Ordered uniform features in micro-/or nano-scale have highly potential applications in biomedical and pharmaceutical academic research and industry. In the past decade, micro-/nano-fabrication technologies have been widely developed and may readily produce such desirable features. However, most of these techniques are limited to inorganic materials (silicon or glass) and low-aspect-ratio features. Biomedical applications typically require high-aspect ratio features based on more biocompatible polymer materials (i.e., PLA, PLGA, and et al.). When transferring these desirable features to polymer, it often raises de-molding challenges because of the large contact area resulting from high density, high aspect ratios features and fragile feature size. Mold release agents may be helpful to assist de-molding for low aspect ratios nano-features (typically less than one). Its effectivity is quite questionable when applying to high-aspect-ratio features. Besides, mold release agents introduce potential contamination for biological applications. Our laboratory has recently developed a process, called Sacrificial Template Imprinting (STI), to eliminate those potentials issues when producing such desirable features. The sacrificial templates can be massively produced quickly and easily. It ensures this process to economically mass produce desirable micro-/nano- features. Figure 1 is the schematic of the procedure. In this process, no clean room facility is necessary. Here we show examples to produce highly ordered monodisperse nanopores and nanoneedles.

The mother master is a conical shape array of probes (Figure2a) . It is fabricated on the distal faces of coherent fiber-optic bundles by differential wet etching. Those tips are around 5 m m high and the diameter of the sharp end is around 50-100nm (Thus the aspect ratio is at least 50; it is ultrahigh aspect ratio features). The aspect ratio and angle of the probe can be further tuned by varying etching conditions. The nanotip array can be used directly as the mold. But it may not last in mass-production and probably face the de-molding issues mentioned above. Therefore, a polymer sacrificial template (Figure 1c), instead of t he optical fiber master, is used to produce those nano-features with a two-step replication. A PDMS female mold is firstly used as the transition mold to replicate the nanotip array by replica molding. Then polymer solutions are cast on PDMS mold to generate sacrificial templates, which have the same pattern with the optical fiber master. This two-step replication can easily mass-produce the sacrificial templates with high replica quality ( Figure 2a and 2c). The replication accuracy depends on the wetting ability of casting fluid to the mold surface and dimension changes during solution drying. Optimizing those factors will highly improve the replica quality.

To make monodisperse membrane with nanopores, a polymer resin (or solutions) is then spun on the sacrificial template. Through adjusting the spin speed and the spin time, a thin polymer layer is obtained with the thickness slightly less than the height of the probes. This ensures that the polymer film has open pores. After curing the resin (or drying the solution), an anisoporous membrane is formed.. The membrane is finally released by dissolving the sacrificial template in hot water. Figure 2d shows the SEM image of this anisoporous membrane from the side with smaller pore diameter. The diameter of the pores on the small pore end is around 200 nm. The pore size depends on the top angle of the probes, polymer layer thickness and the replication accuracy Small variations in pore size result from the size and quality difference of the original optic fibers (shown in Figure 2a). Other factors such as replication accuracy and surface roughness may also contribute to the pore size variance.

The production of ordered nanoneedle array has similar process except using short spinning time and extra treatments. Liquid film, instead of solid film, stays around nanotips after spinning. As the result of surface force drawing and dynamic evaporation, the solution can crawl a little bit along the surface of those nanotips before drying. This helps to form nanoneedles this time. The nanoneedles generated in this way currently have a height around half a micron.

Figure 1. Schematic of polymer membrane with highly ordered anisotropic pores by differential etching of optical fibers and sacrificial template imprinting: (A) Differential wet etching coherent fiber-optic bundles to generate nanotip array; (B) Replicate PDMS female daughter mold; (C) Casting polymer sacrificial template from the PDMS female mold; (D) Spin coating a polymer film on the sacrificial template. (E) Release STI membrane by dissolving the sacrificial template in hot water. (F) Integrate STI membrane onto support material for characterization.

Figure 2. SEM Images of: (A) Nanotip array; (B) PDMS female daughter mold; (C) Polymer sacrificial template; (D) Polymer membrane with highly ordered anisotropic pores