(31c) Dynamic Surfaces to Study and Manipulate Cell Function

Many studies have shown the importance of a substrate's chemical functionality and physical properties on cellular functions such as adhesion, proliferation, migration, and differentiation. An underlying theme to this research is to better understand how cells receive information from their external microenvironment, and oftentimes, sophisticated control of the cell-material interface is required to answer the complex questions about this dynamic relationship. As a result, materials and interfaces with tunable properties are of growing in interest as cell culture platforms. A new direction in the field is to design biomaterial systems for user-directed manipulation of the surface properties to control cell-material interactions or direct the release of soluble factors. This talk will illustrate how tunable material systems allow the experimenter to externally trigger a property change and subsequently follow the resulting biological response.

Specifically, we present an approach to varying the presentation of physical and chemical cues to cells cultured on hydrogel surfaces in a manner that can be controlled both spatially and temporally, as well as in a continuum manner, via the use of light. Specifically, two photodegradable macromolecular monomers were synthesized by incorporating a nitrobenzyl ether derived moiety, which was selected for its photolytic efficiency and previous use in live cell culture and imaging. A multifunctional photodegradable monomer was synthesized by acrylation of the photolabile moiety via a pendant hydroxyl group. This was subsequently attached via a pendant carboxylic acid to poly(ethylene glycol)-bis-amine to create a photocleavable crosslinking macromer from which degradable gels were synthesized. These gels were subsequently degraded using a light exposure gradient and confining the light to the top 50 microns to create a material with a continuously varying surface stiffness. The stiffness of these materials with light exposure was characterized via photorheology and AFM. The initial gel stiffness was 13.6±1.0 kPa, and upon exposure to 10 mW/cm2 of 365 nm light, G' decreased to 79% of its original value with 2.5 mins of exposure, 26% with 5 mins, and 3% with 10 mins. Valvular interstitial cells (VICs), which are known to respond to substrate stiffness, were seeded on these gels and their response to the stiffness gradient was evaluated by examining cell morphology, proliferation, migration and alpha smooth muscle actin (aSMA) production. VIC activation (i.e., aSMA production) decreased in response to decreasing substrate stiffness, and VIC activation on stiff substrates was found to be reversable by photoniduced softening of the extracellular matrix. These properties are important towards understanding and manipulating disease progression in heart valves. To control the chemical functionality of the gel, we developed a strategy for coupling the photodegradable monomer through its pendant carboxylic acid to biomolecules containing a pendant primary amine or hydroxyl group. This approach yields an asymmetric biofunctional acrylic monomer that is tethered to the gel and is released upon light exposure. To demonstrate its utility in regulating the chemical functionality of surfaces, we fabricated gels with the RGD functionality, which is an epitope found in fibronectin, an important protein for cell adhesion. Human mesenchymal stem cells (hMSCs) were cultured on these gels, and the surface chemistry was temporally tuned to direct hMSC chondrogenic differentiation by externally-triggering the photorelease of the RGD functionality.