Abstract
Micro-patterning tools adopted from the semiconductor industry have mostly been optimized to pattern features onto rigid silicon and glass substrates, however, recently the need to pattern on soft substrates has been identified in simulating cellular environments or developing flexible biosensors. We present a simple method of introducing a variety of patterned materials and structures into ultra-flexible polydimethylsiloxane (PDMS) layers (elastic moduli down to 3 kPa) utilizing water-soluble dextran sacrificial thin films. Dextran films provided a stable template for photolithography, metal deposition, particle adsorption, and protein stamping. These materials and structures (including dextran itself) were then readily transferrable to an elastomer surface following PDMS (10 to 70∶1 base to crosslinker ratios) curing over the patterned dextran layer and after sacrificial etch of the dextran in water. We demonstrate that this simple and straightforward approach can controllably manipulate surface wetting and protein adsorption characteristics of PDMS, covalently link protein patterns for stable cell patterning, generate composite structures of epoxy or particles for study of cell mechanical response, and stably integrate certain metals with use of vinyl molecular adhesives. This method is compatible over the complete moduli range of PDMS, and potentially generalizable over a host of additional micro- and nano-structures and materials.
Highlights
Polydimethylsiloxane (PDMS) forms the base of a large proportion of microdevices, and has seen extensive use in microfluidics [1,2], flexible electronics [3,4,5,6,7,8], and in developing cell-material interfaces [9,10,11,12,13,14,15]
The wide range of chemical moieties that can be conjugated to the dextran used as the sacrificial layer for PDMS substrates allows tuning of wetting/surface adsorption characteristics of the elastomer beyond those of standard PDMS. We found that this approach stably integrated epoxy, proteins, metals, and particles into PDMS of variable stiffness, and we utilized composite structures generated by this method to pattern cells, measure deflections induced by cells, and study cell self-patterning in dual material systems
Strong bonding between dextran/protein and the PDMS network is encouraged by a number of interactions, including 1) a strong molding effect due to the high mobility of PDMS monomers during the initial prepolymerization stage of the interaction, and 2) covalent bonding between proteins and the PDMS through a so-called ‘‘poisoning’’ of the catalyst used in Sylgard 184 formulations of PDMS by various protein side chains, namely amino- and thiol- bearing amino acids [28,29], yielding covalent bonds between the siloxane network and the proteins
Summary
Polydimethylsiloxane (PDMS) forms the base of a large proportion of microdevices, and has seen extensive use in microfluidics [1,2], flexible electronics [3,4,5,6,7,8], and in developing cell-material interfaces [9,10,11,12,13,14,15]. A large proportion of traditional devices are fabricated using the standard formulation, a 10:1 ratio of polymer base to crosslinker that possesses an elastic modulus of approximately 2 MPa. Ultra-flexible formulations of PDMS, which can be straightforwardly generated through increasing the base to cross-linker ratio up to 70:1, can typically achieve elastic moduli down below 3 kPa [14] which could have unique advantages for flexible electronics and as cell biology substrates. PDMS at this flexibility is unique to stiffer formulations in that they deflect under significantly lower stresses than standard PDMS, and their viscoelasticity lends a minor selfhealing quality to the layers This could potentially yield a new avenue for flexible electronics, which often utilize composite structures of plastics and membranes of 10:1 PDMS. These substrates are diversely utilized for traction force microscopy [12,13,16] (measuring deflections generated by cells), stem cell differentiation [17], studying cell polarization [9], in which the goal is often to assay how stiffness of the substrata can affect cellular behavior [18]
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