Mechanical Properties of Poly-L-Lysine Hydrogels Across the Helix-Coil Transition*

Poly-L-lysine (PLL) is a biological polyelectrolyte that undergoes structural changes in

response to environmental shifts such as pH. Here, we take advantage of PLL’s structural changes

across the helix-coil transition as a function of pH to develop new functional materials. PLL is

crosslinked with poly (ethylene glycol) diglycidyl ether (EDGE) at crosslink densities, ranging from 3-

6%, and the bulk mechanics of the gel are studied across the coil-helix transition. Dynamic and Dynamic

Small Angle Light Scattering are used to determine the diffusion coefficients of the gel and rheometry is

used to determine shear modulus and strain behavior of the gel as a function of crosslink density, salt,

and pH. Circular Dichroism is used to characterize the helical structure of the PLL in gel. Analyzing the

results of these methods, meshsize is calculated as a function of modulus and volume fraction and

compared to changes in crosslinker % and pH of solution. We find that shear modulus increases as a

function of % crosslinker and mesh size decreases as a function of pH and % crosslinker. By

understanding the hydrogels’ structure and response to environmental changes, we can expand their

potential as functional materials in biomedical applications or soft actuators.