Here, we study cracking of nanometre and sub-nanometre-thick metal lines (titanium, nickel, chromium, and gold) evaporated onto commercial polydimethylsiloxane (PDMS) substrates. Mechanical and electromechanical testing reveals potentially technologically useful effects by harnessing cracking. When the thin film metal lines are subjected to uniaxial longitudinal stretching, strain-induced cracks develop in the film. The regularity of the cracking is seen to depend on the applied longitudinal strain and film thickness—the findings suggest ordering and the possibility of creating metal mesas on flexible substrates without the necessity of lithography and etching. When the metal lines are aligned transversally to the direction of the applied strain, a Poisson effect-induced electrical ‘self-healing’ can be observed in the films. The Poisson effect causes process-induced cracks to short circuit, resulting in the lines being electrically conducting up to very high strains (~40%). Finally, cracking results in the observation of an enhanced transversal gauge factor which is ~50 times larger than the geometric gauge factor for continuous metal films—suggesting the possibility of high-sensitivity thin-film metal strain gauge flexible technology working up to high strains.

Future in vitro life science studies that require in situ functionalities-going beyond the common glass petri dish-can potentially be realized by using microsystems technologies to fabricate bio-microelectromechanical systems (BioMEMS) and laboratory-on-a-chip (LOC).One such material used to construct these microsystems is the commercial photoresist SU-8.Here, the neuronal biocompatibility of SU-8 is examined as a function of its hard-baking thermal treatment. The SU-8 surfaces were non-patterned large uniform thin films-this allowed the biocompatibility and cell adhesion of the SU-8 to be fully tested. In total, 75 SU-8 samples were fabricated for the study using spin-coating and lithography techniques. We observe that a single hard-baking step of 180°C for 2 hours coupled with a coating (poly-Dlysine + laminin) is enough to detoxify SU-8 and promote primary cortical cell adhesion and survival up to 28 days in vitro (DIV). Therefore, the protocol described here makes SU-8 surfaces compatible with the development of neuronal networks from primary neural cells.

Films of polydimethylsiloxane (PDMS) can be obtained by spin coating for a range of applications in microengineering and microsystems. However, the spin coating of PDMS films in the 100-1000 µm thickness range invariably results in the formation of non-negligible 'edge beads'. If not removed, such features could cause problems if the PDMS films were to form part of a multi-step planar process involving e.g. contact photolithography. Here, we study the reflow planarization and edge bead reduction of spin-coated PDMS films obtained in the spin-speed range (250-500 rpm). We provide an optimized technical solution for the planarization and associated edge bead removal-along with useful spin speed versus thickness data of PDMS films. Our approach takes into consideration the reflow planarization time and the polymerization time. We demonstrate the usefulness of our method by conducting photolithography on the planarized PDMS films by using a commercial spin-coated photoresist (AZ®1518)-clearly showing that the photolithographic resolution is improved for planarized PDMS films compared to non-planarized PDMS films. Finally, the findings suggest that if a planarized PDMS film having a thickness of ~1000 µm is required, a single very low spin speed coating followed by an enhanced reflow planarization is preferable to multiple higher spin speed coatings and planarization 2 steps. The findings should be useful to those working to develop stretchable technologies using planar means who wish to integrate PDMS films into more complex systems.

We show how a rigid patterned SU-8 thin film in tandem with the mechanical Poisson effect can be used beneficially to enable robust metallization on stretchable polydimethylsiloxane (PDMS).Two generic planar processes used to form a thin, mechanically-robust strain-shielding photoresist on a soft substrate are developed to demonstrate these ideas. The first process is a self-aligned metallization of photolithographically patterned SU-8 features on PDMS. The second process is a fully photolithographic lift-off metallization of pre-patterned SU-8 features on PDMS. In both cases, the SU-8 has a sub-micrometre thickness (~800 nm)-supporting a thermally evaporated Ti/Au (5 nm/50 nm) thin film metallization. The resulting samples were characterized electromechanically-the results demonstrate that the electrical continuity of metal lines (width = 150 µm and length up to 1 cm) is maintained up to ~70% strain between the lines. The electrical resistance of such lines remains relatively stable: 35 Ω at zero-strain and 63 Ω at 69% average interline strain. The electrical resistivity of the evaporated gold is near to that of pure gold, and remains so even at high strains (49 nΩ m at 10% and 70 nΩ m at 41%)this proves the absence strain-induced micro-cracking of the metal lines. Although a specific 2 evaporated metal combination is used here to demonstrate a working prototype system, in principle, any thin-film material, which can be deposited by microfabrication, e.g. insulators, semiconductors…, could be integrated into the generic processes. The basic lithographic processes expounded here are also potentially extendible to encompass the integration of other microfabrication techniques, e.g. soft lithography, contact printing…, enabling more complex flexible systems to be envisaged and realized.