A series of unique hybrid thermoplastic polyurethanes (TPUs) was synthesized using PEG as soft segment and incorporating an isobutyl-functionalized POSS diol (TMP POSS diol) in the hard segment. The molecular weight of PEG was systematically varied to include 10, 20, and 35 kDa, while the mole ratio of POSS diol (as chain extender) to PEG was in range from 3:1 to 8:1 with samples featuring a PEG molecular weight of 10 kDa. The diisocyanate employed for TPU polymerization was 4,4'-methylenebis(phenyl-isocyanate) (MDI). We found that the hydrophobic hard segments (POSS) can form crystalline structures driven by micro-phase separation, this being due to significant thermodynamic incompatibility between POSS and ethylene oxide units. The POSS nano-crystals thus formed serve as physical crosslinking sites within an inorganic-organic hybrid network. This affords a new hybrid organic-inorganic hydrogel in the water-swollen state. The equilibrium swelling ratio increased monotonically with PEG loading and ranged from ∼70% to ∼600%. The shear modulus, G, of the hybrid hydrogels was observed to span 0.3 < G < 4.0 MPa – C values commonly found for elastomers, not hydrogels. Indeed, the hydrogel stiffness can be finely tuned through the POSS:PEG molar ratio, as this predictably controls swelling in water.

The familiar property responses of isopropylacrylamide hydrogel, cause the hydrogel to undergo a discontinuous volumetric phase across a critical level of stimuli. Poly(N-isopropylacrylamide) hydrogel is a recognized response to global temperature stimuli across the low critical solution temperature (LCST). The basic PNIPAM hydrogel undergoes its phase change at a LCST of 34°C, where water is expelled from the interior of the gel microsphere, as the temperature increases past LCST. The responsiveness of PNIPAM hydrogel offers the potential for controlling optical properties of a medium such as refractive index and scattering index. In the present work we present the refractive index of microsphere/microbeaded hydrogel structures necessary for optical application. The particle size of the hydrogels nanostructures at room temperature is observed to be 300-400 nm as estimated by dynamic light scattering and agree well with scanning electron microscopy measurements. The temperature dependent refractive index change of hydrogel microspheres have been measured using variable angle spectroscopic ellipsometry and shows a 10% change as the temperature changes from 33°C to 34°C.

A promising treatment for multiple neurological disorders including stroke, Huntington's, and Parkinson's is the transplantation of stem cells into the diseased site to promote regeneration of the neural tissue. However, viability of transplanted cells is often low (15-35%) and unpredictable. Cell viability has been directly correlated with functional outcome of the treatment, motivating the development of more reliable cell transplantation procedures. To protect transplanted cells from shear stress during injection and from the hostile, inflammatory environment of the diseased brain tissue, many research groups are exploring physical hydrogels as a protective, growth-permissive matrix to enhance cell viability. However, physical hydrogels require an environmental trigger to induce gelation. These environmental triggers include sudden changes in temperature, pH, or salt concentration - all of which are detrimental to encapsulated cells and proteins and complicate their use in a clinical environment. To address this need, we have designed a two-component, protein-based hydrogel system that can self-assemble under constant physiological conditions.

Both components of the hydrogel system are created using recombinant protein technology, which allows synthesis of exact monomer sequences within monodisperse polymers. The first hydrogel component is a block copolymer made of several repeats of a peptide sequence encoding the WW domain-fold, a short triple-stranded, anti-parallel, beta-sheet. The WW domains are interspersed with a random-coil, hydrophilic spacer to enhance polymer flexibility and solubility. The second hydrogel component is made of several repeats of a polyproline rich peptide sequence interspersed with a random-coil, hydrophilic spacer. Upon mixing the two hydrogel components together, the WW-domains in component 1 and the polyproline rich peptides in component 2 bind together with 1:1 stoichiometry. This binding has an apparent association constant of 2.2×105 M, as measured by isothermal titration calorimetry. This peptide-binding event serves as the physical crosslinks to form a polymeric network composed of the two components. Because gelation is initiated by simply mixing the two components together at physiological pH, temperature, and ionic strength, this system is highly biocompatible and easy to use. Furthermore, the precision of protein engineering allows both components to be easily modified. For example, increasing the length of the hydrophilic spacers will increase the resulting network pore size. Additionally, bioactive peptide sequences, such as the RGD cell-binding domain, have been introduced into the hydrophilic spacers to modify cell-scaffold interactions. Our long-term objective is to design a self-assembling hydrogel system for cell delivery that will both improve cell viability and mimic many of the essential cues in the developmental niche to encourage cell differentiation and outgrowth.

Three-component photoinitiator systems generally include a light-absorbing photosensitizer (PS), an electron donor and an electron acceptor, which is usually an onium salt. To investigate the key factors involved with visible-light free radical polymerizations of three-component photoinitiators, we used thermodynamic feasibility and kinetic considerations with three-component photoinitiator systems containing either rose bengal (RB) or fluorescein (FL) as the photosensitizer. The Rhem-Weller equation was used to verify the thermodynamic feasibility for the photo-induced electron transfer reaction. Using the thermodynamic feasibility, we suggest three different kinetic mechanisms, which are i) photo-reducible series mechanism, ii) photo-oxidizable series mechanism and iii) parallel-series mechanism.

Fiber networks deform nonaffinely due to their inhomogeneous structure. The degree of nonaffinity depends on many factors such as the spatial distribution and properties of fibers, the nature of the applied load, the type of joints and the length scale of observation. The “homogenized” response on given scale depends on the non-affine mechanics on all sub-scales. Here, a method is developed to map the non-affine mechanics of a regular network with a large density of defects into an equivalent continuum, which is then used to determine the homogenized elastic properties of the network. This semi-analytic method establishes a relationship between the structure of the network and its overall elastic properties. Furthermore, we develop a method to quantify the nonaffine strains in a random network and use it to study the dependence of the degree of non-affinity to the scale of observation as well as the dependence on the network architecture.

A quartz crystal microbalance (QCM) has been used to investigate polymer samples. The vapor sorption of three different polymer samples (poly(vinyl acetate), polybutadiene and polydimethylsiloxane) was studied. The change in resonance frequency of the quartz sensor uniformly coated with polymer films was measured as a function of the film thickness and water absorption at different temperatures. The range of linear frequency vs. mass response was determined in the absence of absorbed water. The glass transition temperature of thin poly(vinyl acetate) films (10 nm <thickness< 1000 nm) was found to be in reasonable agreement with published values for macroscopic samples.

The aim of this work is the development of controlled delivery system immobilized by local anesthetics on the basis of modified microparticles of calcium alginate gels. The kazkain, rikhlokain, lidokain and novokain were used as local anesthetics. Modified microparticles were obtained by syringed dropwise a solution of ansthetics in sodium alginate into solution of polymers such as chitosan or polyethyleneimine in calcium chloride. The obtained modified alginate microparticles were contained immobilized anesthetics in core of particles and a surface layer of polymer. Effects of polymer concentration and exposure duration on the thickness of polymer coating were determined. The release of anesthetics from the modified alginate gel particles into a physiological solution with different thickness of coating were studied. All the release data show the typical pattern for a matrix controlled mechanism. The cumulative amount of drug released from alginate gels was linearly related to the square root of the time and the release rate decreased this time. The process is controlled by the diffusion of anesthetics through the polymeric coating. The data shown a possibility of the regulation of the rate of anesthetics release from the modified alginate particles by way of alternation of thickness of the polymer coating. The anesthetic effect of the alginate microparticles containing drugs was tested on rats by method “tail flick” according two criteria: full analgesia – the absence of reaction on pain and sufficient analgesia – exceeding of pain threshold sensibility two and more times. Medical-biological tests show that the duration of anesthetic activity for the drug-containing alginate beads increases at 5-8 times in comparison of free drugs.

AFM micro- or nanoindentation is a powerful technique for mapping the elasticity of materials at high resolution. When applied to soft matter, however, its accuracy is equivocal. The sources of the uncertainty can be methodological or analytical in nature. In this paper, we address the lack of practicable nonlinear elastic contact models, which frequently compels the use of Hertzian models in analyzing force curves. We derive and compare approximate force-indentation relations based on a number of hyperelastic general strain energy functions. These models were applied to existing data from the spherical indentation of native mouse cartilage tissue as well as chemically crosslinked poly(vinyl alcohol) gels. For the biological tissue, the Fung and single-term Ogden models were found to provide the best fit of the data while the Mooney-Rivlin and van der Waals models were most suitable for the synthetic gels. The other models (neo-Hookean, two-term reduced polynomial, Fung, van der Waals, and Hertz) were effective to varying degrees. The Hertz model proved to be acceptable for the synthetic gels at small strains (<20% for the samples tested). Although this finding supports the generally accepted view that many soft elastic materials can be assumed to be linear elastic at small strains, we propose the use of the nonlinear models when evaluating the large-strain indentation response of gels and tissues.

Re-orientational dynamics of liquid crystal molecules in a polymer network subjected to an electric field is studied by means of light diffraction [1]. When the optical pitch of the electric-field induced cholesteric phase is small compared to the optical wavelength of light, dynamic light scattering (DLS) can be performed to extract the relaxation dynamics of the chiral nematic molecules in the presence of the polymer network. Intriguingly, the reactive mesogenic type of polymer network exhibits a confinement effect, which can be probed within the limited range of scattering angles that comply with the structural correlation length in the system [2].

Diffusive mass transport of molecules through a rod network can be studied via fluorescence correlation spectroscopy (FCS) and DLS. Long time self-diffusion of tracer spheres (silica and proteins) in isotropic and nematic colloidal-rod networks (fd-viruses) is systematically studied for various tracer-sphere sizes as compared to the mesh size of the network [3]. In addition, by varying the salt concentration, the relative contribution of electrostatic interactions can be varied. A theory is developed where the diffusion coefficient is expressed in terms of the hydrodynamic screening length of the highly entangled rod-network. The hydrodynamic screening length of rod networks is extracted from diffusion data as a function of the rod concentration both for isotropic and nematic networks [4-5].

Historically, the low thermal conductivity of silica aerogels has made them of interest to the aerospace community for applications such as cryotank insulation. However, recent advances in the application of conformal polymer coatings to these gels have made them significantly stronger, and potentially useful as lightweight structural materials. In this work, we perform multiscale computer simulations to investigate the compressive strength and failure behavior of silica aerogels.

The gels' nanostructure is simulated via the diffusion limited cluster aggregation (DLCA) process, which produces fractal aggregates that are structurally similar to experimentally observed gels. The largest distinct feature of the clusters is the so-called secondary particle, typically tens of nm in diameter, which is composed of primary particles of amorphous silica an order of magnitude smaller. The secondary particles are connected by amorphous silica bridges that are typically smaller in diameter than the particles they connect.

We investigate compressive failure via the application of a uniaxial compressive strain to the DLCA clusters. In computing the energetics of the compression, the detailed structure of the secondary particles is ignored, and the interaction among secondary particles is described by a Morse pair potential parameterized such that the potential range is much smaller than the secondary particle size; an angular potential contribution is included in some of the simulations as well. The Morse and angular parameters are obtained by atomistic simulation of models of the interparticle bridges, with the compressive and bending behavior of these bridges modeled via molecular statics. We consider the energetics of compression and compressive failure, and compare qualitative features of low-and high-density gel failure.

We have developed an optical chamber for studying the effect of swelling or shrinking of a gel on the translational diffusion of fluorescent nanoprobes embedded in the gel. On one side of the chamber, the gel is in contact with a hydrating or dehydrating polymeric solution through a porous membrane, allowing control of the rate of hydration or dehydration of the gel. On the other side, a laser beam is focused into the gel to excite the fluorescence of the nanoprobes, which is continuously monitored to reveal possible structural changes of the stressed gel. Using fluorescence correlation spectroscopy we measure correlation functions of the nanoprobes at various times following the contact of the gel with the hydrating or dehydrating solution, and determine changes of both the average concentration and the apparent diffusion time of the nanoprobes as the gel is shrinking or swelling. We have tested the chamber using the fluorophore, TAMRA (MW = 430 Da), embedded in a poly(vinyl-alcohol) gel that is being dehydrated by a solution of poly(vinyl-pyrrolidone) (28% w/w). As expected TAMRA moves slower as the gel shrinks. However, the changes in the diffusion time of TAMRA as a function of the PVA concentration of the shrinking gel appear to be different than those measured on TAMRA diffusing in PVA gels prepared at different PVA concentrations but with the same cross-link density.

Atomic Force Microscopy (AFM) was employed to probe the internal structure of living HepG2/C3A cells grown on various commercially-available substrates. In order to understand the driving mechanisms behind the different cell morphologies, the surface properties of these substrates was characterized with AFM and related techniques. The roughness of a 10μm×10μm region of a series of substrates was determined and found to be independent of both coating and culture media, with the exception of thick hydrogel-like coatings. Probing with functionalized tips could not distinguish relative degrees of hydrophobicity under cell culture media, presumably because Debye shielding masks the substrate surfaces. Force spectroscopy was performed on the surfaces to determine exposed surface proteins/polymers intrinsic to the substrate and adsorbed from culture media. Preliminary investigation of cell-mediated substrate reconstruction suggests that the cells secrete large (1000kDa) polymeric molecules at the substrate interface.