A peptide-based hydrogelation strategy has been developed that allows homogenous encapsulation and subsequent delivery of C3H10t1/2 mesenchymal stem cells. Structure-based peptide design afforded MAX8, a 20-residue peptide that folds and selfassembles in response to DMEM resulting in mechanically rigid hydrogels. The folding and self-assembly kinetics of MAX8 have been tuned so that when hydrogelation is triggered in the presence of cells, the cells become homogeneously impregnated within the gel. A unique characteristic of these gel-cell constructs is that when an appropriate shear stress is applied, the hydrogel will shear-thin resulting in a low-viscosity gel. However, after the application of shear has stopped, the gel quickly resets and recovers its initial mechanical rigidity in a near quantitative fashion. This property allows gel/cell constructs to be delivered via syringe with precision to target sites. Homogenous cellular distribution and cell viability are unaffected by the shear thinning process and gel/cell constructs stay fixed at the point of introduction, suggesting that these gels may be useful for the delivery of cells to target biological sites in tissue regeneration efforts.hydrogel ͉ self-assembly ͉ stem cell

A design strategy to control the thermally triggered folding, self-assembly, and subsequent hydrogelation of amphiphilic beta-hairpin peptides in a pH-dependent manner is presented. Point substitutions of the lysine residues of the self-assembling peptide MAX1 were made to alter the net charge of the peptide. In turn, the electrostatic nature of the peptide directly influences the solution pH at which thermally triggered hydrogelation is permitted. CD spectroscopy and oscillatory rheology show that peptides of lower net positive charge are capable of folding and assembling into hydrogel material at lower values of pH at a given temperature. The pH sensitive folding and assembling behavior is not only dependent on the net peptide charge, but also on the exact position of substitution within the peptide sequence. TEM shows that these peptides self-assemble into hydrogels that are composed of well-defined fibrils with nonlaminated morphologies. TEM also indicates that fibril morphology is not influenced by making these sequence changes on the hydrophilic face of the hairpins. Rheology shows that the ultimate mechanical rigidity of these peptide hydrogels is dependent on the rate of folding and self-assembly. Peptides that fold and assemble faster afford more rigid gels. Ultimately, this design strategy yielded a peptide MAX1(K15E) that is capable of undergoing thermally triggered hydrogelation at physiological buffer conditions (pH 7.4, 150 NaCl, 37 degrees C).

We have previously shown that properly designed lysine and valine-rich peptides undergo a random coil to beta-hairpin transition followed by intermolecular self-assembly into a fibrillar hydrogel network only after the peptide solutions are heated above the intramolecular folding transition temperature. Here we report that these hydrogels also undergo a stiffening transition as they are cooled below a critical temperature only when boric acid is used to buffer the peptide solution. This stiffening transition is characterized by rheology, dynamic light scattering, and small angle neutron scattering. Rheological measurements show that the stiffening transition causes an increase in the hydrogel storage modulus (G') by as much as 1 order of magnitude and is completely reversible on subsequently raising the temperature. Although this reversible transition exhibits rheological properties that are similar to polyol/borax solutions, the underlying mechanism does not involve hydroxyl-borate complexation. The stiffening transition is mainly caused by the interactions between lysine and boric acid/borate anion and is not driven by the changes in the secondary structure of the beta-hairpin peptide. Addition of glucose to boric acid and peptide solution disrupts the stiffening transition due to competitive glucose-borate complexation.