PATTERNING BELOW THE LENGTH SCALE OF HETEROGENEITY: NANOMETER-SCALE CHEMICAL PATTERNING OF ELASTOMERIC SURFACES

There is a plethora of applications that require chemical patterning on the molecular scale. While the surface science community has made tremendous progress in achieving this level of control on hard, crystalline interfaces, significant challenges are associated with extending this progress to less “perfect” systems such as soft, amorphous interfaces. Applications ranging from soft robotics and wearable electronics to regenerative medicine often utilize polymeric materials such as polydimethylsiloxane (PDMS) and hydrogels. These materials have advantageous properties, including biocompatibility and mechanical tunability. Biological applications, for example, often require the display of functional groups with precise spatial resolution. Cellular behavior is dictated by biochemical and biophysical cues in the extracellular matrix; therefore, substrate properties, including stiffness and ligand density, must be independently tunable. Soft, polymeric materials are highly heterogenous with pore sizes ranging from 10 nm to 1 µm and hence, particularly difficult to pattern below the length scale of substrate heterogeneity. Furthermore, deconvolving mechanical properties such as elastic modulus from the density of surface-active functional groups is especially challenging, with softer materials typically corresponding is lower ligand densities. Additionally, many traditional surface science characterization and patterning methods are incompatible with soft interfaces (e.g. amorphous surface structure, low mechanical strength, hydrated environment). Recently, we have reported a method capable of achieving high-resolution chemical patterning of PDMS and hydrogels. Long studied within the scanning probe community, amphiphiles with long alkyl chains self-assemble into lying down stripe phases on highly ordered pyrolytic graphite (HOPG), generating 1-nm-wide stripes of functional headgroups between 5-nmwide stripes of exposed alkyl chains. Stripe phases of functional diacetylenes (DA) are photopolymerized, producing a polydiacetylene backbone that tethers together adjacent molecules, generating a PDA film on HOPG (sPDA). We have shown that PDA films on HOPG can be transferred to PDMS as well as polyacrylamide hydrogels. When PDMS is cured in contact with sPDAs, the PDA backbones can act as a site for hydrosilylation, the same reaction responsible for PDMS curing, covalently linking sPDAs to the PDMS mesh. Careful exfoliation reveals nm-scale functional patterns on the surface layer of PDMS. 10 Here, we examine the impact of PDMS structural components on the efficiency of interfacial reactions between sPDAs and the PDMS network. We also illustrate the impact of PDAfunctionalized PDMS on the adhesion and spreading behavior of C2C12 murine myoblasts.