is usually short, since they will be removed quickly via lymphatic system. Therefore, intense laser irradiation for one-shot treatment or in combination with chemotherapy is necessary to enhance the efficacy of cancer therapy. However, the tumor accumulation of nanoparticles and drug is still limited due to the blood circulation and organ clearance. To address the problem, injectable hydrogel systems have been used as drug delivery system, which can significantly prolong the retention time of nanoparticles in vivo. Besides, hydrogel can also be used as drug controlled-release platform to enhance the therapeutic effect without damage of normal tissues. Thus, the development of multifunctional hydrogel systems is extremely valuable, especially for combination cancer therapy.
Herein, we developed a novel injectable hydrogel system, by embedding gold nanoshells (GN, NIR-light absorber) and doxorubicin (DOX, anti-cancer drug) into a biodegradable natural polymer, silk fibroin. First, the mixed SF solution was orthotopically injected into the mice bearing with 4T1 breast cancer and followed by laser irradiation. The light-induced heat from the GN would increase the local temperature and induced in situ formation of SF hydrogel through sol-gel transition. In this case, most of GN and DOX could be trapped well within the tumor and thus the retention time of both agents would be significantly prolonged. We observed that the absorbance peak of GN in SF was ca. 800 nm and the local temperature would elevate quickly under NIR-laser irritation. The content of β-sheets in SF increased more rapidly at 50 ^oC, as compared to that at 37 ^oC, indicating the laser-induced heating is necessary to induce formation of SF hydrogel. Besides, we also found that Dox could be released successively from SF/DOX hydrogel up to 71 % within 12 days because of the acidic environment, degradation of SF hydrogel, and heat-enhanced diffusion. Then, the photothermal and chemo cytotoxicity of multifunctional hydrogel system in 4T1 breast cancer cells was evaluated with WST-1 and LIVE/DEAD assay. The WST-1 result clearly pointed up that the silk/GN/DOX group exhibited the highest cytotoxicity effect due to the synergistic photothermal-chemotherapy. Further material characterization and in vitro/in vivo evaluations are in progress and will be reported in the conference.

Suckerin-12 (S12) is a protein derived from the sucker ring teeth of the Humboldt squid. As a family, the Suckerins offer unique properties and potential advantages over those of silk fibroin. Recently, S12 has been investigated for its shape changing properties. Under specific buffer conditions, S12 hydrogels contract reversibly into a dehydrated state where mechanical properties can be modulated by varying salt anions. Additionally, green fluorescent protein from Branchiostoma floridae (bfloGFP) has been identified as a strong adsorber to S12 hydrogels. In this work, recombinantly-expressed bfloGFP is tested as a fusion partner for the enzymatic functionalization of S12 hydrogels. Enzyme-bfloGFP chimeras have been created through genetic fusion of both elements in a single open reading frame as well as post-translationally using the SpyTag/SpyCatcher system. The functionalization of S12 hydrogels through bfloGFP-mediated adsorption has allowed the testing of the condensed hydrogels for their ability to stabilize the adsorbed enzymes during thermal stress. Bioprinting of bfloGFP-fused enzymes onto suckerin-12 hydrogel substrates has also been used to investigate how spatial localization of esterases can affect localized anion production and sclerotization. These results highlight the unique properties of S12 hydrogels as a binding matrix for enzymes and other biomolecules towards the creation of responsive biomaterials.

Both positive and negative feedback mechanisms are vital in a number of biologically relevant processes. For example, feedback loops are essential for limb development in mammals and are responsible for constraining growth in plants to the specific regions. Controlling feedback mechanisms within the fully synthetic materials is important for a range of biomimetic functionalities. Herein, using three-dimensional Gel Lattice Spring Model-based simulations, we focus on the two types of hydrogels, pure Poly(N-isopropylacrylamide) (PNIPAAm) hydrogels and PNIPAAm gels functionalized with light-sensitive trisodium salt of copper chlorophyllin. Prior experimental studies had shown that illumination of the latter gels results in their heating and in discontinuous volume phase transitions; the results of our simulations are in a good agreement with these experimental studies. We consider thin hydrogel membranes under different confinements and under the influence of such external stimuli as temperature gradients and light. We focus on pattern development during the volume phase transitions as the samples re-swell under the confinement. Our results demonstrate that these gels exhibit rich steady-state bucking patterns (including well-ordered patterns) for the chosen gel parameters and confinement conditions. We show that one can effectively utilize external stimuli (light or temperature gradients) to control specific pattern selection and well as feedback mechanisms in these systems.

Direct methanol fuel cells (DMFC) typically use low concentrations of methanol (1-2 M) in water as fuel so as to reduce methanol cross-over and consequent losses in efficiency. Here we report successfully using methanol-polymer gels coated with a non-porous polysulfone coating to deliver methanol to the DMFC a concentrations low enough to reduce methanol crossover and significantly reduce the amount of water required for DMFC operation. After finding that many polymeric hydrogels were not compatible with pure methanol, we discovered that gels could be made from 2-hydroxylethyl methacrylate (HEMA) cross-linked with 5-20 mol% ethylene glycol dimethacrylate (EGDMA) in methanol (75wt%). The monomers in methanol were polymerized with AIBN (0.05 mol%) at 65 °C. The un-modified, cross-linked gels released methanol over 5-10 hours; not slowly enough to replace traditional aqueous methanol. Release rates were dramatically reduced by coating the gels with a nonporous polysulfone membrane. Subsequently we showed DMFC fuel cells operated significantly longer using methanol from the polysulfone-modified methanol gels than from a comparable mass of 1-2 M solutions of methanol in water.