Double-network (DN) hydrogels, with their unique combination of mechanical strength and toughness, have emerged as promising materials for soft robotics and tissue engineering. In the past decade, significant effort has been devoted to synthesizing DN hydrogels with high stretchability and toughness; however, shaping the DN hydrogels into complex and often necessary user-defined two-dimensional (2D) and three-dimensional (3D) geometries remains a fabrication challenge. Here, we report a new fabrication method based on optical projection lithography to print DN hydrogels into customizable 2D and 3D structures within minutes. DN hydrogels were printed by first photo-crosslinking a single network structure via spatially modulated light patterns followed by immersing the printed structure in a calcium bath to induce ionic cross-linking. Results show that DN structures made by this method can stretch four times their original lengths. We show that strain and the elastic modulus of printed structures can be tuned based on the hydrogel composition, cross-linker and photoinitiator concentrations, and laser light intensity. To our knowledge, this is the first report demonstrating quick lithography and high-resolution printing of DN (covalent and ionic) hydrogels within minutes. The ability to shape tough and stretchable DN hydrogels in complex structures will be potentially useful in a broad range of applications.

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We report a small exploratory study of a methodology for real-time imaging of chemical and physical changes in spinal cords in the immediate aftermath of a localized contusive injury. One hundred separate experiments involving scanning NIR images, one-dimensional, two-dimensional (2-D), and point measurements, obtained in vivo, within a 3 × 7 mm field, on spinal cords surgically exposed between T9 and T10 revealed differences between injured and healthy cords. The collected raw data, i.e., elastic and inelastic emission from the laser probed tissues, combined via the PV[O]H algorithm, allow construction of five images over the first 5 h post injury. Within the larger study, a total of 13 rats were studied using 2-D images, i.e., injured and control. A single 830-nm laser (100-μm diameter round spot) was spatially line-scanned across the cord to reveal photobleaching effects and surface profiles possibly locating a near surface longitudinal artery/vein. In separate experiments, the laser was scanned in two dimensions across the exposed cord surface relative to the injury in a specific pattern to avoid uneven photobleaching of the imaged tissue. The 2-D scanning produced elastic and inelastic emission that allowed construction of PV[O]H images that had good fidelity with the visually observed surfaces and separate line scans and suggested differences between the volume fractions of fluid and turbidity of injured and healthy cord tissue.

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We produce 2 kinds of images of spinal cord in the immediate aftermath of a localized contusive injury, noninvasively and in vivo in a rat model using remitted light from a single spatially scanned 830 nm laser. Two different processing algorithms/modalities were employed 1) the PVOH algorithm producing images based on the turbidity of the spinal cord tissue which includes the cerebrospinal fluid (CSF) and 2) Raman spectroscopy which allows images based on pH. Whereas PVOH requires relatively little data processing e.g., no separation of Raman and fluorescence emission for implementation, quantitative use of spontaneous Raman spectra to calculate e.g., pH requires careful separation and accounting for the presence of underlying fluorescence. We utilize an unbiased procedure i.e., no assumptions are made, that an unsupervised machine can execute based on the inherent differences in line widths for pure vibrational and vibronic transitions in fluid media, and the overall effect of heterogenous spectral broadening at physiological temperature. The fundamentally stronger signal to noise ratio for PVOH imaging allows much greater spatial resolution for the same collection times needed to construct the Raman based pH images. This proof of principle study is consistent with a hypothesis that fast localized change in the CSF pH, induced by the mechanical disruption of tissues during the injury, disrupts the delicate balance of chemical factors stabilizing the CSF phase stability. We suggest that this combination of new methodologies for real-time imaging of chemical and physical changes in spinal cords and other tissues could have many uses.