Protein Hydrogels Engineered to Promote Cell Growth
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develop organic diodes have resulted in devices with maximum frequency response of no more than 10 kHz and with output current densities of less than 1 A/cm2 under an applied ac voltage. Since response speed is governed by the capacitance of a device, the researchers predict that reduced device areas will yield a 10-fold or more improvement in response time when scaling the device to micron-scale dimensions. The scientists attribute the enhanced performance to the heat treatment, prior to which the performance was poor. They believe that the heat treatment causes the Cu to diffuse into the C60 layer, forming a stable metallic interface to the C60 layer. The device’s strong electron acceptor properties lead to a conducting charge-transfer complex similar to the heavily doped interface in silicon technology to form a good ohmic contact. This situation allows for efficient electron injection from the Cu cathode into C 60, increasing by about three orders of magnitude after heat treatment. Atomic force microscopy revealed that the C60 recrystallized, enhancing carrier mobility. Al, on the other hand, forms covalent bonds to C60, resulting in the observed work-function increase from 4.2 eV to 5.2 eV, which is consistent with the observed I–V curve reversal. Organic diodes, with their response speed in ac mode below 10 kHz, are not stable in air. The C60-based organic diode, however, did not show any noticeable performance decay, even after a 40 h stress test in air without encapsulation at 2.4 ac voltage and 1 MHz frequency, whereas normal organic diodes were found to have a reported lifetime of no more than 17 h even under current conditions that are three orders of magnitude less severe. ALFRED A. ZINN
Protein Hydrogels Engineered to Promote Cell Growth A research team at The Johns Hopkins University (JHU) has created a class of artificial proteins that self-assemble into a
Figure 1. Schematic illustration depicting a hydrogel network with three distinct bioactive domains (D) formed by the self-assembly of modular proteins. Illustration by Will Kirk.
gel that can be tailored to send different biological signals stimulating the growth of selected types of cells. Tissue engineers use hydrogels to provide a framework or scaffold upon which to grow cells. The researchers hope to advance their technique to the point that it can be used to treat medical ailments by growing replacement cartilage, bones, organs, and other tissue in the laboratory or within a human body. “We’re trying to give an important new tool to tissue engineers to help them do their work more quickly and efficiently,” said James L. Harden, whose laboratory team, L. Mi and S. Fischer, developed the biomaterial. Harden, an assistant professor in the Department of Chemical and Biomolecular Engineering at JHU, reported on his work at the 227th national meeting of the American Chemical Society in Anaheim, Calif., on March 28. Harden’s hydrogel is made by mixing specifically designed modular proteins in a buffered water solution. Each protein consists of a
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