Soft Lithography, a Tool to Address Single-Objects Investigations

Nowadays, research is following a new trend where nanotechnology meets biology and vice-versa. In fact, nanotechnology offers remarkable tools to improve our understanding of biology’s mechanisms but at the same time, biology shields precious clues that could inspire nanotechnology to answer to today’s and tomorrow’s issues and needs. The study of cells and molecules at the single-object level has become essential in this complementary exchange and multidisciplinary effort. But, to track single proteins at work, watch nanoscale biomachines or control cell adhesion and differentiation, one of the main challenges is still the integration of these bioentities onto desired locations on a substrate. In other terms, one has to be able to single-out the object of interest from a given media, organize it and fix it on a surface while preserving its functionality. This integration of objects further implies being efficient and having a high-throughput capacity in order to conciliate single event recording with statistical analysis. Thus, efforts on creating defectfree and well-ordered assemblies are crucial for the study of single objects. To define the number, position, and distances of the objects on a surface, it is often necessary to direct their assembly by means of suitable prepatterned substrates or stamps. Nanopatterned surfaces have been fabricated by nanoimprinting (Wang et al., 1999), contact printing (Kumar & Whitesides, 1993; Xu et al., 2007), microtransfer molding (Zhao et al., 1996), pattern replication induced by an electric field (Morariu et al., 2003), lithographically induced self-construction (Chou et al., 1999), grid-assisted self-organization (Cavallini et al, 2002), and self-assembly (Chou & Zhuang, 1999) among others. Most of these techniques rely on a soft mould or stamp (Quake & Sherer, 2000). Soft-lithography emerged in the early 90’s as a technique based on the use of an elastomer material named polydimethylsiloxane (PDMS). This polymer can be poured onto a siliconbased master with microor nanostructures made by photolithography or electron beam lithography respectively, to obtain a PDMS replication of the master’s structures. The corresponding PDMS stamp can then be inked with a molecule and put in contact with a target surface to transfer the molecules into a patterned layer. This is what we call microcontact printing (Kumar & Whitesides, 1993; Xia & Whitesides, 1998). The major advantages of microcontact printing are that the contact with the surface is made in a very gentle manner, so the bioentities are transferred without damaging them. The flexibility of the