The increasing demand for clean energy sources that do not add more carbon dioxide and other pollutants to the environment has resulted in increased attention worldwide to the possibilities of a "hydrogen economy" as a long-term solution for a secure energy future based on potentially renewable resources. [1][2][3] Some of the greatest challenges are the discovery and development of new on-board hydrogen-storage materials and catalysts for fuel-cell-powered vehicles. New materials that store both high gravimetric (! 90 gm H 2 kg À1 ) and high volumetric (! 82 gm H 2 L À1 ) densities of hydrogen that can be delivered at temperatures between À20 and 85 8C are needed by the year 2015. [4] The volumetric constraints eliminate from consideration pressurized hydrogen systems and guide towards the development of solid storage materials. [5] There are several broad classes of solid hydrogenstorage materials that are currently being investigated as potential on-board storage materials: 1) metal materials, hydrides (e.g., MgH 2 ), [6] imides (e.g., LiNH 2 ), [7] and organic frameworks (e.g., Zn 4 O(1,4-benezenedicarboxylate)), [8] 2) complex hydrides (e.g., NaAlH 4 ), [9] and 3) carbon materials (e.g., carbon nanofibers, [10] single-wall carbon nanotubes). [11] The most thoroughly studied complex hydride, NaAlH 4 , has been shown to release hydrogen at 110 8C when doped with Ti; [12] however, the kinetics are very slow and hydrogen-storage densities are too low (56 gm H 2 kg À1 ) to meet long-term targets. The temperatures for H 2 release from carbon materials are too low, and the reported storage densities are controversial. [13] The hydrolysis of metal hydrides is being explored, but the unfavorable thermodynamics for regeneration of the spent material prevents their widespread application. For example, the reaction NaBH 4 +4 H 2 O!NaB(OH) 4 +4 H 2 is exothermic by À250 kJ mol À1 . Reaction enthalpy for hydrogen loss is an important property since near-thermoneutral thermodynamics will be critical for materials for reversible H 2 storage. To date, few of these materials meet the long-term gravimetric requirements and provide rapid hydrogen release at temperatures between À20 and 85 8C; thus, new materials and novel approaches are needed. Herein we show that the kinetics of hydrogen release are significantly enhanced at low temperatures for a new hybrid material, ammonia borane infused in nanoporous silica, and that the hydrogen purity is increased. These findings suggest that hydrogen-rich materials infused in nanoscaffolds offer a most promising approach to on-board hydrogen storage.Chemical hydrogen-storage materials that release H 2 by thermolysis without generating CO 2 may offer an attractive alternative to other systems studied. For example, the NH x BH x family of compounds [14] should provide favorable gravimetric densities of 245, 196, 140, and 75 gm H 2 kg À1 for x = 4, 3, 2, and 1, respectively. As the NB unit is isoelectronic with CC, these materials are viewed as inorganic analogues of hydrocarbons. Howeve...

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Recently, tissue engineering approaches using injectable, in situ gel forming systems have been reported. In this review, the gelation processes and several injectable systems that exhibit in situ gel formation at physiological conditions are discussed. Applications of selected injectable systems (alginate, chitosan, hyaluronan, polyethylene oxide/polypropylene oxide) in tissue engineering are also described. Injectable polymer formulation can gel in vivo in response to temperature change (thermal gelation), pH change, ionic cross-linking, or solvent exchange. Kinetics of gelation is directly affected by its mechanism. Injectable formulations offer specific advantages over preformed scaffolds such as: possibility of a minimally invasive implantation, an ability to fill a desired shape, and easy incorporation of various therapeutic agents. Several factors need to be considered before an injectable gel can be selected as a candidate for tissue engineering applications. Apart from tissue-specific cell-matrix interactions, the following gel properties need to be considered: gelation kinetics, matrix resorption rate, possible toxicity of degradation products and their elimination routes, and finally possible interference of the gel matrix with histogenesis. Anat

We recently developed a cell printer (Wilson and Boland, 2003) that enables us to place cells in positions that mimic their respective positions in organs. However, this technology was limited to the printing of two-dimensional (2D) tissue constructs. Here we describe the use of thermosensitive gels to generate sequential layers for cell printing. The ability to drop cells on previously printed successive layers provides a real opportunity for the realization of three-dimensional (3D) organ printing. Organ printing will allow us to print complex 3D organs with computer-controlled, exact placing of different cell types, by a process that can be completed in several minutes. To demonstrate the feasibility of this novel technology, we showed that cell aggregates can be placed in the sequential layers of 3D gels close enough for fusion to occur. We estimated the optimum minimal thickness of the gel that can be reproducibly generated by dropping the liquid at room temperature onto a heated substrate. Then we generated cell aggregates with the corresponding (to the minimal thickness of the gel) size to ensure a direct contact between printed cell aggregates during sequential printing cycles. Finally, we demonstrated that these closely-placed cell aggregates could fuse in two types of thermosensitive 3D gels. Taken together, these data strongly support the feasibility of the proposed novel organ-printing technology. Anat Rec Part A 272A: 497-502, 2003.