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The integration of color filters with microfluidics has attracted substantial attention in recent years, for on-chip absorption, fluorescence, or Raman analysis. We describe such tunable filters based on the micro-flow of liquid crystals. The filter operation is based on the wavelength-dependent liquid crystal birefringence that can be tuned by modifying the flow velocity field in the microchannel. The latter is possible both temporally and spatially by varying the inlet pressure and the channel geometry, respectively. We explored the use of these optofluidic filters for on-chip absorption spectroscopy in poly(dimethylsiloxane) microfluidic systems; by integrating the distance-dependent color filter with a dye-filled micro-channel, the absorption spectrum of a dye could be measured. Liquid crystal microflows substantially simplify the optofluidic integration, actuation and tuning of color filters for lab-on-a-chip spectroscopic applications.

We report an optofluidic photoswitchable grating, based on a polydimethylsiloxane periodic structure on a glass substrate, separated by a thin liquid crystal film. The polydimethylsiloxane microstructure was realized via high resolution replica molding and was employed to both confine and align a photosensitive nematic liquid crystal. In the absence of any surface treatment, the liquid crystal exhibited homeotropic alignment. By inducing planar alignment on the glass substrate, a hybrid orientation of the liquid crystal was achieved, inducing polarization sensitive transmission. The photosensitivity of the liquid crystal enabled the all-optical control of the grating transmission and 20% diffraction efficiency was measured. © 2010 American Institute of Physics. ͓doi:10.1063/1.3377801͔Light responsive materials are promising candidates for the realization of optically addressed devices. In particular, dye-doped nematic liquid crystal ͑NLC͒ has attracted significant attention for its potential applications in the areas of all-optical holographic gratings 1,2 and optical phase conjugation.3 These light sensitive liquid crystals ͑LC͒ contain photochromic molecules, which can undergo reversible trans-cis isomerization. This transition can be induced with UV/visible light and reversed by heating or irradiation with light at longer wavelength. 4,5 The combination of the NLC properties with on-chip photonic structures have been widely explored, 6 however the integration of NLCs with costeffective and high-resolution nanostructures remains an open challenge. To this end, polydimethylsiloxane ͑PDMS͒ has attracted significant attention as a high-quality optical material, namely, due to its transparency, low surface energy, low dielectric constant and Young's modulus, and thermally and optically enabled polymerization. The emergence of optofluidics, where microfluidics are integrated with optics, has sparked renewed interest in PDMS for optical applications. 7,8 The birefringent properties of LCs with the enhanced optical characteristics of PDMS have been recently combined to realize tuneable high-Q microresonators and transmission gratings.9,10 In these, the NLC is dispersed in a PDMS matrix and the photonic structure was defined either via self-assembly, or holographically, placing thus a limit in the morphology and shape control. In this letter, we report the integration of NLCs with a PDMS microstructure realized by cast-molding lithography and demonstrate an optically controlled transmission grating. Such hybrid device combines the advantage of cost-effective and high resolution diffractive structures with the tuneability of LCs. Its applications may involve optical switching or lab-on-a-chip optofluidic applications. The construction strategy of the device was based on high-resolution cast molding lithography, opening thus an alternative path toward cost-effective and highly multiplexed optical functionalities for lab-on-a-chip and information processing applications. 11 We found that the low surface energy of the PDM...

We report an optical switch based on a diffraction grating by combining PDMS microstructures with a photo-responsive Nematic Liquid Crystal (NLC). The grating was realized via replica molding and was subsequently coated with a thin SiO layer. SiO induced a full planar alignment of the liquid crystal. The induced parallel alignment of the LC reduces the response time of the structure by approximately an order of magnitude compared to the same structures without SiO. We explored the effect of the pump intensity on the transmission properties and time response of the switch and identified a strong dependence on the probe polarization, due to the full planar alignment in this structure. The aforementioned inclusion of the SiO layer enables enhanced performance of optical devices based on the fusion of nematogens with soft and flexible substrates.

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Prdm1 mutant mice are one of the rare mutant strains that do not develop whisker hair follicles while still displaying a pelage. Here, we show that Prdm1 is expressed at the earliest stage of whisker development in clusters of mesenchymal cells before placode formation. Its conditional knockout in the murine soma leads to the loss of expression of Bmp2, Shh, Bmp4, Krt17, Edar, and Gli1, though leaving the β-catenin-driven first dermal signal intact. Furthermore, we show that Prdm1 expressing cells not only act as a signaling center but also as a multipotent progenitor population contributing to the several lineages of the adult whisker. We confirm by genetic ablation experiments that the absence of macro vibrissae reverberates on the organization of nerve wiring in the mystacial pads and leads to the reorganization of the barrel cortex. We demonstrate that Lef1 acts upstream of Prdm1 and identify a primate-specific deletion of a Lef1 enhancer named Leaf. This loss may have been significant in the evolutionary process, leading to the progressive defunctionalization and disappearance of vibrissae in primates.

Whiskers (vibrissae) are miniaturized organs that are designed for tactile sensing. Extremely conserved among mammals, they underwent a reduction in primates and disappeared in the human lineage. Furthermore, whiskers are highly innervated and their mechanoceptors signal to the primary somatosensory cortex, where a column of neurons called barrel represents each of them. This structure, known as barrel cortex, occupies a large portion of the somatosensory cortex of the rodent brain. Strikingly, Prdm1 conditional knockout mice are one of the rare transgenic strains that do not develop whisker hair follicles while still displaying a pelage (Robertson et al. 2007). Here we show that Prdm1 is expressed early on during whisker development, more precisely in clusters of mesenchymal cells before placode formation. Its conditional knockout leads to the loss of expression of Bmp2, Shh, Bmp4, Krt17, Edar, Gli1 though leaving the beta-catenin driven first dermal signal intact. Furthermore, we prove that Prdm1 expressing cells not only act as a signaling center but also as a multipotent progenitor population contributing to the formation of the dermal papilla, dermal sheath and pericytes of the vascular sinuses of vibrissae. We confirm by genetic ablation experiments that the absence of motile vibrissae (macro vibrissae) formation reverberates on the organization of nerve wiring in the mystacial pads and organization of the barrel cortex. We prove that Lef1 acts upstream of Prdm1 and identify a potential enhancer (named Leaf) that might be involved in the evolutionary process that led to the progressive reduction of snout size and vibrissae in primates.

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