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Science kl AA AS An Electronic Second Skin Zhenqiang Ma Science 333, 830 (2011); DOI: 10.1126/science.1209094 This copy is for your personal, non-commercial use only. If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this infomation is current as of August 15, 2011): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://i.vww.sciencemag.org/content/333/6044/830.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/333/6044/830.full.html#related This article cites 7 articles. 2 of which can be accessed free: http://www.sciencemag.org/content/333/6044/830.full.html#ref-list-1 This article appears in the following subject collections: Materials Science http://www.sciencemag.org/cgi/collection/mat sci Downloaded from www.sciencemag.org on August 15. 2011 Science (print ISSN 0036-8075: online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2011 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS. EFTA01120562 PERSPECTIVES for germ cell development, and the sex of the germ cells must match that of the soma for proper gametogenesis to occur (2). A key question facing researchers is how the germ line coordinates signals from the soma with its own sex chromosome constitution to achieve proper sexual identity. Hashiyama et at demonstrate that in Drosophila, a gene named Sex lethal (Sxl) acts as a key switch in regulating gennline sex determination. Expression of &I is suf- ficient to allow germ cells with an XY geno- type, which would normally be male, to pro- duce eggs when they are transplanted into a female soma (ovary)—something that a male germ cell would normally never do. There are several reasons why this work is exciting. First, Sxl expression is able to over- come the incompatibility between a male germ line and a female soma. This offers insight into how the two distinct inputs from the germ line and soma contribute to germ- line sex determination. Second, the fact that Sr/ is sufficient to activate female germline identity is interesting given that it is also the key switch gene in determining the sex of the soma (3), yet it acts differently in the germ line. Lastly, Hashiyama et at show that sex determination in the germ line occurs ear- lier than was thought. Previous studies showed that germ cells exhibit sex-specific behaviors and gene expression at the time they first associate with the somatic gonad (2). However, female-specific expression of Sr/ begins much earlier (1)—as soon as the germ cells are believed to generally activate zygotic transcription (4'). Thus, germ cells have a sexual identity even before they are influenced by sex-specific signals from the somatic gonad. This exciting work raises as many ques- tions as it answers. How is Sxl activated in female germ cells? Although Sr! expres- sion in both the soma and the germ line is regulated by the number of X chromosomes present, it appears that the way the cell's biochemical machinery "counts" X chro- mosomes in the germ line differs from the way it counts them in the soma (5, 6). Fur- ther, what are Srts targets in the germ line? Sr/ encodes an RNA binding protein; in the soma, it acts as a regulator of alternative RNA splicing and translation, and controls both sexual identity [by regulating trans- former (rra)] and X chromosome dosage compensation (by regulating male specific lethal 2). However, neither of these key Sri targets in the soma are important in the germ line (7, 8), indicating that Sri must regulate other, unknown factors there. Hashiyama et at also found that activation of Sri in male germ cells did not interfere with nor- mal spermatogenesis; when Sxl-expressing male germ cells were left in a male envi- ronment (testis), they produced sperm, as observed previously (9). Thus, Sr! cannot feminize a germ cell in all respects, because a female germ cell is unable to make sperm in a male environment. Compatibility between the germ line and the soma is also an issue for mammalian germ cells. In humans who are XXY (Kline- fetter's syndrome), the soma is male because of the masculinizing influence of the Y chro- mosome. However, the presence of two X chromosomes is incompatible with male germline development and these individuals are sterile; their testes have severely reduced germline characteristics, including loss of premeiotic spermatogonia and spermatogo- nial stem cells (10). This defect is due to the number of X chromosomes in germ cells; any foci of spermatogenesis observed in these patients are from germ cells that have lost one X chromosome (11, 12). Germ cell defects are also seen in females with Turner's syn- drome, which is characterized by the pres- ence of only a single X chromosome (XO) (13). Although recent studies have identified signals by which the somatic gonad influ- ences germline sex determination in mam- mals, how the sex chromosome constitution affects this process remains unknown. The appearance and function of sperm and egg are similar throughout the animal kingdom, which suggests that the process of germline sexual development may be highly conserved. Thus, a better understand- ing of how gennline sexual identity is reg- ulated by the soma, and by the germ line's own sex chromosome constitution, will have far-reaching implications for our knowledge of animal development and human fertility. Hashiyama et al. have now brought us one step closer to this goal. References 1. K. Hashiyama, Y. Hayashi, S. Kobayashi, SIWACe 333, 885 12011). 2. S. M. Murray, S. Y. Yang, M. Van Doren, Cua. Opm. Cell Biol. 22, 722 (2010). 3. T. W. Chne, Dem Mot 72, 266 (1979). 4. M. Van Omen, A. L. Williamson, R. Lehman, Cum 8ml. 8, 243 (1998). 5. M. Steinmann-back% Development 117, 763 (1993). 6. 8. Grenadine, P. Santamana,1. Sanchez, Development 118, 813 (1993). 7. ). I.. Marsh, E. Wieschaus, Notate 272, 249119781. 8. 0. Bachiller, L. Sanchez, Oev. 8/of. 118, 379 (1986). 9. ). H. Hager, 1. W. Cline, 0evelopmen 124, 5033 (1997). 10. A. M. Wiksirbm, L Dunkel, Ham. Res 69,317 (2008). 11. R. B. Sourano et al., Hum. Remod. 24, 2353 (2009). 12. M. Bemire et d., Hum. Reproof. 17, 32 (2802). 13. K. Reynaud Prof., felt Sten! 81.1112 (2004). 14. N. Camara, C. Whinuerth, M. Van Doren, Can. Top. Dee. Biol. 83, 6512038). 15. rn.trameink et of., Herat 436, 563 (2005). MATERIALS SCIENCE to.fizetscience.tz 10282 An Electronic Second Skin Zhenqiang Ma Small, flexible devices that attach to the skin without adhesives or gels monitor physiological signals. I n clinical health monitoring, the diagnos- tic machines that perform physiological measurement and stimulation through skin are connected to patients with wires and cables. Such complicated wiring can be inconvenient and distressing for both patients and physicians. For example, a patient who may have heart disease is usually required to wear a bulky monitor for a prolonged period (typically a month) in order to capture the abnormal yet rare cardiac events. The cur- rent best electrodes are gel-coated adhesive pads. Many people, particularly those who have sensitive skins, will develop a rash, and Department of Electrical and Computer Engineering, Uni- versity of Wisconsin, Madison, WI 53706, USA. E-mail: mamipenor.wisc.edu the electrode locations have to be constantly moved around, interrupting monitoring. Clin- ical physicians strongly desire more compact and even wireless health monitoring devices. An electronic skin recently developed by Kim et at (1), reported on page 838 in this issue, will help solve these problems and allow monitoring to be simpler, more reliable, and uninterrupted. These devices were made through "transfer printing" fabrication pro- cesses that create flexible versions of high- performance semiconductors that are brittle as bulk materials. The electronic skin concept was initially developed for applications in robotics (2-4). Robots could be provided with pressure sens- ing ("touch") that would allow them to grip objects securely without damaging them (the Downloaded from www.sciencemag.org on August 15, 2011 830 12 AUGUST 2011 VOL 333 SCIENCE www.sciencemag.org Published byAAAS EFTA01120563 PERSPECTIVES Transistors Photodetectors Circuits and sensors Strain gauges Temperature sensors Po yester skin Human skin N Information caught by film. A flexible electronic device attaches to the skin like a bandage tape and can be used to acquire physiological information without bulky electrodes. Kim et at. developed an electronic skin in the form of a highly stretchable net, consisting of various sensors and electronics of serpentine shapes, that is sandwiched between two protection layers of equal thickness. The device layer sits on a polyester layer that was engineered with mechanical properties to match those of natural skin. "picking up an egg" problem). These elec- tronic skins, which mainly consist of pres- sure-sensing materials and associated elec- tronic devices for pressure reading, might also provide touch sense to prosthetic devices such as artificial legs or arms. One challenge for making these devices is that the transistors (and the semiconduc- tors in them) that amplify weak signals must be flexible in order to act like skin. The abil- ity of transistors to amplify signals—their gain—depends on the mobility of the charge carriers in their semiconductor under the gate layer (or in their gated semiconductor layer). Doped single-crystalline silicon wafers are used in most computer chips because of their high carrier mobility, which allows opera- tion with low applied voltage and low power. However, the wafers are brittle, so alterna- tive materials have been pursued. Some of the candidate flexible semiconductors, such as conducting polymers (2, 3), have much lower carrier mobilities. The higher voltages needed to use these materials as transistors may not be suitable for electronic skin that makes direct contact with a patient's skin, and may quickly exhaust small power supplies. Another approach is to convert brittle semiconductors into more flexible forms. For example, silicon and germanium are highly flexible as nanowires (4, 5). However, their carrier mobility, although much higher than that of conducting polymers (2, 3), is still much lower than that of doped silicon. With these types of materials, it is difficult or impossible to achieve the performance needed to amplify very weak signals acquired from natural skin. The electronic skin demonstrated by Kim et aL uses thin single-crystal silicon that has superior flexibility and a mobility equivalent to that of the silicon used in personal porta- ble devices. The approach, a printing method developed previously by Rogers's group (6), could be called "inking and printing." A thin silicon layer is bonded to a silicon dioxide release layer. The silicon layer is cut into a lattice of micrometer-scale "chiplets,- and a transfer stamp layer is then attached to the top of the divided silicon. The transfer layer and chiplets are then lifted and transferred to a flexible substrate. Attaching electronic skin to natural skin is more difficult than attaching it to robots or prosthetics. Natural skin is soft and del- icate and already has touch-sensing func- tions. The electronic skin that can be used for physiological monitoring must have a supporting layer with mechanical properties that match those of natural skin to avoid any discomfort resulting from long wearing. The electronic skin must not be too thick, too rigid, too hard, or too heavy, but must have conformal contact, intimate integration, and adequate adhesion with the natural skin. Special materials that are properly designed through accurate modeling were needed to achieve these properties. The sup- port layer of the electronic skin is an elasto- meric (rubbery) polyester engineered to have mechanical properties well matched to those of natural skin. The circuitry part of the elec- tronic skin consists of two protection layers that sandwich a multifunctional middle layer (see the figure). With their equal thicknesses, the protection layers develop opposite strains that cancel, so the middle circuit layer expe- riences little stress no matter which direction the device is bent. The middle layer consists of the metal, semiconductor, and insulator components needed for sensors, electronics, power supplies, and light-emitting compo- nents, all of which are in the serpentine shape that forms a stretchable net. The serpentine shapes allow the net to deform drastically with little effect on its functionality. This innovative design contains all of the neces- sary components in an ultrathin layer about the thickness of a human hair. The electronic skin designed by Kim et at can be simply mounted onto or peeled off natural skin in the same way as bandage tape. Physiological information has been collected from heart, brain, and skeletal muscles with a quality equivalent to that collected with bulky electrodes and hardware. Other forms of physiological information collection based on the electronic skin are readily feasible because they could use components that have more sophisticated functions. The transfer-printing fabrication approach (6) has proved to be viable and low-cost in this demonstration, which will greatly facilitate the practical clinical use of the electronic skin. Because of the higher quality of the transferable thin silicon, wire- less communication directly from the elec- tronic skin should be feasible, given recent demonstrations of this capability in other devices (7). Other types of electronic skins with applications beyond physiology, such as body heat harvesting and wearable radios, may also point to interesting directions for future work. References 1. 0.-H. Kim et of, Science 333, 838 (20111. 2. 1. 5omeya et at, Proc. Not!. Arad. so. tr.sA 101, 9966 (200O. 3. S. C. B. Manndetdet of., Nat. Mato. 9, 859 (2010). 4. Z. Fan a of Nano Left. 8, 20 (2008). 5. K. Takei et or.. Nat Mato. 9, 821(2010). 6. E. Menu& R. G. teuuo.1. k Rogers, Appt Phys. Lett 86, 09350712005). 7. t. Sun ed.Small 6, 2553 (2010). 10.1126/Sibente.1209094 Downloaded from www.sciencemag.org on August 15, 2011 www.sciencemag.org SCIENCE VOL 333 12 AUGUST 2011 pubranedayams 831 EFTA01120564