Many biological processes, such as stem cell differentiation, wound healing and development, involve dynamic interactions between cells and their microenvironment. The ability to control these dynamic processes in vitro would be potentially useful to fabricate tissue engineering constructs, study biological processes, and direct stem cell differentiation. In this paper, we used a parylene-C microstencil to develop two methods of creating patterned co-cultures using either static or dynamic conditions. In the static case, embryonic stem (ES) cells were co-cultured with fibroblasts or hepatocytes by using the reversible sealing of the stencil on the substrate. In the dynamic case, ES cells were co-cultured with NIH-3T3 fibroblasts and AML12 hepatocytes sequentially by engineering the surface properties of the stencil. In this approach, the top surface of the parylene-C stencil was initially treated with hyaluronic acid (HA) to reduce non-specific cell adhesion. The stencil was then sealed on a substrate and seeded with ES cells which adhered to the underlying substrate through the holes in the membrane. To switch the surface properties of the parylene-C stencils to cell adhesive, collagen was deposited on the parylene-C surfaces. Subsequently, a second cell type was seeded on the parylene-C stencils to form a patterned co-culture. This group of cells was removed by peeling off the parylene-C stencils, which enabled the patterning of a third cell type. Although the static patterned co-culture approach has been demonstrated previously with a variety of methods, layer-by-layer modification of microfabricated parylene-C stencils enables dynamic patterning of multiple cell types in sequence. Thus, this method is a promising approach to engineering the complexity of cell-cell interactions in tissue culture in a spatially and temporally regulated manner.

The patterned deposition of cells and biomolecules on surfaces is a potentially useful tool for in vitro diagnostics, high-throughput screening, and tissue engineering. Here, we describe an inexpensive and potentially widely applicable micropatterning technique that uses reversible sealing of microfabricated parylene-C stencils on surfaces to enable surface patterning. Using these stencils it is possible to generate micropatterns and copatterns of proteins and cells, including NIH-3T3 fibroblasts, hepatocytes and embryonic stem cells. After patterning, the stencils can be removed from the surface, plasma treated to remove adsorbed proteins, and reused. A variety of hydrophobic surfaces including PDMS, polystyrene and acrylated glass were patterned using this approach. Furthermore, we demonstrated the reusability and mechanical integrity of the parylene membrane for at least 10 consecutive patterning processes. These parylene-C stencils are potentially scalable commercially and easily accessible for many biological and biomedical applications.

development of next generation of compact and flexible electronics. [1] The increase in computer usage and ever-growing dependence on cloud systems require better methods for dissipating heat away from electronic components. The important ingredients of thermal management are the thermal interface materials (TIMs). Various TIMs interface two uneven solid surfaces where air would be a poor conductor of heat, and aid in heat transfer from one medium into another. Two important classes of TIMs include curing and noncuring composites. Both of them consist of a base, i.e., matrix materials, and thermally conducting fillers. Commonly, the studies of new fillers for the use in TIMs start with the curing epoxybased composites owing to the relative ease of preparation and possibility of comparison with a wide range of other epoxy composites. Recent work on TIMs with carbon fillers have focused on curing composites, which dry to solid. [2][3][4][5][6][7] Curing TIMs are required for many applications, e.g. attachment of microwave devices, but do not cover all thermal management needs. Thermal management of computers requires specifically noncuring TIMs, which are commonly referred to as thermal pastes or thermal greases. They are soft pliable materials, which unlike cured epoxy-based composites, or phase change materials, remain soft once applied. This aids in avoiding crack formations in the bond line due to repeated thermal cycling of two connected materials with different temperature expansion coefficients. Noncuring TIMs also allow for easy reapplication, known as a TIM's reworkability property. Noncuring TIMs are typically cost efficient-an essential requirement for commercial applications. Various applications in electronics, noncuring grease-like (soft) TIMs are preferred. Examples of the applications include but are not limited to cooling of servers in large data centers [8] and personal devices which are the primary targets for these applications. Current commercially available TIMs perform in thermal conductivity range of 0.5-5 Wm −1 K −1 with combination of several fillers at high loading fractions. [9] State-of-the-art and next-generation electronic devices require thermal pastes with bulk thermal conductivity in the range of 20-25 Wm −1 K −1 . [10,11] This study focuses specifically on noncuring TIMs with graphene and few-layer graphene (FLG) fillers.Curing and noncuring TIMs consists of two main components-a polymer or oil material as its base and fillers, Development of next-generation thermal interface materials (TIMs) with high thermal conductivity is important for thermal management and packaging of electronic devices. The synthesis and thermal conductivity measurements of noncuring thermal paste, i.e., grease, based on mineral oil with a mixture of graphene and few-layer graphene flakes as the fillers, is reported. The graphene thermal paste exhibits a distinctive thermal percolation threshold with the thermal conductivity revealing a sublinear dependence on the filler loading. This behavior contrasts wi...