Solutions for determining equibiaxial substrate strain for dynamic cell culture.

In this work, empirical and analytical solutions of equibiaxial strain on a flexible substrate are derived for a dynamic cell culture system. The empirical formula, which fulfills the mechanistic conditions of the culture system, is based on a regression analysis from finite element analyses for a substrate undergoing large strains (<15%). The analytical (closed-form) solution is derived from the superposition of two elastic responses induced in the equibiaxial strain culture system after applying pressure to a substrate undergoing small strains (microstrains). There is good agreement between the strain predicted from the solutions and from the direct measurement. Using material and geometric properties of the culture system, the solutions developed here are straightforward and can be used to circumvent experimental measurements or finite element analysis to establish substrate pressure-strain relationships.

Stimuli-responsive polypeptide vesicles by conformation-specific assembly.

In biology, lipids are well known for their ability to assemble into spherical vesicles. Proteins, in particular virus capsids, can also form regular vesicle-like structures, where the precise folding and stable conformations of many identical subunits directs their self-assembly. Functionality present on these subunits also controls their disassembly within the cellular environment, for example, in response to a pH change. Here, we report the preparation of diblock copolypeptides that self-assemble into spherical vesicular assemblies whose size and structure are dictated primarily by the ordered conformations of the polymer segments, in a manner similar to viral capsid assembly. Furthermore, functionality was incorporated into these molecules to render them susceptible to environmental stimuli, which is desirable for drug-delivery applications. The control of assembly and function exhibited in these systems is a significant advance towards the synthesis of materials that can mimic the precise three-dimensional assembly found in proteins.

Effect of chemistry and morphology on the biofunctionality of self-assembling diblock copolypeptide hydrogels.

Amphiphilic, diblock copolypeptides of hydrophilic lysine or glutamic acid and hydrophobic leucine or valine have been observed to self-assemble into rigid hydrogels in aqueous solution at neutral pH and very low volume fraction of polymer, > or =0.5 wt % polypeptide. Laser scanning confocal microscopy and ultra small angle neutron scattering revealed a heterogeneous microstructure with distinct domains of hydrogel matrix and pure water pores. In situ nanoscale characterization, using cryogenic transmission electron microscopy, revealed a porous, interconnected membranous network of assembled polypeptides. At concentrations of polypeptide below gelation, diblocks containing lysine were cytotoxic to cells, whereas those containing glutamic acid were noncytotoxic. At higher polypeptide concentrations, within rigid gel scaffolds, both lysine and glutamic acid based diblocks were noncytotoxic but did not support cell attachment/proliferation. The cationic chemistry observed as cytotoxic in the fluid state was essentially inert in the intact, rigid hydrogel state.

Responsive hydrogels from the intramolecular folding and self-assembly of a designed peptide.

A general peptide design is presented that links the pH-dependent intramolecular folding of beta-hairpin peptides to their propensity to self-assemble, affording hydrogels rich in beta-sheet. Chemical responsiveness has been specifically engineered into the material by linking intramolecular folding to changes in solution pH, and mechanical responsiveness, by linking hydrogelation to self-assembly. Circular dichroic and infrared spectroscopies show that at low pH individual peptides are unstructured, affording a low-viscosity aqueous solution. Under basic conditions, intramolecular folding takes place, affording amphiphilic beta-hairpins that intermolecularly self-assemble. Rheology shows that the resulting hydrogel is rigid but is shear-thinning. However, quick mechanical strength recovery after cessation of shear is observed due to the inherent self-assembled nature of the scaffold. Characterization of the gelation process, from the molecular level up through the macroscopic properties of the material, suggests that by linking the intramolecular folding of small designed peptides to their ability to self-assemble, responsive materials can be prepared. Cryo-transmission electron and laser scanning confocal microscopies reveal a water-filled porous scaffold on both the nano- and microscale. The environmental responsiveness, morphology, and peptidic nature make this hydrogel a possible material candidate for biomedical and engineering technology.

Rapidly recovering hydrogel scaffolds from self-assembling diblock copolypeptide amphiphiles.

Protein-based hydrogels are used for many applications, ranging from food and cosmetic thickeners to support matrices for drug delivery and tissue replacement. These materials are usually prepared using proteins extracted from natural sources, which can give rise to inconsistent properties unsuitable for medical applications. Recent developments have utilized recombinant DNA methods to prepare artificial protein hydrogels with specific association mechanisms and responsiveness to various stimuli. Here we synthesize diblock copolypeptide amphiphiles containing charged and hydrophobic segments. Dilute solutions of these copolypeptides would be expected to form micelles; instead, they form hydrogels that retain their mechanical strength up to temperatures of about 90 degrees C and recover rapidly after stress. The use of synthetic materials permits adjustment of copolymer chain length and composition, which we varied to study their effect on hydrogel formation and properties. We find that gelation depends not only on the amphiphilic nature of the polypeptides, but also on chain conformations--alpha-helix, beta-strand or random coil. Indeed, shape-specific supramolecular assembly is integral to the gelation process, and provides a new class of peptide-based hydrogels with potential for applications in biotechnology.