Microrheology and Microstructure of Fmoc-Derivative Hydrogels

Anders Aufderhorst-Roberts,William J Frith,Athene M Donald,Mark Kirkland

doi:10.1021/la5005819

Abstract

The viscoelasticity of hydrogel networks formed from the low-molecular-weight hydrogelator Fmoc-tyrosine (Fmoc-Y) is probed using particle-tracking microrheology. Gelation is initiated by adding glucono-δ-lactone (GdL), which gradually lowers the pH with time, allowing the dynamic properties of gelation to be examined. Consecutive plots of probe particle mean square displacement (MSD) versus lag time τ are shown to be superimposable, demonstrating the formation of a self-similar hydrogel network through a percolation transition. The analysis of this superposition yields a gel time t(gel) = 43.4 ± 0.05 min and a critical relaxation exponent n(c) = 0.782 ± 0.007, which is close to the predicted value of 3/4 for semiflexible polymer networks. The generalized Stokes-Einstein relation is applied to the master curves to find the viscoelastic moduli of the critical gel over a wide frequency range, showing that the critical gel is structurally and rheologically fragile. The scaling of G'/G″ as ω(0.795±0.099) ≈ ω(3/4) at high frequencies provides further evidence for semiflexible behavior. Cryogenic scanning electron micrographs depict a loosely connected network close to the gel point with a fibrillar persistence length that is longer than the network mesh size, further indications of semiflexible behavior. The system reported here is one of a number of synthetic systems shown to exhibit semiflexible behavior and indicates the opportunity for further rheological study of other Fmoc derivatives.

Highlights

The formation of hydrogels from low-molecular-weight components through physical cross-linking is an attractive strategy for the practical design of materials for applications such as wound treatment,[1] drug delivery,[2] and tissue engineering.[3]
Self-similar behavior is observed for hydrogels formed from the low-molecular-weight hydrogelators (LMWH) Fmoc-Y
Time-cure superposition is successfully applied to the mean-squared displacement (MSD) curves using the method of Larsen and Furst.[22]

Summary

Introduction

The formation of hydrogels from low-molecular-weight components through physical cross-linking is an attractive strategy for the practical design of materials for applications such as wound treatment,[1] drug delivery,[2] and tissue engineering.[3]. Time-cure superposition is used to construct an MSD master curve, allowing the precise gel point tgel and the critical exponent nc to be determined.

Results

Conclusion