Enzyme responsive surfaces: towards a smart cell-material interface

Enzymes are promising stimuli for the development of responsive biomaterials for biomedical applications. Enzymes are inherently present in the biological environment thus cleverly designed materials for biomedical applications may require no external stimuli to ellicit the required material response. They have been targeted as stimuli in self assembly of bulk materials owing to the material changes in chemical composition afforded by the enzyme interaction. The first examples of autonomous self-regulated drug delivery systems have been reported via the development of reversible enzyme responsive materials that undergo a material change regulated by the enzymes in their environment. Although enzyme responsive surfaces have been reported there are no examples of reversible enzyme response surfaces. The surface is the first point of contact between the biological environment and a biomedical device/implant. Improving this interaction will improve the integration of these biomaterials in biological systems and it has been proposed that biomimetic surfaces are a promising method for full biomaterial integration in the biological environment. The body strives towards homeostasis and this is frequently achieved by enzymatic activating and deactivation of proteins in the body. This process is repeatable and reversible. Herein we address the absence of reversible and repeatable synthetic enzyme responsive surfaces towards the improvement of biomaterial integration. We aim to develop a truly autonomous system wherein enzymes present in the environment can interact with the modified surface to mediate a reversible material response. This goal was achieved by modifying surfaces with copolymers that contain the recognition sequence for Casein kinase II and Alkaline phosphatase to undergo enzymatic phosphorylation and dephosphorylation. Co and homo polymers of serine and glutamic acid were synthesised in solution and conformation/composition relationship was determined by analysis with NMR, GPC and FTIR. Polymerisation from the surface with NCA-Glu and NCA-Ser was achieved as characterised by FTIR, ToF SIMS, XPS and WCA. Enzymatic mediated phosphorylation (CKII) and dephosphorylation (AP) was monitored by surface analysis (ToF SIMS), by monitoring ATP to ADP conversion and phosphate cleavage from the surface using luminescence and colorimetric assays. Conformational changes mediated by enzymatic interactions with the surface was monitored indirectly using a FRET system incorporated in the surface modification. The modified surfaces were able to support cell culture and osteogenesis. This project has made advances in several fields, 1) The use of NCA-ROP as a method to modify surfaces with copolymers, in particular for a random/ alternating amino acid sequence. 2) The use of NCA-ROP as a method to develop stimuli responsive surfaces, specifically, this is the first report of an enzyme responsive surface prepared from NCA-amino acid derivatives. 3) The use of enzymes as stimuli, specifically, this is the first report of a reversible enzymatic responsive surface. In this system reversible phosphorlyation and dephosphorylation was monitored via changes in fluorescence output indicative of induced conformational changes