Paper Number
AR18
Session
Applied Rheology and Rheology Methods
Title
Using hydrogel autofluorescence to determine elastic modulus in spatially nonuniform hydrogels
Presentation Date and Time
October 12, 2021 (Tuesday) 11:05
Track / Room
Track 2 / Ballroom 7
Authors
- McGlynn, John A. (Lehigh University, Department of Chemical and Biomolecular Engineering)
- Schultz, Kelly M. (Lehigh University, Chemical and Biomolecular Engineering)
Author and Affiliation Lines
John A. McGlynn and Kelly M. Schultz
Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, PA 18015
Speaker / Presenter
McGlynn, John A.
Keywords
experimental methods; applied rheology; biological materials; gels; rheology methods
Text of Abstract
Hydrogels are designed as implantable scaffolds to promote tissue regeneration by delivering additional human mesenchymal stem cells (hMSCs) to a wound because hMSCs are involved in all stages of regeneration and improve healing outcomes. Many areas of the body have spatial variations in elastic modulus, G’, such as where tendon meets bone. To use hMSC-laden hydrogels in wound repair, spatial variation in implant G’ must match the tissue. Unfortunately, traditional measurement techniques are unsuitable for characterizing nonuniform materials. For example, bulk rheology averages over the sample and cannot measure the spatial properties of these materials. Microrheological techniques measure low moduli regimes limiting their ability to characterize tissue mimics. Finally, atomic force microscopy is affected by tip geometry, probe stiffness and often relies on assumptions of infinitesimal strain theory, which can be violated in biological materials. We present a new method for measuring spatial variation in G’ by relating G’ to gel autofluorescence. We demonstrate this techniques efficacy using a well-studied hydrogel for hMSC delivery. Our gels consist of 4-arm poly(ethylene glycol)-norbornene which cross-links with a peptide upon exposure to UV light. Uniform gels with different G’ are made by controlling extent of reaction by changing UV exposure time. G’ is measured with a frequency sweep on a bulk rheometer. Gels from the same solution are then made and autofluorescence is measured as a function of extent of reaction. We measure a linear relationship between G’ and brightness, enabling measurements of brightness to be used to accurately determine G’. We then prepare hydrogels with a step-change in G’ by varying UV light exposure spatially using a photomask and measure the resulting spatial variation in brightness and calculate G’. This work introduces a technique for measuring spatial variations in G’ in nonuniform materials, providing a new way to characterize hydrogels designed for use in wound healing.