Date of Award
Master of Science (MS)
Roberts , Mark
Kitchens , Christopher
Bioresponsive hydrogels are emerging with technological significance in targeted drug delivery, biosensors and regenerative medicine. Conferred with the ability to respond to specific biologically derived stimuli, the design challenge is in effectively linking the conferred biospecificity with an engineered response tailored to the needs of a particular application. Moreover, the fundamental phenomena governing the response must support an appropriate dynamic range and limit of detection. The design of these systems is inherently complicated due to the high interdependency of the governing phenomena that guide the sensing, transduction, and the actuation response of hydrogels. To investigate the dynamics of these materials, model systems may be used which seek to interrogate the system dynamics by uni-variable experimentation and limit confounding phenomena such as: polymer-solute interactions, polymer swelling dynamics and biomolecular reaction-diffusion concerns.
To this end, a model system, α-chymotrypsin (Cht) (a protease) and a cleavable peptide-chromogen (pro-drug) covalently incorporated into a hydrogel, was investigated to understand the mechanisms of covalent loading and release by enzymatic cleavage in bio-responsive delivery systems. Using EDC and Sulfo-NHS, terminal carboxyl groups of N-succinyl-Ala-Ala-Pro-Phe p-nitroanilide, a cleavable chromogen, were conjugated to primary amines of a hydrated poly(HEMA)-based hydrogel. Hydrogel discs were incubated in buffered Cht causing enzyme-mediated cleavage of the peptide and concomitant release of the chromophore for monitoring. To investigate substrate loading and the effects of hydrogel morphology on the system, the concentration of the amino groups (5, 10, 20, and 30 mol%) and the cross-linked density (1, 5, 7, 9 and 12 mol%) were independently varied. Loading-Release Efficiency of the chromogen was shown to exhibit a positive relation to increasing amino groups (AEMA). The release rates demonstrated a negative relation to increasing cross-linked density attributed to decreasing void fractions and increasing tortuosities. The diffusion coefficient of Cht, D0,Cht, was determined to be 6.9 ± 0.5 _ 10-7 cm2 s-1, and the range of Deff of Cht for 1 to 12 mol% TEGDA was determined to 6.9 _10-8 to 0.1 _ 10-8cm2 s-1. We show how these parameters may be optimized and used to achieve programmed release rates in engineered bio-responsive systems.
The field of bioresponsive hydrogels is continuing to expand as the need for such materials persists. Future work will enable more control over the loading and release of therapeutic and diagnostic moieties. Continued research regarding in enzymatically actuated hydrogels will involve pre-polymerization loading methodologies; in silico diffusion-reaction multiphysics modeling; enzyme actuated degradation of the polymer; and substation of various mediating enzyme, cleavable peptides, and release molecules.
Wilson, Andrew, "Enzyme Actuated Bioresponsive Hydrogels" (2012). All Theses. 1431.