Matthew S. Lamm

Controlling hydrogelation kinetics by peptide design for three-dimensional encapsulation and injectable delivery of cells

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 self-assembles 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.

Tuning the pH Responsiveness of β-Hairpin Peptide Folding, Self-Assembly, and Hydrogel Material Formation

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).

Peptide–Silica Hybrid Networks: Biomimetic Control of Network Mechanical Behavior

Self-assembly represents a robust and powerful paradigm for the bottom-up construction of nanostructures. Templated condensation of silica precursors on self-assembled nanoscale peptide fibrils with various surface functionalities can be used to mimic biosilicification. This template-defined approach toward biomineralization was utilized for the controlled fabrication of 3D hybrid nanostructures. The peptides MAX1 and MAX8 used herein form networks consisting of interconnected, self-assembled beta-sheet fibrils. We report a study on the structure--property relationship of self-assembled peptide hydrogels where mineralization of individual fibrils through sol--gel chemistry was achieved. The nanostructure and consequent mechanical characteristics of these hybrid networks can be modulated by changing the stoichiometric parameters of the sol--gel process. The physical characterization of the hybrid networks via electron microscopy and small-angle scattering is detailed and correlated with changes in the network mechanical behavior. The resultant high fidelity templating process suggests that the peptide substrate can be used to template the coating of other functional inorganic materials.