Tirrell Group: Tissue Engineering

Members of the Tirrell lab at both the University of California, Berkeley and the University of Chicago have developed injectable synthetic extracellular matrices for regenerative medicine, specifically for peripheral nerve regeneration applications. Using the Tirrell lab’s platform material, peptide amphiphiles, short peptides attached to fatty acid tails, a new class of materials was designed to self-assemble into long worm-like micelles when specific environmental triggers were applied. These worm-like micelles then entangle at high concentration to form a fibrous hydrogel which can be tuned to reflect the stiffness of the native tissue of interest.

One design by the Tirrell Lab sought to use physiological pH as the gelation trigger. To achieve this, peptide amphiphiles were designed in a branched architecture with histidine and serine amino acid side arms. Below pH 6, histidines are predominately acidic and form weak hydrogen bonds with serines as proton donors. Above pH 6.5, histidines are primarily basic and form strong hydrogen bonds as proton acceptors with the serine amino acids. As a result, the peptide amphiphiles form weak fibers at low pH and above pH 6.5, strong fibers form. Weak fibers in solution resemble a low viscosity liquid and strong fibers form self-supporting hydrogels.

Evidence of this switch from weak to strong fiber can be seen in the storage modulus of the solutions. At low pH solutions behaved as viscous liquids with low moduli. At high pH, the same sample had moduli several orders of magnitude larger, indicating the formation of stiff entangled fibers. This moduli also scales with concentration, allowing us to tune the stiffness of the gel. At 1 wt% in water, the peptide amphiphiles formed hydrogels that achieved a storage modulus (G’) of approximately 10 kPa. Hydrogels formed by C16-GSH resembled a fibrous and entangled network similar in structure to many naturally occurring extracellular matrix proteins. It has since been confirmed that these gels perform very well as a synthetic extracellular matrix, allowing for improved cell spreading, proliferation, and migration in comparison to other common tissue culture standards, like collagen gel and tissue culture plastic.

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