The swelling and mechanical properties of networks are intimately tied together through the free energy function as it is affected either by chain stretching (elastic free energy) or through the mixing of the polymer with the solvent (usually through a Flory-Huggins estimate). Frenkel, then Flory and Rehner (FFR) developed a simple way of looking at this problem by simply balancing the elastic and mixing contributions to the chemical potential at swelling equilibrium. Much work has been done to examine this model, often in a way that suggests that something is missing or that the model itself is incorrect. In this presentation we examine the FFR model and show that it is correct for polymer networks swollen in organic liquids. We demonstrate that anomalous peaks in the swelling activity parameter S (or dilational modulus) are artifacts of experimental methods and we further show that, once the peak in S is removed, quantitative agreement is obtained between mixing and elastic contributions to the free energy with the simple addition of a cross-link dependent interaction parameter.

Slide-ring (SR) gels with movable cross-links along network strands have received much attention as a novel class of polymer gels. The network topology of SR gels is variable in response to imposed stress or deformation whereas that of classical gels with fixed cross-links is invariable. The polyrotaxane-based SR gels are synthesized by intermolecular cross-linking of alpha-cyclodextrin contained in poly(ethylene glycol). The SR gels are expected to exhibit novel properties and functions resulting from the movable cross-links. In this talk, we focus on the fluid-permeation behavior of the membranes of SR gels under imposed pressure (p), and the nonlinear elasticity revealed by biaxial stretching. It has been well known that the fluid permeation of classical gels obeys the Darcy's law: The steady-state fluid velocity (v) is linearly proportional to p. The proportionality constant (f) corresponding to the friction coefficient between gel network and fluid is independent of p for the classical gels. Here, we demonstrate that the membranes of SR gels exhibit a peculiar p dependence of f, where f sharply varies between two different values within a narrow p range. It indicates that SR gels are promising polymer membrane materials that enable the on-off control of fluid permeation by imposed pressure, which can be developed to separation membranes and drug delivery systems with novel functions. Further, we show a unique feature in nonlinear elasticity for SR gels revealed by the experiments using unequal biaxial strains. The effect of the strain in one direction on the stress in the other direction in SR gels is much smaller than that in classical gels. This feature results in no explicit strain-coupling term in the strain energy density function of SR gels.

Recently, we have developed a novel gel system (Tetra-PEG gel) by new network formation method, “AB-type crosslink-coupling”; the network is formed by the combination of two mutually reactive tetra-arm prepolymers with same shape. Our previous study revealed that the Tetra-PEG gel has a homogeneous polymer network with small amount of structural defects. Although the connectivity and spatial heterogeneity was observed, the degree of heterogeneity is extremely smaller than that of conventional gels. In this study, we focus on the correlation between mechanical properties and structural parameters of polymer gels. We tuned the structural parameters including the polymer volume fraction (f_0), polymerization degree of network strands (M = 5k-40k g/mol), and reaction conversion (p). We investigated the values of p by infrared (IR) measurement, elastic modulus (G) and ultimate elongation ratio (lambda;max) by stretching measurement, and fracture energy (T0) by tearing measurement.
First, we compared G measured by a stretching measurement, and that predicted from the reaction efficiency (p) using affine (G_af ) and phantom (G_ph) network models. As for the 10k and 20k Tetra-PEG gels, G_ph and G corresponded well with each other in a wider range than the other gels, suggesting that their elasticities is roughly predicted by the phantom network model. As for the 5k Tetra-PEG gel, the downward deviation of G from G_ph was increasingly pronounced with decreasing f_0. On the other hand, the 40k Tetra-PEG gel shows distinct behavior; G was above G_ph and near G_af. In order to discuss the whole tendency, G/G_af was plotted against f_0/ f*, f* is the overlapping polymer volume fraction of the prepolymer. In this figure, all of the data fall onto a single curve. In the range from f* to 3.0f*, the elastic moduli were well predicted by the phantom network model. The downward deviation below f* is due to the formation of elastically ineffective loops. In the range above 3.0f*, G = G_af increased with an increase in f_0 and approached to 1.0. This data strongly suggests that trapped entanglements are introduced to the network or that the model shifts to the affne network model, or both in the region above 3.0f*. Only from these results, we cannot distinguish whether the deviation from the phantom network model prediction is originated from trapped entanglements or from the change in models. Thus, we investigate the trapped entanglements by the tearing measurement. The results in tearing measurement indicated that there are few trapped entanglements. Taking into account these data, it is strongly suggested that the model predicting the elastic modulus shifts model with increasing _0 and N. In the presentation, the results of ultimate elongation ratio will be also discussed.

Responsively tough, injectable biomaterials are potentially useful for the minimally-invasive surgical implantation of durable matrices, either for delivering cellular or molecular cargo, or to act as robust fillers for soft tissue reinforcement. Associative protein hydrogels are well-suited for use as injectable materials, exhibiting cyto-protective shear-banding behavior and immediate recovery post-injection. However, the low yield stress required for the injectability of these materials is typically incompatible with the high resistance to deformation and fracture that is needed to maintain implant integrity under physiological stresses. One approach to accomplish both injectability and toughness is to engineer a shear thinning hydrogel with a low yield stress at low temperatures that is reinforced at body temperature due to thermoresponsive block copolymer self-assembly. Toward this end, triblock copolymer hydrogels containing artificially engineered associative protein midblocks and poly(N-isopropylacrylamide) (PNIPAM) endblocks have been developed, exhibiting responsive and reversible toughening over clinically-relevant temperature ranges. The PNIPAM endblocks exhibit lower critical solution behavior in this hybrid protein-polymer gel, and heating above the transition temperature to 37°C leads to endblock desolvation and aggregation into nanoscale domains. The formation of these reinforcing domains leads to large increases in the gel&’s elastic modulus and yield stress in shear, as well as improved resistance to compressive failure, erosion, and creep. The relationship between macromolecular design, nanostructure formation, and gel mechanics has been investigated in detail using small-angle neutron scattering (SANS) and oscillatory shear rheology, revealing important principles for controlling network self-assembly and achieving improved reinforcement. In particular, large micellar cores, high PNIPAM volume fractions, and high densities of midblock associations led to the stiffest hydrogels, with elastic moduli reinforced by a factor of 14 to approximately 130 kPa at 37°C. Stress relaxation times were seen to increase by up to 50-fold for gels with the largest micelles and packing fractions. For large enough endblocks, midblock associations were seen to promote endblock segregation even below the PNIPAM transition temperature, and these swollen micellar structures introduced long relaxation times into the gels. These studies demonstrate that control of the double network structure is critical for tuning the gel&’s mechanical behavior over a broad range for use in various biomedical applications.

We propose here a generic method to reinforce weak elastomers without using fillers. This method could be particularly interesting in the biomedical or high tech applications where pure polymers with specific physical properties (transparency, resistance to UV or temperature) are used but have poor mechanical properties.
In recent studies on the reinforement of hydrogels. Gong et al have shown that hydrogels synthesized with two interpenetrating networks with very different levels of crosslinking and conformations of chains have a fracture toughness significantly enhanced relative to a single homogeneous network[1][2][3]. Their mechanical properties are enhanced through breaking the bonds of the more crosslinked and highly stretched minority network while avoiding crack propagation through the less crosslinked and unstreched majority network[4].
We applied this method to acrylic elastomers and succesfully prepared poly(alkyl acrylate) elastomers containing isotropically prestretched chains at different volume fractions, using sequential swelling/polymerization steps. Samples containing prestretched chains show an impressive enhancement of properties compared to networks polymerized in one step. Both initial modulus and fracture toughness are enhanced while retaining a negligible hysteresis and residual deformation upon unloading, which is impossible in simple networks.. Our best samples show a 50 time increase in true stress at break and in fracture toughness, making those material as tough as some filled elastomers. Our methodology holds great promise to improve the extensibility, toughness and tune the non-linear elasticity of elastomers previously thought to be mechanically too weak to be used in mechanically demanding applications.

[1] Gong, J. P.; et al, Y. Adv. Mater. 2003, 15, 1155-1158.
[2] Tanaka, Y.; et al J. of Physical Chemistry B 2005, 109, 11559-11562.
[3] Gong, J. P. Soft Matter 2010, 6, 2583.
[4] Brown, H. R. Macromolecules 2007, 40, 3815-3818.

We investigated the effects of swelling and deswelling on the mechanical properties of Tetra-PEG ion gels with variable polymer volume fractions at the measured condition (phi;_m). Tetra-PEG gels were prepared by the AB type crosslink-coupling between the two symmetrical tetra-arm prepolymers with tuning the network strands length and polymer volume fractions. The use of an ionic liquid as diluent enables us to measure the mechanical properties of strongly deswollen Tetra-PEG gels above the melting point of PEG. The drastic increase in the elastic modulus was observed in the high phi;_m region due to the unusually contracted conformation of the network strands, called supercoiling. The Obukhov model, which predicts the elasticity of polymer networks using the concentration at preparation (phi;_0) and measured condition (phi;_m) as variables, can describe the phi;m-dependence of the elastic modulus in all phi;_m regions. Furthermore, we analyzed the stress-elongation relationships for the swollen and deswollen networks. We estimated the fractal dimensions based on the Pincus blob concept. We, for the first time, observed the crossover of the phi;_m-dependence of the fractal dimension from normal scaling to supercoiling, and the dependence of the fractal dimension on the strand length. The extensibility at break increased with an increase in phi;_m and an increase in the network strand length. These results did not obey the familiar Kuhn model, but were better explained by our model. These findings will help to understand the structure and formation mechanism of supercoiling.

The piezoelectric-rubber is expected to be a material that is used at the places where conventional piezoelectric materials cannot be applied, because it has flexibility and can be formed into arbitrary shape. However, so far there have been the following issues that in order to increase the piezoelectric property of piezoelectric-rubber, a large amount of the large size piezoelectric-ceramic particles must be mixed in the rubber, resulting in loss of the elastic property of the piezoelectric-rubber.
In the previous investigation, it was confirmed that the orientation of the piezoelectric-ceramic particles in the direction normal to the surface of the rubber was effective in increasing d33 which indicates the piezoelectric coefficient when the input load is applied and the output electric charge is taken both in the direction of thickness of rubber. On the other hand, it is considered that the piezoelectric property of the “Oriented-Type” in which the piezoelectric-ceramic particles is oriented in the rubber is strongly affected by rubber&’s property as the matrix of the Oriented-type, because the volume of the rubber of Oriented-type is larger than that of the PZT particles and the stress on the oriented particles is affected by the property of matrix.
Therefore the influence of the matrix property on piezoelectric property of Oriented-Type was investigated. In the investigation, the Young&’s modulus of matrix was focused as the matrix property. The lower the Young&’s modulus of matrix is, the higher d33 of Oriented-Type is, because the stress applied to oriented PZT particles in Oriented-type increases. Four kinds of matrix materials were investigated: Silicone gel, Silicone rubber, Urethane rubber and Poly-methyl-methacrylate. The Young&’s modulus of matrix materials are as follows: Silicone gel is 5MPa; Silicone rubber, 70 MPa; Urethane rubber, 180MPa; and Poly-methyl-methacrylate, 600MPa. The piezoelectric-ceramic particles to be mixed with the matrix materials are the same particles of Lead Zirconate Titanate (PZT).
As the result of investigation, the sample made of the silicone gel which had the lowest Yang&’s modulus in the selected matrix materials had the largest d33. The large increase as compared with the previous investigation was observed. The d33 of the fabricated sample was more than 60 pC/N when the concentration of PZT particle was about 10 Vol%. The Oriented-type using silicone gel as matrix is expected to be the vibration-isolating materials having the property of sensor, because the volume of silicone gel is 90 Vol% and Young&’s modulus is 12MPa even if the sample is the piezoelectric-rubber of Oriented-Type.

Poly(N-isopropylacrylamide) (PNIPAM) is a thermosensitive polymer that is well-known for its lower critical solution temperature (LCST) around 305K. Below the LCST, PNIPAM is soluble in water and above this temperature polymer chains collapse prior to aggregation. In the presence of methanol, experiments suggest that, LCST of PNIPAM is depressed up to certain mole fraction of methanol (0.35 mole fractions) and it is speculated that addition of methanol affects the PNIPAM-water interactions. Above 0.35 moles fraction of methanol, LCST gets elevated to temperatures above 32°C and cannot be detected up to 100°C. In the present study we have used MD simulations to investigate the effect of solvent mixed with methanol on conformational transitions and the LCST of PNIPAM. We employ polymer consistent force-field (PCFF) and CHARMM force-field to study the PNIPAM and water-methanol mixtures of different mole fractions of methanol namely, 0.018, 0.09, 0.27, 0.5, and 0.98 mole fractions, at fully atomistic level were carried out at 260, 278, 310, and 340 K. Simulated trajectories were analyzed for different structural properties such as radius of gyration of PNIPAM, extent of hydration of PNIPAM etc. Different dynamical properties such as diffusion coefficient, hydrogen bonding life-times, residence time of water and methanol near PNIPAM were also studied at 260, 278, 310, and 340 K.

Natural Rubber latex (NRL) is a colloidal system constituted of rubber particles (20-45%) ranging from 3 nm to 5 µm dispersed in a liquid serum [1]. The NRL has bioactive properties that have recently been explored to produce films using polyelectrolyte multilayers technique [2]. However, the production of homogenous films may be influenced by the distribution of rubber particles size. In this sense, we separated the NRL to keep structures bellow 200 nm, aiming the production of homogeneous thin films using layer-by-layer technique. The separation of NRL occurred by a sequence of centrifugations. First centrifugation at 12,225 G (9,000rpm) for 2h removed the bigger particles; followed by a sequence of 3 centrifugations at 42,206 G (22,000 rpm) for 1h. On each step the liquid on the bottom was collected and submitted to the next centrifugation step. By this mean, the NRL was reduced in 75% of the initial weight. This serum was finally filtrated in 0.2 µm membrane and the solution obtained was analyzed. The presence of poly-cis-isoprene in the sample was confirmed by FTIR peaks attributed to CH2 (2858, 2926 cm-1), CH3 (2964 cm-1), C=C (1664 cm-1) and C=CH (840 cm-1). The presence of proteins was confirmed by the FTIR peaks of C=O (1737 cm-1), C-O (1099 cm-1), O-O (1013 cm-1) and NH (1580 cm-1). Furthermore, there is a broad peak at 3330 cm-1 that indicates the presence of both NRL (=CH, 3033 cm-1) and proteins (N-H 3280 cm-1 and -OH, 3440 cm-1) [3]. The presence of proteins together with rubber nanoparticles was also detected by field-flow fractionation with asymmetric cross-flow (FFF). Besides, the separated material was evaluated by dynamic light scattering (DLS) revealing sizes of 40 to 220 nm and zeta potential of -46 mV. However, only particles smaller than 70 nm were identified in AFM images of the NRL deposited on polyethylenimine (PEI) layer. The NRL layer immobilized on surface was 3 nm thick when analyzed by ellipsometry, which was similar to the height of the structures detected by AFM. Therefore, it can be concluded that nano-sized natural rubber particles remain on the serum after centrifugation.
1. d'Auzac, J., J.-L. Jacob, and H. Chrestin, Physiology of rubber tree latex. 1989, Boca Raton: CRC press, Inc. 488.
2. Davi, C.P., et al., Natural rubber latex LbL films: Characterization and growth of fibroblasts. Journal of Applied Polymer Science, 2012. 125(3): p. 2137-2147.
3. Rippel, M.M., et al., Skim and cream natural rubber particles: colloidal properties, coalescence and film formation. Journal of Colloid and Interface Science, 2003. 268(2): p. 330-340.

In previous studies, pH-sensitive and ionic-strength-sensitive hydrogels have been produced by adding pendant acidic or basic functional groups to the polymer chains. These hydrogels swell/shrink in response to appropriate pH and ionic strength changes in aqueous media. The swelling/shrinking properties of these hydrogels are depended on electrostatic repulsion. Generally, these hydrogels containing weakly ionic groups, such as carboxylic groups and amino groups, swell/shrink reversibly in response to pH because the dissociation state of these functional groups change to pH. We expected that the strongly ionic groups such as a sulfonic acid and alky ammonium should be suitable for novel hydrogels enabling the fairly swelling/shrinking characteristics at wider pH range.
In this study, we aim to develop a novel pH- and molecular-sensitive hydrogels which is prepared with copolymerization of ionic functional monomers and PEG-based crosslinker having good thermal/pH stability and high mobility. We evaluated the water adsorption ability of various PEG-based hydrogels by changing the molecular weight of PEG-dimethacrylate (PEG-DMA) as a crosslinker, porogenic solvents and its compositions. Moreover, we evaluated the specific swelling/shrinking based on the molecular recognition by measuring the volume change of PEG-based hydrogels toward various ionic compounds. Additionally, we examined the drastic volume change of the hydrogels by dual interactions containing ionic interactions of hydrogels-solutes and hydrophobic interactions of solutes-solutes which were adsorbed onto the hydrogels.

One of the ultimate goals of polymer science is to understand the relationship between the structure and the physical properties of hydrogels. Recently, we developed a near-ideal network, Tetra-PEG gel, which is formed by the combination of two mutually reactive tetra-arm prepolymers with the same shape. Our previous study determined the exact models that predict the relationship between the structural parameters and the physical properties of Tetra-PEG gels. In contrast, the mechanical properties of conventional hydrogels remain difficult to predict because conventional hydrogels have a significant degree of the heterogeneity. The heterogeneities are categorized into spatial, connectivity and topological heterogeneities. These heterogeneities relate with each other and make variety of substructures in polymer network, which complicates the structural parameters and mechanical properties. In this study, we focused on one of the simplest heterogeneities (i.e., mesh size heterogeneity) and investigated the influence on physical properties. We prepared Tetra-PEG gels with bimodal distribution in strand length (Tetra-PEG bimodal gels) by combining different molecular weight of Tetra-PEG prepolymers, and measured the physical properties of the hydrogels (elastic modulus, and maximum deformation, and fracture energy). The samples formed above the overlapping concentration of prepolymers had few heterogeneities regardless of heterogeneous distribution in strand length. The physical properties of Tetra-PEG bimodal gels were also well described by the models for conventional Tetra-PEG gels with the average of polymerization degree between crosslinks and initial volume fraction. We conclude that the mechanical properties of hydrogels that have heterogeneous distribution in strand length can be predicted from the average strand length in the range of this study.

[Introduction]
We have investigated poly(vinyl alcohol) (PVA) hydrogel as a biomaterial for a replacement for artificial articular cartilage because of its excellent mechanical properties, biocompatibility, and low friction coefficient.
Generally, PVA hydrogels is physical gels with small crystalline as crosslinking points and have been prepared by low temperature crystallization method. When water is used as a sole solvent during freezing, phase separation into a PVA-poor and PVA-rich phase may take place, resulting in PVA crystallization in the phase separated state to give a translucent weak hydrogel.
In order to make PVA hydrogel having high mechanical properties, a mixed solvent of dimethyl sulfoxide (DMSO) and water should be used to prevent freezing of the solvent and phase separation during freezing. Such hydrogel is transparent and has high mechanical properties. But DMSO has toxicity, and it also has the effect of accelerating the absorption of harmful substances.
Therefore, in this study, we intended to eliminate DMSO in the process to make PVA hydrogel with high mechanical properties.
[Material and methods]
PVA hydrogels were prepared by low temperature crystallization with DMSO method and novel heat pressing method. In low temperature crystallization method, PVA (Japan VAM & POVAL Co. Ltd., degree of polymerization 1700, degree of saponification 99.7%) was dissolved in a solvent mixture of DMSO and H2O with weight ratio of 80/20 at 120 C at a concentration of 10w/w%. The solution was poured into a mold and cooled to -20 C for 24h. In heat pressing method, PVA was mixed with same weight of H2O (50w/w%). The swollen PVA/H2O was pressed at 0~20MPa and 90C for 30 min.
[Results and discussion]
PVA hydrogels from a highly concentrated aqueous solution was prepared by a heat pressing method. By this method, transparent PVA hydrogels can be obtained because of the fast crystallization, even at room temperature. And water content was well controlled by heating of dried gel due to the increasing of crystallinity. Fourier transform infrared spectroscopy (FTIR) and dynamic mechanical analysis (DMA) measurement revealed that heat pressing PVA hydrogel with only water showed slower crystallization and gelation than DMSO containing hydrogels. Small angle X-ray scattering (SAXS) shows all PVA hydrogel has a microcrystalline structure, showed quite similar structure of both low temperature crystallization and heat pressing hydrogels. The mechanical properties of PVA hydrogels were remarkably dependent on their water contents after gelation, regardless of the preparation method. It was concluded that successfully we were able to make PVA hydrogel with high mechanical properties by a heat pressing method without using DMSO.

We report unique methodologies for the construction of supramolecular fluorescent dendron nanotubes of which the surface is covered with cyclodextrins (CDs). In particular, their sensory characterisctics will be discussed.
The organic nanotubes with unique fluorescence characteristics were fabricated by self-assembly of the amide dendrons and cyclodextrins (CDs). In addition, the electron transfer property of the hybrid array of the nanotubes with metal nanoparticles was utilized for sensory application. The dendritic building blocks with focal pyrene unit self-organize into vesicles in aqueous phase. The in-situ inclusion of the focal pyrene units into the cavity of β- or γ-CD induced formation of self-assembled fluorescent nanotubes. The surface of the nanotube is covered with CDs. Therefore, the functional group on the surface of the nanotube is controlled precisely by modifying the functionality of CD. This work provides an efficient methodology not only to create a new class of CD-covered organic nanotubes. The hybrid array of the tubes with Au nanoparticles can be conveniently prepared by using the interaction of the surface functionality of the tube with that of Au nanoparticle. The electron transfer from the organic nanotube to Au nanoparticle induces fluorescence quenching which can be effectively utilized for sensing application. In this work, biosensory characteristics of the dendron-CD nanotube was demonstrated representatively by using the strepavidin-biotin system. In addition, we demonstrated that dendron-CD nanotube can be an efficient metal sensory platform by introducing coumarin-Gly-His (GH) dipeptide unit onto the surface of the nanotube.

A novel three-dimensionally ordered macroporous (3DOM) hydrogel with immobilized-enzyme was synthesized, characterized, and used for protein digestion. The 3DOM hydrogel was prepared by the copolymerization of poly(ethylene glycol) methacrylate (PEOMA) and poly(ethylene oxide) dimethacrylate (PEODMA) in the presence of latex colloidal crystal as the template. The colloidal crystal was synthesized by surfactant-free emulsion polymerization, and has a uniform pore size typically in the range of 100-1000 nm. After being used as the sacrificing template, the colloidal crystal was dissolved in acetone to generate the 3D ordered macropores with interconnected windows in the crosslinked hydrogel, which facilitated the liquid transport through the pores. The trypsin was introduced onto the pore surfaces through condensation reactions. The structure of the functionalized 3DOM hydrogel was characterized by scanning electron microscopy (SEM) and nanoscale 3D X-ray microscopy (XRM). It was demonstrated that the porous structure of the hydrogel was able to undergo reversible shrinkage and expansion by drying and swelling. The trypsin-immobilized hydrogel was loaded in a column and showed high activity for enzyme digestion when an aqueous solution of Nα-benzoyl-L-arginine p-nitroanilide (BAPNA) or bovine serum albumin (BSA) passing through it. This study indicates that the colloidal crystal templated 3DOM hydrogel is a useful enzyme immobilization substrate for protein digestion.

I am characterizing structural changes that arise in the polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) amphiphilic triblock copolymers (commercially known as Pluronics) as a function of temperature through Differential Scanning Calorimetry (DSC). Pluronic® P-105 (BASF Corp) (Mw = 6.5Kg/mol) has been evaluated most extensively. I am also measuring how adding small amounts of Methyl Paraben (MP) perturb the structure and the driving force for micelle formation in aqueous PEO-PPO-PEO solutions with different block lengths. Micelle formation is an aggregate of surfactant molecules dispersed in solution. Hydrophilic heads interact with surrounding aqueous solvent while hydrophobic tails form the micelle center. This new evolving structure can be resolved by Differential Scanning Calorimetry (DSC). DSC is a thermo analytical technique where the difference in the amount of heat required to increase the temperature of a sample and a reference is tracked with temperature. Methyl Paraben is commonly used as preservatives by the cosmetic and pharmaceutical industries. It is found in fruits where it acts as an antimicrobial agent and kills or inhibits the growth of microorganisms. Future experiments will focus on resolving the driving force for micelle formation in the presence of drug mimics. We are ultimately trying to resolve how drugs incorporated into dispersion-based drug delivery vehicles affects their solidification characteristics when heated to body temperature.

Like cellular proteins that form fibrillar nanostructures (e.g., cytoskeletal filaments), small hydrogelator molecules self-assemble in water to generate molecular nanofibers.1 While most endogenous protein filaments are crucial for normal cellular functions (e.g., cell movements and division)2 and some have been associated with human illnesses (e.g., Alzheimer's, Pick's, and Parkinson&’s diseases),3 the mechanism of self-assembly of small molecules in the cell and the fate of intracellular molecular nanofibers remain largely unknown.4 Here we interpret the self-assembly behavior of peptide-based hydrogelators in terms of the phase diagram determined from light scattering and NMR measurements made as a function of concentration and temperature. We visualize by fluorescence imaging the enzyme-triggered self-assembly of the non-fluorescent hydrogelator with the doping method. Cell fractionation experiments, fluorescent imaging, and electron microscopy indicate that the molecular nanofibers localize to the endoplasmic reticulum (ER) and are likely processed via the cellular secretory pathway (i.e., ER-Golgi-lysosomes/secretion). This work not only provides the spatiotemporal profile of molecular nanofibers inside cells, but also may lead to a new paradigm for regulating cellular functions based on the interactions between molecular nanofibers and organelles as well as a new model system for understanding neurodegenerative diseases caused by molecular aggregates.
References:
1. Yang, Z. M.; Liang, G. L.; Xu, B. Acc. Chem. Res. 2008, 41, 315.
2. Alberts, B. Essential Cell Biology; 3rd ed.; Garland Science: New York, 2009.
3. Spillantini MG; Crowther RA; Jakes R; Hasegawa M; Goedert M. Proc. Natl. Acad. Sci. USA 1998, 95, 6469.
4. Gao, Y.; Shi, J. F.; Yuan, D.; Xu, B. Nat. Commun. 2012, 3, 1033.

Cartilage is a gel-like biological tissue consisting of solid and fluid components. The solid component is composed of a collagen network, which contains highly swollen negatively charged proteoglycan assemblies. The predominant cartilage proteoglycan is the bottle-brush shaped aggrecan. In the presence of hyaluronic acid (HA) and link protein aggrecan molecules condense on the HA chain and form a secondary bottle-brush structure. The swollen microgel-like assemblies enable cartilage to support high loads. The collagen network plays limited role in load bearing. Its principal role of is to immobilize the aggregan/HA assemblies and provide tensile stability to the cartilage.
Cartilage can be divided into three zones based on the orientation of fibers. Close to the surface, collagen fibers are aligned parallel to the articulating surface of the joint. In this region, designated as superficial zone, the collagen fibers support the tensile stresses generated when compressive loads are applied to the tissue. In the intermediate zone the collagen fibers are randomly oriented. In the deep zone, the fiber orientation is perpendicular to the bone surface. To quantify the consequences of the above morphological differences on the macroscopic properties we made elastic modulus measurements and osmotic swelling pressure measurements on the three layers. The measurements were made on bovine cartilage of a 2 year-old femur head combining atomic force microscopy with tissue-osmometry.

Physically-associating telechelic polymer networks are of particular interest not only for tissue engineering applications due to their ability to shear thin, but also as model systems for further study of network behavior under external forces. Models of these gels are based on kinetic theories of transient networks. In the 1940s, Green and Tobolsky postulated a theory for the mechanical behavior of viscoelastic materials meant to enhance classic Maxwell, Voigt, and other classical relaxation theories by looking more closely at the molecular origins of material behavior. Half a century later, Tanaka and Edwards took the ideas presented in Green and Tobolsky&’s paper and applied them to a telechelic polymer network in solution comprised of long chains with hydrophobic stickers at either end capable of forming elastically active network junctions. Most importantly, Tanaka and Edwards&’ theory incorporated mechanical dependence on deformation history. These two studies provided the basis for the more recent paper by Tripathi, Tam and McKinley, where an additional solvent drag term was added to the chain relaxation, and the presence of both dangling chains and bridging chains contribute to the final equation for the stress tensor. A limitation of current theories is that they do not enable easy tracking of the polymer chain end distribution. This ability is important for full understanding of network behavior - specifically with respect to material recovery following deformation.
We have developed a theory for polymer chain dynamics in a polymer network that models the entire chain end-to-end distribution using a Smoluchowski equation combined with force-activated transient network kinetics according to Bell&’s Law. This equation is solved numerically, calculating the response of a telechelic polymer gelator in shear flow. With this computationally simple theory, we can determine the locations and types of chain present (fraction of chains present in loops, in bridges, and as free chains) in the system under steady shear and oscillatory shear flows at equilibrium. In particular, the decrease in loop fraction at higher Deborah numbers has already given valuable insight into the creation and destruction of loops in steady shear flow. By looking at the evolution of loop, free, and bridge chain fractions over time, we can also show how the system recovers following shear. Our theory shows potential for expansion to more complex systems such as brush polymers, multiblock copolymers, and protein-polymer block copolymers.
Works Cited:
Green, M. S.; Tobolsky, A. V. The Journal of Chemical Physics 1946, 14, 80
Tanaka, F.; Edwards, S. F. Macromolecules 1992, 25, 1516.
Tripathi, A.; Tam, K. C.; McKinley, G. H. Macromolecules 2006, 39, 1981.

Photonic hydrogels are an emerging family of materials that combine periodic nanoscale ordering giving rise to Bragg-like reflection of visible light with the responsive properties of hydrogels. These composites are often based around self-assembled close-packed arrays of spherical nanoparticles, giving rise to brilliant reflected colors that changes as the hydrogel swells and contracts in response to external stimuli. Chiral nematic (CN) liquid crystalline phases can act as photonic structures due to their helical, periodic layering, and exhibit unique optical properties that are desirable in the form of a responsive hydrogel. Here, I will discuss our recent work in preparing new responsive hydrogels with photonic CN ordering through the self-assembly of cellulose nanocrystals. This approach can be considered a general route to prepare photonic hydrogels with CN ordering that respond to various external stimuli.

Solar energy conversion has been expected as one of the most urgent and promising technologies for sustainable energy production. Many types of solar cells have been developed, but unfortunately all of these solar cells have a critical disadvantage, in that the photoreceptors cannot absorb the entire photo-band of sunlight, which leads to inefficient energy conversion. A reasonable solution for this problem can be realized by applying a spectral conversion film (SCF) from unused UV-A to visible wavelengths suitable for solar cells. We demonstrate a new strategy for fabricating effective SCF, which is based on formation of a polymer/monomer phase-separated structure through self-assembling fluorophores functionalized by L-glutamide (g) as a low-molecular-weight gelator.
We prepared a g-functionalized pyrene derivative as a fluorophore (g-Pyr) according to our previously reported procedure. The g-Pyr was doped in a polymer film by a casting method from a mixed solution of g-Pyr and polymer such as polystyrene and EVA. The resultant transparent polymer film showed typical excimer emission at lambda;max = 450 nm by absorption of UV-A light while monomeric emission was observed at 350 and 380 nm. TEM and CD observations indicated that g-Pyr was phase-separated as nano-fibrillar aggregates with highly ordered stacking structures in polymer.
In this system, the emission band can be tuned by controlling the phase-separated structure of a fluorophore in polymer. For example, by changing the alkyl groups in g-unit, the emission band (lambda;max) was adjusted between 420 and 475 nm. By choosing phenyl anthracene and alkynyl anthracence derivatives as fluorophoric moieties instead of pyrene, the excimeric emission appeared at lambda;max = 480 nm and 500 nm, respectively. Moreover, it was confirmed that the emission band of g-derivatives could be controlled by the polymer composition and casting condition (concentration, solvent and temperature). As a result, when the g-Pyr-doped polystyrene film was put on the top of a compound semiconductor solar cell and the power-conversion efficiency was measured by using the solar simulator, 5 - 6 % enhancement of the power conversion efficiency was observed through matching of spectral sensitivity.

Aerogel is an open-celled, microporous, solid foam composed of a network of interconnected structures obtained either by supercritical drying or freeze-drying the wet gels to replace the solvent with air. The high aspect ratio of carbon nanotubes (CNTs) enables the formation of a percolating network at relatively low loading (< 1%). Further, integrating the intrinsic mechanical and electrical properties of CNTs with those of an aerogel provides a new class of material with unique multifunctional properties which may find applications in fuel cells, super capacitors, batteries, catalyst supports, energy absorption materials, chemical and biological sensors (Gui et al., Adv. Mater. 2010, 22, 617-621). A binder such as polyvinyl alcohol may be added to the CNT aerogels as reinforcement, but this may increase the junction resistance and result in lower thermal and electrical conductivities (Bryning et al., Adv. Mater. 2007, 19, 661-664). In this presentation, we will report our recent attempts in improving the mechanical integrity by first functionalizing the CNTs with carboxylic (-COOH) groups and subsequently cross-linking the CNTs using diamines. The density of -COOH groups on the CNTs has been carefully quantified using titration to determine an appropriate stoichiometric ratio for the amidation reactions. Preliminary results on the rheological properties of the wet gels during cross-linking and the mechanical properties of the final CNT aerogels will also be presented.

Multi-sensitive microgels have special properties that can be controlled via the chemical composition as well as the morphology of the particle. [1] Core-shell microgels are used as templates for the formation of more complex structures. Dissolution of the core leads to hollow microgel particles the structure of which depends on the type of the core. Particles with temperature dependent spherical or anisotropic shape can be prepared, thus shape transitions can be induced. Small angle neutron scattering employing contrast variation provides detailed information on the structure of the hollow microgels.
Microgels can also be employed as responsive template for the formation of stable polyelectrolyte complexes. We will first discuss the influence of charge distribution inside microgels on their properties. Then we will show how microgels with different morphology can be used for the formation of polyelectrolyte complexes and as substrate for polyelectrolyte multilayers.
[1] Richtering, W.; Pich, A. The special behaviours of responsive core-shell nanogels. Soft Matter 2012, 8, 11423-11430.

Microgels are hydrogel particles with micron and sub-micron diameters. They have been developed, studied, and exploited for a broad range of applications because of their unique combination of size, soft mechanical properties, and controllable network properties. We have been using microgels to modulate the properties of biomaterials surfaces to differentially control their interactions with tissue cells and bacteria. The long-term goal is to create biomaterials that promote healing while simultaneously inhibiting infection. Because poly(ethylene glycol) [PEG] is used in a number of FDA-approved products and has well-known antifouling properties, we work primarily with PEG-based microgels. We render these anionic either by copolymerization with monomeric acids or by blending with polyacids during suspension photopolymerization. Both methods produce pH-dependent negative charge. Surfaces, both planar 2-D surfaces as well as topographically complex 3-D surfaces, can be modified using a hierarchy of non-line-of-sight electrostatic deposition processes that create biomaterials surfaces whose cell adhesiveness is modulated by a sub-monolayer of microgels. Average inter-microgel spacings of 1-2 microns exploit natural differences between staphylococcal bacteria and tissue cells, which open the opportunity to differentially control surfaces interactions with them. After deposition, the microgels can be loaded with a variety of small-molecule, cationic antimicrobials. The details of loading depend on the relative sizes of the antimicrobials and the microgel network structure as well as on the amount and spatial distribution of electrostatic charge within both the microgel and the antimicrobials. The exposed surface between microgels can be further modified by the adsorption of adhesion-promoting proteins such as fibronectin again by electrostatic interaction. This approach combines a rich interplay of microgel structure and chemistry as a key component in a simple and translatable approach to differentially modulate the surface properties of next-generation biomaterials.

Nanocomposites based on synthetic polymers grafted from kraft lignin with average particle size of 5 nm were synthesized using atom transfer radical polymerization. Lignin macroinitiators were prepared at average numbers densities of 2, 7, and 16 per lignin particle, and polystyrene and poly(methyl methacrylate) were grafted from the particles using atom transfer radical polymerization. Tensile testing of the samples showed a decreased modulus but enhanced toughness of all nanocomposites compared to unfilled homopolymers, but the poly(methyl methacrylate)-grafted samples had nearly twice the ultimate elongation as the polystyrene grafts at high graft density. Both types of grafted nanocomposites had toughness values that were 20 times that of the corresponding blend, indicating the importance of the grafted architecture in achieving these mechanical properties. Dynamical mechanical analysis was used to measure softening temperatures, and both the polystyrene-grafted and poly(methyl methacrylate)-grafted nanocomposites had a peak in the loss modulus that was higher than the corresponding homopolymer, consistent with strong polymer-lignin interactions. Polymer-grafted lignin could be an important structural material based on an inexpensive, renewable feedstock that offers unique mechanical properties compared with many other nanocomposites based on inorganic nanoparticles.

Polymersomes are vesicles consisting of amphiphilic block-co-polymers; they are well suited delivery vehicles. They are typically formed using re-hydration techniques where block-co-polymers self-assemble into vesicular structures. However, only certain types of block-co-polymers self-assemble into these structures; this precludes the incorporation of many different types of block-co-polymers into the vesicular membranes. Polymersomes can also be formed through microfluidic techniques. This assembly route uses double emulsion templates to form vesicles; it therefore enables the incorporation of amphiphilic molecules into the polymersome membrane that would not spontaneously assemble into vesicular structures. It allows us to design thermo-responsive polymersomes by incorporating thermo-responsive block-co-polymers into the vesicular membrane. These vesicles can be further converted into photo-responsive polymersomes by incorporating hydrophobic gold nanoparticles into the polymersome membrane. The resulting photo-responsive polymersomes enable triggered release of encapsulants through laser irradiation.

Reversible shift of the pKa/pKb value of acids/base in enzymes plays a crucial role in their functions. For instance, binding of oxygen to hemoglobin induces drastic conformation change of the protein, lowering pKa of ammonium and imidazolium cations in/on the proteins. The shift of pKa induces release of protons from hemoglobin, pushing out bicarbonate ions from blood effectively. Recently, we have reported that temperature responsive gel particles which contains amine groups, reversibly absorbs CO2 in the response of temperature-induced volume phase transition. Conformation change of pNIPAm induced by the phase transition lowered dielectric constant around the ammonium cations in the particles, resulting significant decrease of pKa. The shift of pKa results in release of protons from the NPs, pushing out CO2 from the solution. However, general guidelines to prepare NPs with large pKa shift have not been revealed. In this study, we report that nanoparticles that show large pKa shift can be prepared by copolymerizing N-isopropyl acrylamide and acrylic acid in the presence of proton as a template.

Polyelectrolyte multilayers (PEMs) that contain an excess of amines are known to swell and shrink significantly and repeatably in response to changes in ambient pH. Nanoparticles can be incorporated onto and into such films, and can then be actuated by the swelling and shrinking of the films. Such structures are of interest for applications in areas such as photonics and biosensing, and can also be used to probe the properties of the film itself. We report on a series of such experiments, where the vertical position of particles was determined to high accuracy either through fluorescent lifetime measurements or by what is known as a plasmon nanoruler. With these methods, we can dynamically monitor the film thickness, and have for instance been able to show that while thicker PEMs tend to switch abruptly between discrete thicknesses while exhibiting large hysteresis, thin films consisting of a single polycation layer instead vary smoothly in thickness with pH, showing a minimum of hysteresis. These same techniques can also be used to study the interaction between a surface and a PEM. For example, when larger charged particles are combined with a PEM, Coulomb forces embed the particles into the body of the film, so that the gap between particle and surface is only a fraction of the unstrained film thickness. Although the film is highly mechanically compliant, this gap can nonetheless be reliably modulated with pH, so that relatively thick amine-rich PEMs can be used to dynamically tune distance of only a few nm or less.

In this research, we show that poly(vinyl alcohol) (PVA), a synthetic water-soluble polymer with wide ranging attributes including, a high level of biocompatibility, and ease of chemical functionalization and cross-linking, can be successfully assembled into a multilayer thin film by hydrogen bonding interactions. The presence of hydroxyls group in PVA allows hydrogen bonding with carboxylic acid groups present in weak polyacids such as poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) at low pH conditions. Among the systems investigated, PVA/PAA multilayers functionalized with poly(ethylene glycol methyl ether) (PEG) exhibited two interesting characteristics: one is zwitter-wettable behavior whereby the multilayer film exhibits a facile, rapid absorption of water from the gas phase while simultaneously exhibiting very high contact angles for drops of liquid water placed on the surface. The second notable characteristic of these films is their transient water contact angle behavior. Time-dependent wetting behavior of these coatings results from the surface rearrangement of hydrophilic functional groups towards the surface in response to exposure to a liquid water environment and has been assessed by both goniometry and dynamic tensiometry. Also, the zwitter-wettable behavior and its use in antifrost and antifogging coatings have been investigated in detail.

An ideal drug delivery system (DDS) is expected to satisfy the conflicting requirements of high stability in extracellular fluid during the in vivo circulation, however, being labile after targeting to a desirable site, followed with the release of therapeutic agents. The aim of this communication is to present original results on reversibly crosslinked (RCL) nanogels as a new DDS, consisting of thermo-responsive copolymer of poly(vinyl alcohol)-b-poly(N-vinylcaprolactam) and boronate-functionalized maghemite nanoparticles. These multifunctional RCL nanogels were constructed via the reversible boronate/diols bonding strategy, and found to combine all these properties:
(i) Significantly improved stability under physiological pH (7.4) in aqueous medium, compared to that of phenylboronic acid-crosslinked nanostructures reported before;
(ii) Minimal premature drug release (< 8% in 24 h) was found at physiological condition (5 mM glucose, pH 7.4); however, triggered release was observed upon exposure to acidic pH (6.0 or 5.0) and/or presence of glucose;
(iii) Magnetically-induced local heating could be generated under alternative magnetic field (AMF, 755 kHz, 14 mT), and AMF application could also contribute to an accelerated release, while with a mild heating to the release medium (< 1°C). Moreover, enhanced T2-weighted contrast performance was also observed;
(iv) Cytotoxicity against fibroblastic L929 and melanoma MEL-5 cell lines disclosed a good biocompatibility of these RCL nanogels. In vitro pH- and/or glucose-triggered drug release of tamoxifen with the cell culture was evidenced with the enhanced cell-proliferation inhibition and cleavage of nanogel nanostructure within the endosomes by TEM.
This multi-functional DDS, combing with stimuli-triggered release potential, therapeutic agent and MRI contrast agent, is expected to be delivered as a versatile platform for on-demand releasing drugs to those acidic cellular compartments or disease microenvironments, such as diabetes, and biomedical diagnosis.

Poly(2-oxazoline)s (POx) are an interesting, yet less well-known class of polymers although they are thermo-responsive [1], non-toxic and possess similar stealth properties as PEG [2]. Thus, they represent highly promising materials for the development of intelligent drug delivery devices in various kinds of compositions, such as linear, block or comb-like macromolecules, and hydrogels. Thereby, the living cationic ring-opening polymerization (CROP) mechanism allows a straightforward design of the polymer properties by copolymerization as well as the attachment of cell targeting ligands or labels.
A range of thermo-responsive comb and graft copolymers are accessible by end-capping the living oxazolinium species from the CROP of 2-ethyl-2-oxazoline (EtOx) and reversible addition fragmentation chain transfer (RAFT) polymerization of the obtained macromonomers. On the one hand, this allows insights into the lower critical solution temperature (LCST) behavior of polymers that exist as macromolecular bottle brushes in aqueous solution [3]. On the other hand, the LCST properties can be brought into the physiological range by copolymerization with methacrylate-based monomers, such as methyl methacrylate and methacrylic acid [4,5]. Due to the introduced carboxylic acid moieties, the latter graft polymers are responsive not only to temperature but in to the pH value as well. In addition, the advanced polymeric architectures have been functionalized with cell-targeting sugars selectively at the end of the side chains [6].
Another way to introduce thermo-responsiveness into POx is the variation of the side group of linear polymers by the co-polymerization of functional 2-oxazolines. The introduction of primary amine groups using a statistical co-polymerization results in a system whose LCST depends on the pH value. The gelation of these phase separated polymer solutions by cross-linking of the amine moieties leads to hydrogels with a distinct microstructure. The resulting cationic networks can be used to selectively bind and release genetic material for the purpose of purification or detection in microarrays and DNA chips.[7]
1. C. Weber, R. Hoogenboom, U. S. Schubert, Prog. Polym. Sci. 2012, 37, 686-714.
2. K. Knop, R. Hoogenboom, D. Fischer, U. S. Schubert, Angew. Chem. Int. Ed. 2010, 49, 6288-6308.
3. C. Weber, S. Rogers, A. Vollrath, S. Hoeppener, T. Rudolph, N. Fritz, R. Hoogenboom, U. S. Schubert, J. Polym. Sci.; Part A: Polym. Chem. 2013, 51, 139-148.
4. C. Weber, C. R. Becer, R. Hoogenboom, U. S. Schubert, Macromolecules 2009, 42, 2965-2971.
5. C. Weber, C. R. Becer, W. Guenther, R. Hoogenboom, U. S. Schubert, Macromolecules 2010, 43, 160-167.
6. C. Weber, J. A. Czaplewska, A. Baumgaertel, E. Altuntas, M. Gottschaldt, R. Hoogenboom, U. S. Schubert, Macromolecules 2012, 45, 46-55.
7. M. Hartlieb, D. Pretzel, K. Kempe, C. Fritzsche, R. M. Paulus, M. Gottschaldt, U. S. Schubert, Soft Matter 2013, 9, 4693-4704.

The dynamic control over materials shape plays a key role in the complex biological environment and has attracted considerable attention in science, engineering, and medicine. We report on a dynamic volume transitions exhibited by ultrathin hollow hydrogel microcontainers of cubical shapes, also called hydrogel capsules. These cubical capsules were produced as poly(methacrylic acid)-based hydrogel (PMAA) replicas of cubical sacrificial templates and obtained from chemically cross-linked multilayers of hydrogen-bonded ultrathin films. We found that pH-induced volume transitions in those systems were strongly influenced by capsule shell thickness, composition, and crosslink density. While one-component (PMAA)20 cubical capsules turned into bulged spherical-like structures when transitioned from acidic to basic pH values, more rigid two component PMAA-poly(vinylpyrrolidone) capsules retained their cubical shape and increased in size instead. This pH-triggered size and shape changes were completely reversible. The drastic difference in pH-triggered shape responses was rationalized through the difference in hydrogel rigidity expressed as the ratio of the polymer contour length between the neighboring cross-links to persistence polymer length. Those ratios of 22.7 and 2 for (PMAA) and (PVPON-PMAA) systems, respectively, suggested that dual-component system is more rigid and therefore expand uniformly in all directions upon pH-triggered swelling. We believe that these results provide new prospects for developing polymeric materials with predictable shape and size-changing properties for controlled drug delivery, cellular uptake, and in pH-regulating transport behavior in microfluidic devices.

The nanostructure and nanomechanical properties of aggrecan monomers extracted and purified from human articular cartilage from donors of different ages have recently been visualized and quantified via atomic force microscopy (AFM)-based imaging, force spectroscopy, and high bandwidth nano-rheology. AFM imaging enabled direct comparison of full length monomers at different ages. The demonstrably shorter glycosaminoglycan (GAG) chains observed in adult full length aggrecan monomers, compared to newborn monomers, also reflects markedly altered biosynthesis with age. Direct visualization of aggrecan subjected to chondroitinase and/or keratanase treatment revealed conformational properties of aggrecan monomers associated with chondroitin sulfate and keratan sulfate GAG chains. Furthermore, compressive stiffness of chemically end-attached layers of adult and newborn aggrecan was measured in various ionic strength aqueous solutions, suggesting the importance of both electrostatic and non-electrostatic interactions in nanomechanical stiffness. These results provide molecular-level evidence of the effects of age on the conformational and nanomechanical properties of aggrecan, with direct implications for the effects of aggrecan nanostructure on the age-dependence of gel-like properties of cartilage tissue. Poroelastic interactions of interstitial fluid flow with cartilage extracellular matrix, in particular aggrecan proteoglycans, has been hypothesized to be the underlying mechanism in many critical functions of hydrated soft tissues such as transport, energy dissipation, and self-stiffening. However, the quantification and detailed mechanism of molecular level fluid-solid interactions in these systems is largely unknown. We recently studied brush layers of aggrecan from these same different aged human donors, and utilizing a new high frequency atomic force microscope (AFM)-based rheology system quantified the dynamic mechanical behavior of the solid-fluid interactions in the 1Hz to 10 kHz frequency range. Most dramatically, the magnitude and phase angle of the dynamic stiffness showed frequency-dependent trends remarkably similar to those of intact native cartilage tissue. For the first time, we were able to estimate the hydraulic permeability of aggrecan networks. These results confirm that aggrecan is the key molecule in determining the fluid-flow-dependent properties of cartilaginous tissues.

Connective tissues such as tendon, ligament, fascia, and cartilage require the presence of large, organized collagen fiber bundles to achieve sufficient load bearing capacity in tension and shear. This assembly of collagen fibers can also result in high degrees of mechanical anisotropy. While such structure and properties are routinely achieved in natural tissues, they are often lacking in tissue-engineered implants. How such structures are formed developmentally is not fully understood. However, their formation coincides with the transition of stem cells from a rounded shape and undifferentiated state to an elongated shape associated with a fibrous phenotype. It has been known for more than three decades that fibroblastic cells exert traction forces that cause mechanical remodeling of collagenous matrices. We hypothesized that these cellular traction forces could be directed to control collagen matrix assembly through the manipulation of mechanical boundary conditions imposed on tissue-engineered implants during extended cell culture.
We explored this hypothesis in experiments aimed at tissue engineering of two types of fibrocartilage, the annulus fibrosus and the meniscus. In both tissues, large collagen fiber bundles are oriented circumferentially in the plane orthogonal to the direction of loading. To guide the formation of such fibers in engineered tissues, fibrochondrocytes from these tissues were seeded into collagen gels in concentrations ranging from 1 to 20 mg/ml. Collagen gels were molded into shapes that were appropriate for the target tissues - a flat ring for the annulus fibrosus and a semi-lunar wedge with a triangular cross section for the meniscus. For both shapes, cellular collagen gels were cultured for up to 4 weeks under varying static mechanical constraints. Controls for both group were unconstrained and allowed to contract freely, while a rigid, impermeable cylinder was placed at the inner edge of the annulus fibrosus constructs and the meniscus constructs were clamped at the edges (i.e. “horns”) to direct contraction.
For both cell types, mechanically unconstrained gels contracted dramatically, shrinking to <10% of their original volumes. This contraction was associated with the formation of small (1-3 um), unorganized fiber bundles. The compressive and tensile moduli of these samples increased slightly, but the resulting tissue was isotropic. The mechanically constrained implants produced networks of circumferentially organized bundles of fibers similar in size to those in the native tissues (10-20 um). The tensile modulus of constrained gels increased up to 10-fold over 4 weeks and exhibited significant anisotropy, being 3-4 fold higher in the circumferential direction compared to radial direction.
These studies demonstrate the importance of cellular traction forces on the development of collagen organization in fibrous tissues, and point to new methods to control this process in tissue engineering.

Two primary paradigms have been used to understand and explain the functional properties of cartilage, such as its load bearing and lubricating ability. One family of phenomenological models describe cartilage as a charged, poroelastic continuum (like a charged clay) with a charged elastic network phase that can bear mechanical load, and an interpenetrating ionic fluid phase that can develop a hydrostatic pore pressure. Historically, these models have not distinguished among the soft molecular constituents comprising cartilage&’s elastic network, which includes collagen, proteoglycans, and polysaccharides. Another approach tries to explain functional material properties of these tissues as arising from polymeric interactions between and among the different molecular constituents within the tissue. This approach treats the tissue effectively as a complex molecular composite containing highly charged polysaccharide microgels trapped within a fine collagen meshwork. We have been developing a multi-scale experimental and theoretical framework to explain key material properties of cartilage by studying their constituents and interactions among them at a variety of length and time scales. We use this approach to address important biological questions, such as, Why doesn&’t cartilage collapse like other polyelectrolytes, in the presence of the high calcium ion concentrations present in joints, particularly near the bone-cartilage interface? Why is the diffusivity of aggrecan so much larger than that measured in free solution? How do aggrecans self-aggregate and what is the functional significance of the microgel structures they form? Is there any evidence of cross-linking between the collagen network and the polysaccharides they entrap? One novel application we plan to focus on here is the use of various complementary non-invasive magnetic resonance imaging (MRI) methods to characterize different components and compartments within cartilage and the different water environments associated with each one, in an attempt to provide a comprehensive picture of the mechanical/chemical state of cartilage.

Regenerative medicine is an emerging interdisciplinary field. Of the three essential elements for tissue engineering (i.e., stem cells, differentiation signals and scaffolds), the recent progress in the development of cell sources and in the identification of essential signals has been remarkable. With regard to scaffolds, hydrogels are promising materials because their structural compositions are similar to those of the soft tissues in the human body. Certain hydrogels can be injected in solution and transformed into the gel state with the required shape, which is an indispensable prerequisite for minimally invasive surgery. However, no injectable hydrogel has reached a practical stage as structural materials, mainly due to the fact that all the conventional hydrogels inevitably “swell” under physiological conditions (i.e., in an aqueous environment at 37°C), which drastically spoils their mechanical properties. Moreover, their morphological changes induce slippage