Metallization and biopatterning on ultra-flexible substrates via dextran sacrificial layers

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.

Figure 1. Process flow for dextran-based surface micromachining of ultra-soft PDMS (50 to 70∶1).

Dextran is spun and dried on silicon pieces, before deposition of various micro- and nano-structures. PDMS is subsequently cross-linked directly above these substrates, micromachined PDMS is released in water.

Figure 2. Contact angle of various formulations of dextran-PDMS.

Introduction of dextran onto the PDMS surface increases the wetting of the substrate, depending on formulation of PDMS, and absorption of water into the embedded dextran.

Figure 3. Adsorption characteristics of various formulations of dextran-PDMS (60∶1).

Unmodified dextran-embedded PDMS possesses similar adsorption to native PDMS, while amino-modified dextrans heavily adsorb fibrinogen.

Figure 4. Cell patterning.

a) and b) Live-cell images of cells cleanly patterned on fibronectin islets following 2 days of culture for both 100 kDa and 6 kDa dextran formulations. Scale bar is 60 µm.

Figure 5. Cell self-patterning above epoxy-PDMS substrates.

a) and b) Cells self-patterning on epoxy-PDMS. Cells strongly prefer PDMS portions of the substrate due to preferential adsorption of protein onto these regions, and perhaps develop adhesions towards “stiff” regions adjacent to epoxy. Scale bar is 60 µm.

Figure 6. Embedding fluorescent particles directly in PDMS. Particle displacements generated by a single cell.

Scale bar is 20 µm.

Figure 7. Dextran-mediated transferring of microstructured metal onto PDMS composed of lithographically patterning metal, and silanization by allyltriethoxysilane.

a) Thin film metal features (lines and elements, 200 nm thick) of Titanium/Copper/Titanium efficiently transferred onto ultra soft PDMS (60∶1). b) Scotch tape test verifying adhesive capabilities of allyl- based silanes on PDMS (10∶1). Scale bar is 60 µm.