Title: Probing Cellular Response to Heterogeneous Rigidity at the Micro- and Nanoscale
Description: Physical factors in the environment of a cell regulate cell function and behavior and are involved in the formation and maintenance of tissue. There is strong evidence that substrate rigidity plays a key role in determining cell response in culture. Previous studies have demonstrated the importance of rigidity in numerous cellular processes including migration and adhesion and stem cell differentiation. Immune cells have been shown to respond differently to surfaces having different rigidities. Atypical response to rigidity is also a characteristic of cancerous cells. Understanding the mechanisms that support cellular rigidity sensing can lead to new tissue engineering strategies and potential new therapies based on rigidity modulation. A new technique was developed for the creation of biomimetic surfaces comprising regions of heterogeneous rigidity on the micro- and nanoscale. The surfaces are formed by exposing an elastomeric film of polydimethylsiloxane (PDMS) to a focused electron beam to form patterned regions of micro- and nanoscale spots.

This thesis involves the formation of theses surfaces, characterization of their physical and chemical properties as a consequence of the electron beam exposure and investigation of how cells behave when plated on these surfaces. Cellular response to different patterns of heterogeneous rigidity is performed for several cell types. Human mesenchymal stem cells plated upon electron beam-exposed PDMS in a pattern of spots with diameters ranging from 2 µm to 100 nm display differential focal adhesion co-localization to the exposed features, depending on both rigidity and feature size. This behavior persists as the area of the exposed regions is reduced below ~1 µm. On spots with diameters of ~ 250 nm and smaller, focal adhesion co-localization is lost. This supports the notion that there is a length scale for cellular rigidity sensing, with the critical length in the range of a few hundred nanometers. When the heterogeneous rigidity surfaces are applied to CD4+ T cells, accumulations of proteins including TCR and pCasL on the exposed features are observed as a function of feature size.

The pCasL appeared to significantly accumulate on 2 µm spots; For spots ~ 1 µm and below, cells appeared unable to identify the rigid regions. Further, Ca2+ release, a functional indicator of immunoresponse, is significantly enhanced on mixed-rigidity patterned PDMS relative to both soft and hard PDMS. Possible signaling pathways of TCR activation have been verified on e-beam exposed PDMS substrates with heterogeneous rigidity. These results are suggestive of possible new approaches to adoptive immunotherapy based on rigidity modulation. Studies on breast cancer cells indicate that on patterned substrates, sub-cellular processes are also significantly modulated. Integrin recruitment is enhanced on the rigid regions. Understanding the role of geometry in cellular rigidity response will point the way toward revealing its functional response and will shed light on the mechanistic underpinnings of this process.