The fabrication of high aspect ratio (5 and above) microstructures based upon UV embossing of mixtures containing poly(ethylene glycol) diacrylate (PEGDA) is described. UV embossing is a quick and convenient replication technique using low pressure and room temperature. The biocompatibility and cell- and protein-resistance of PEGDA make deep three-dimensional (3-D) micropatterned PEGDA films potentially useful for many biological applications such as protein delivery, tissue engineering, drug delivery, and biosensors. Microarrays of deep microchannels and microcups separated by PEGDA walls with aspect ratios of 7 and 5, respectively, were successfully embossed. UV embossing was found to faithfully replicate the lateral periodicity and height of the mold. We also successfully UV embossed a mixture having equal weight proportions of hydrophilic PEGDA and hydrophobic poly(propylene glycol) diacrylate and demonstrated the use of this microarray of microcups for encapsulation of a model protein (bovine serum albumin) within a UV cured PEGDA hydrogel; a protein encapsulated within a hydrogel 3-D microarray was fabricated. Although high aspect ratio UV embossing has many attractive features, it is a difficult process to implement, requiring precise control and optimization of mold, process, and material parameters. Successful high aspect ratio UV embossing was achieved using two molds: a rigid nickel mold and a silicone rubber mold. The latter did not require any surface treatment, but the nickel mold was found to require coating with a cured silicone resin to obtain a suitable nonstick surface. The UV exposure time was controlled to optimize the hardened resin strength while avoiding excessive brittleness. Peel-off of the hardened microstructures was performed at a small angle to avoid breakage of the molded structures. A mold release additive was necessary for successful demolding. Trimethylolpropane triacrylate, a high shrinkage monomer, also facilitated demolding.
In ultraviolet (UV) embossing, a substrate with a coating of liquid or semi-solid UV curable resin mix is pressed against a patterned embossing mold. The resin mix is irradiated with UV before demolding of the hardened microstructures. UV embossing can be done at room temperature and low pressure. However, demolding of UV embossed high aspect ratio microstructures from a metallic mold is typically difficult since there is no differential thermal contraction between the mold and the embossing. Several factors have been identified to influence demolding of UV embossed microstructures: (1) Roughness of mold, (2) Taper angle of microstructures of mold, (3) Chemical interaction between mold and embossing, (4) Tensile and crosslinking shrinkage properties of the irradiated resin and (5) Uniformity of crosslinking process through the thickness of the molded microstructures. By controlling these five parameters, a microarray with an aspect ratio of 5 was demonstrated using a Formulation containing epoxy acrylate, IrgacureÒ 651, silicone acrylate and other acrylates. The embossed microstructures replicated the features of the mold very well. It was also shown that by controlling the amount of irradiation, the tensile modulus of the UV formulation increased whilst the elongation decreased. An optimum irradiation is needed for clean demolding from the mold without microcracking. IntroductionPolymeric micro-electromechanical systems (MEMS) are important as low-cost alternatives to silicon-based MEMS technologies for a range of present and future commercially viable products. These include applications in Life sciences such as DNA microarrays or disposable devices on biocompatible substrate [1], micro-optics like diffractive optical elements or waveguide elements [2, 3] and display technology like optical backplane [4]. Making polymeric MEMS typically involves using a microstructured mold to shape the polymeric material with features in the micrometer range. In this paper, ultraviolet (UV) embossing is introduced as a technique for making polymeric MEMS components with high aspect ratio. The details of the process and the factors to consider for successful embossing are discussed.Microstructures with high aspect ratio are needed in many applications since they offer a higher active area per unit substrate surface area, the possibility of higher packing density of microstructural elements and higher throughput in continuous flow systems due to higher cross-sections per unit substrate area [1]. Larger surface area and multilevel microstructures are particularly important for chemical or biochemical applications like microreactors, micromixers, chromatographic columns or DNA concentrators to allow long microchannel length for good analysis performance and multitasking within a small area. Similar considerations are important in micro-optics, for example in waveguide applications, which typically involve comparatively large devices with dimensions of several centimeters. Higher packing densities and planar multifunctionality are ...
Films (ca. 150 microm thick) of twelve acrylate mixtures, which contained various proportions of hydrocarbon acrylates [mainly oligo(ethylene glycol) diacrylate, (OEGDA)] and small amounts of a silicone hexaacrylate (in proportion of 5% or less), were cured on a nickel substrate, and X-ray photoelectron spectroscopy analysis of the nickel-side surface compositions showed that for formulations with and without the silicone hexaacrylate, this surface was enriched with OEGDA and saturated (up to 50%) with the silicone hexaacrylate, respectively. The silicone hexaacrylate phase-separated and formed micelles which migrated to the resin-nickel interface. Silicone hexaacrylate, inherently less reactive, also significantly slowed the photopolymerization of the mixtures. The sequential homopolymerization of OEGDA and silicone hexaacrylate in a formulation was elicited using real-time Fourier transform infrared spectroscopy. The design-of-experiment approach was used to quantify the influence of the components on gelation time and the nickel-side surface composition as well as provide the statistical models to predict these two properties for new compositions.
Summary: A novel experimental set‐up has been devised to measure simultaneously, in real time, the conversion and shrinkage of multi‐acrylates during photopolymerization. The data show that the current practice of assigning the excess volume entirely as excess free volume is inappropriate as this leads to an increasing fractional free volume with conversion. We propose to partition the excess volume into free and occupied volume components. The new model produces satisfactory results.Experimental set‐up for the simultaneous collection of shrinkage and conversion data.magnified imageExperimental set‐up for the simultaneous collection of shrinkage and conversion data.
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