3-D PRINTING
Engineered tissue goes bigger Crafty chemistry and bioprinting creates thicker tissue with vasculature
LEWIS LAB/WYSS INSTITUTE/HARVARD SEAS (PRINTED TISSUE); ACS CENT. SCI. (SCHEMATIC)
Engineered, living tissue could help scientists and doctors test for drug safety or even repair injured tissue. Researchers have already used cells and biocompatible polymers to build materials that look and behave like living tissue in many ways. But they have struggled to create tissue that is thicker than about 1 mm and that can survive for more than a couple of weeks, says Jennifer A. Lewis of Harvard University. Lewis and her team have now turned to three-dimensional printing and an assortment of innovative “bioinks” to create tissue thicker than 1 cm that can live longer than six weeks (Proc. Natl. Acad. USA 2016, DOI: 10.1073/pnas.1521342113). The team can direct these 3-D printed constructs to behave like bone-generating tissue found in people. In the past, researchers including Lewis have worked with gelatinous polymers that cross-link under ultraviolet light to mimic the biological environment in which cells live in the body. But UV light also scatters from the polymer it
Vasculature glows red and stem cell ink emits yellow-green light in this printed tissue, shown from above (top) and in cross section (below). The tissue is about 1 cm thick. cures, limiting how thick researchers can make their synthetic matrices, Lewis says. To get around this problem, Lewis’s team relied on enzymes—thrombin and transglutaminase—and multiple bioinks. One ink contains gelatin, fibrinogen proteins, and stem cells from bone marrow, and another is a so-called fugitive ink, which researchers can remove from the tissue after it sets
to create empty channels for vasculature. The researchers print a 3-D lattice, made up of layers of cell ink and fugitive ink. They then fill the lattice with more gelatin containing fibrinogen and the enzymes. These biomaterials diffuse and mingle with the printed features. The thrombin converts fibrinogen to insoluble fibrin, which the slower-acting transglutaminase cross-links with the gelatin to lock in the tissue’s structure. After removing the fugitive ink, the researchers can then coat the vasculature left behind with endothelial cells and perfuse nutrients through the tissue to sustain it for many weeks. To further demonstrate the potential of this approach, Lewis and her team flowed growth factors through the tissue to direct the printed stem cells to behave like bone-growing cells and make their own collagen and calcium phosphate. These are “very cool, exciting results,” says Lisa E. Freed, a tissue engineering researcher with Draper, a not-for-profit company, and an affiliate research scientist with MIT. She adds that the researchers brought together novel materials in an exciting way.—MATT DAVENPORT
DIAGNOSTICS
PCR offers new way to detect disease antibodies To diagnose some conditions, such as autoimmune disease, doctors often look for certain antibodies in patients’ blood. Unfortunately, these proteins can be few and far between. Now, researchers have developed a polymerase chain reaction (PCR) method for detecting such antibodies that is 1,000 times as sensitive as enzyme-linked immunosorbent assay (ELISA), the gold-standard test (ACS Cent. Sci. 2016, DOI: 10.1021/acscentsci.5b00340). In ELISA, antigen proteins sit on a plastic slide and bind to antibodies present in a sample, capturing them for detection. But “some antigen proteins unfold when bound to the ELISA plate,” which prevents antibodies from binding them, says Carolyn R. Bertozzi, a chemist at Stanford University and editor-in-chief of ACS Central Science.
Also, ELISA isn’t usually sensitive enough to detect the low antibody levels in saliva or urine or in blood during the early stages of a disease. In the new study, Bertozzi’s team wanted to detect antibodies associated with autoimmune thyroid disease. These antibodies bind the protein thyroglobulin. The researchers first attached thyroglobulin (beige) to one of two different DNA se-
quences (red, green). Then they incubated these conjugates with a blood sample containing thyroglobulin antibodies (Y shapes). Because the antibodies bind two thyroglobulin molecules, the antibodies bring the tethered DNA strands close to each other. The scientists added a piece of DNA bridging the two types of strands (shown). The team used PCR to amplify a piece of DNA that contained the two linked sequences. They detected it using gel electrophoresis. The method detected 100 to 100,000 antibodies in 2 µL of serum, Bertozzi says . Mark W. Pandori of the University of California, San Francisco, says this approach appears to be sensitive enough to detect antibodies in saliva, which would be helpful for diagnostics in resource-poor areas.—ERIKA
GEBEL BERG, special to C&EN MARCH 14, 2016 | CEN.ACS.ORG | C&EN
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