TRECC SUPPORTS DEVELOPMENT OF DURABLE SMART SKIN AT UIUC

Sensors -- thermometers, barometers and pressure gauges, motion sensors, anemometers, direction sensors -- have become a ubiquitous part of our everyday life. But biological skin -- even more commonplace and ordinary -- outshines them all as a natural marvel of sensor technology.

Optical micrograph of an array of sensors on a flexed polymer substrate (Kapton). The sensor array consists of multiple nodes, each capable of sensing several different variables. The flexed state of the substrate is mapped using curvature sensors located between nodes. view larger

The reason for this is that single-point sensors, at best, extrapolate about an environment using random samplings of a feature or property at a given time or place. Skin, however, is a natural distributed-sensor system, every square inch containing large numbers of miniscule sensor nodes capable of communicating large quantities of information complex entire two-dimensional and three-dimensional measurements of its environment. "If you take a measurement of a flow field over a complex object, you need distributed sensors," says Jon Engel, a graduate student at the Micro- and Nanotechnology Laboratory at the University of Illinois, where he and his colleagues are trying to replicate the sensing properties of skin.

The project is one of twelve currently supported by the Technology Research, Education, and Commercialization Center (TRECC). TRECC is a UIUC program funded by the Office of Naval Research (ONR) and administered by the National Center for Supercomputing Applications (NCSA). It supports innovative research in advanced information technologies and their application for the Navy R&D community. "Emerging technologies like this one are important for future naval missions and security," says E. J. Grabert, program manager for TRECC.

Various sensors under development at UIUC's Micro- and Nanotechnology Laboratory: (a) SEM micrograph of artificial haircell sensors; (b) SEM micrograph of off-plane hot-wire anemometers; (c) optical micrograph of a polymer pressure sensor; (d) optical micrograph of a polymer-based shear stress sensors. view larger

What Engel and his advisor, Dr. Chang Liu, director of the laboratory, are working toward is the creation of a flexible, artificial skin that can be attached to or wrapped around objects and tools. "Our short-term goal is to develop a skinlike flow sensor that can be made available for underwater vehicles to detect complex fluid conditions and use the information for navigation and stabilization," says Liu. "Our longer-term goal would be to enable skinlike sensors that could be used for broader applications, like sensing flow and stabilization for aerial vehicles and producing large arrays of sensors." Other possible applications include workspaces and tables that can determine what has been placed on them based on comparative weight, texture, and thermal conductivity and robotic arms capable of exploring environments or performing delicate tasks.

In the coming months, TRECC will provide more than just financial support for Liu and his research group. TRECC will help match them up with potential partners in industry and defense. "We're very interested in making connections with governmental agencies and industrial partners," says Liu. "I think TRECC can help us make these connections."

An autonomous underwater vehicle (AUV) that could benefit from having a flow-sensing skin on the outer curved surface. view larger

Currently, Engel is working on a smart skin prototype consisting of flexible polymer sheets that contain a variety of sensors, spaced about five millimeters apart. Strain sensors, square, raised, and hollow inside, each cover a total area of around 100 square microns. Resembling miniature trampolines, they determine pressure based on measurements of change and resistance through microscoping, silicon-based MEMS (Micro-Electro-Mechanical Systems) sensors. The wafers are as thin as possible to permit flexibility without sacrificing signal-processing capability.

The smart skin can play host to a unique kind of flow sensor, about three millimeters high and resembling a coarse hair. Its design also comes from nature -- in this case, the physiology of fish. Fish have an organ called a lateral line sensor -- a long, thin sensory organ located on the side of the fish made up of millions of tiny, flexible hairs that can send information back to the fish's brain about current force and velocity in an underwater flow field. Thin and flexible, Engel's flow sensors are around three millimeters tall and can be spaced 100 microns apart. The applications here are numerous: flow sensors could be attached to the walls of a wind tunnel, for instance, for the purposes of studying aerodynamic flow; they could also be used to help underwater autonomous vehicles, such as submarines, track targets and avoid obstacles and severe turbulence.

Schematic diagram of one potential application where a vehicle tracks a moving target underwater by measuring wake created by the target. view larger

Creating a skin that mimics the sensitivity, flexibility, and durability of biological skin can be a difficult problem to solve. Acuity -- the sharpness or quality of a sensation, is an important characteristic of an effective smart skin, says Engel. Silicon is both fragile and inflexible, which makes integrating the silicon-based MEMS sensors into the polymer sheets a challenge. The sheets are made of Kapton, a durable, high-performance polymer film that will withstand the harsh processes of metal deposition and etching, as well as exposure to temperatures around 400 degrees Celsius. However, rather than conform pliably to the surface of complex objects to which Engel is interested in attaching a sensor skin, Kapton tends to work best in two-dimensions, wrapped around cylindrical and square objects. "Other people have done work with stretchier, more flexible materials, which you can stretch over shapes," says Engel. "However, their performance isn't as good as the Kapton in terms of withstanding our processes.We need to consider the material properties, but also allow for making the sensors smaller and getting adequate acuity."

Durability may, perhaps, be the most important quality in the case of military applications, where use on the battlefield necessitates sensors that can withstand subjection to extreme battering. "We want a flow sensor that's indestructible," says Engel. While sensors made entirely out of silicon yield improved performance, "you really can't touch them, because if you're not careful you'll break them. If you're going to put something between a robotic hand and an object that has to be squeezed, it needs to be pretty tough. What's good about our smart skin is that when we set objects on it, it doesn't break."

The center of the human fingertip can sense separate pin pricks as close together as two millimeters; at five millimeters apart, Engel's smart skin sensors provide a fairly close approximation. "The performance and robustness of natural skin is still much better than artificial skin," says Engel. "But we're working on it."

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