Enhanced mechanical properties of photo-clickable thiol-ene PEG hydrogels through repeated photopolymerization of in-swollen macromer.

Current hydrogels used for tissue engineering are limited to a single range of mechanical properties within the replicated tissue construct. We show that repeated in-swelling by a single hydrogel pre-cursor solution into an existing polymerized hydrogel followed by photo-exposure increases hydrogel mechanical properties. The process is demonstrated with a photo-clickable thiol-ene hydrogel using a biocompatible precursor solution of poly(ethylene glycol) dithiol and 8-arm poly(ethylene glycol) functionalized with norbornene. The polymer fraction in the precursor solution was varied by 5, 10, and 20 percent by weight and an off-stoichiometric ratio of thiol : ene was used, leaving free enes available for subsequent reaction. Multiple swelling and exposure cycles for the same precursor solution were performed. The compressive modulus increased by a factor between three and ten (formulation dependent), while volume swelling ratio decreased by a factor of two, consistent with increased crosslink density. The modified hydrogels also demonstrate increased toughness by fracturing at compressive forces five times greater than the initial hydrogel. We attribute the increased toughness to subsequent increases in crosslink density created by the repeated photopolymerization of in-swollen macromer. This technique demonstrates the ability to significantly modify hydrogel network properties by exploiting swelling and polymerization processes that can be applied to traditional three-dimensional printing systems to spatially control local mechanical properties.

Three-dimensional optical disk data storage via the localized alteration of a format hologram.

Three-dimensional optical data storage is demonstrated in an initially homogenous volume by first recording a reflection grating in a holographic photopolymer. This causes the entire volume to be weakly reflecting to a confocal read/write head. Superposition of two or three such gratings with slightly different k-vectors creates a track and layer structure that specialized servo detection optics can use to lock the focus to these deeply-buried tracks. Writing is accomplished by locally modifying the reflectivity of the preexisting hologram. This modification can take the form of ablation, inelastic deformation via heating at the focus, or erasure via linear or two-photon continued polymerization in the previously unexposed fringes of the hologram. Storage by each method is demonstrated with up to eight data layers separated by as little as 12 microns.

Acousto-optic photonic crossbar switch. Part I: design.

We present the design of a 12 × 12 photonic crossbar interconnection network constructed using a single three-dimensional acousto-optic crystal. Previous crossbars based on bulk acousto-optic cells require multichannel deflectors with one deflector per optical input; in contrast the design presented here angularly multiplexes these independent deflectors into a single-transducer acousto-optic device. A Fourier-optics analysis of an acoustically lossy Bragg deflector is coupled to a momentum-space analysis that permits the derivation of complete design equations for the switch. As a concrete example, the complete design of a 12 × 12 crossbar is presented. Finally, a coupled-mode analysis of the first- and second-order diffractions in the angularly multiplexed Bragg cell reveals the fundamental efficiency bounds of the switching network.