Sensitive skin is a large-area, flexible array of sensors with data processing capabilities, which can be used to cover the entire surface of a machine or even a part of a human body. Depending on the skin electronics, it endows its carrier with an ability to sense its surroundings via the skinâ„¢s proximity, touch, pressure, temperature, chemical/biological, or other sensors. Sensitive skin devices will make possible the use of unsupervised machines operating in unstructured, unpredictable surroundings among people, among many obstacles, outdoors on a crowded street, undersea, or on faraway planets. Sensitive skin will make machines cautious and thus friendly to their environment. This will allow us to build machine helpers for the disabled and elderly, bring sensing to human prosthetics, and widen the scale of machinesâ„¢ use in service industry. With their ability to produce and process massive data flow, sensitive skin devices will make yet another advance in the information revolution. This paper surveys the state of the art and research issues that need to be resolved in order to make sensitive skin a reality.
CHAPTER Page No:
1. INTRODUCTION 5
1.1. Machines In Unstructured Environments 5
1.2. Societal Needs And Concerns Of Sensitive Skin 6
1.2.1. Health Industry 6
1.2.2. Environment Friendly Technology 6
1.2.3. Difficulties Of Acceptance 6
2. SYSTEM CONCEPT 7
3. SKIN METERIALS 8
3.1. Areas Of Discussion 8
3.2. Substrate / Interconnect Issues 9
3.2.1. Stretching and Bending 9
3.2.2. Saran Wrap / Soccer Ball / Panty Hose â€œModel 10
3.3. Adding Sensing / Intelligence / Actuation 11
3.3.1. A Hybrid Approach 11
3.3.2. An Integrated Approach 12
3.3.3. Distributed Intelligence Approach 12
4. DEVICES FOR SENSITIVE SKIN 13
4.1. Device Capabilities Sought For Sensitive Skin 13
4.2. Large-Area Electronics Is Coming Of Age 14
4.2.1. Organic Electronics And Optoelectronics On
Flexible Substrates 15
4.2.2. Thin Film MEMS On Flexible Substrates 16
4.2.3. Nanostructures On Flexible Substrates 16
4.3. Manufacturing Of Large-Area Sensitive Skin 18
4.3.1 Direct Printing 18
4.3.2. Laser Writing 20
4.3.3. Nanoimprinting 20
5. SIGNAL PROCESSING 23
5.1. Fault Tolerance 23
5.2. Data Reduction 24
5.3. Data Processing 25
6. APPLICATIONS 25
6.1. Human Skin Or Wearable Skin 25
6.2. Sensitive Skins For Machines 26
6.3. Environmental Sensitive Skin 27
6.4. Actuated Sensitive Skin 27
7. CONCLUSION. 27
8. REFERENCE 28
9. APPENDIX 30
This seminars focuses on the principles, methodology, and prototypes of sensitive skin-like devices, and the related system intelligence and software that are necessary to make those devices work. Sensitive skin represents a new paradigm in sensing and control. These devices will open doors to a whole class of novel enabling technologies, with a potentially very wide impact. Far-reaching applications not feasible today will be realized, ranging from medicine and biology to the machine industry and defense. They will allow us to fulfill our dream for machines sensitive to their surroundings and operating in unstructured environment.
Some applications that sensitive skin devices will make possible are yet hard to foresee. Flexible semiconductor films and flexible metal interconnects that will result from this work will allow us to develop new inexpensive consumer electronics products, new types of displays, printers, new ways to store and share information (like electronic paper and upgradeable books and maps). New device concepts suitable for large area flexible semiconductor films will lead to new sensors that will find applications in space exploration and defense, specifically in mine detection and active camouflage.
An ability of parallel processing of massive amounts of data from millions of sensors will find applications in environmental control and power industry. These areas will be further developed because of the highly interdisciplinary nature of the work on sensitive skin, which lies at the intersection of information technology, mechanical engineering, material science, biotechnology, and micro- and nano electronics. Availability of sensitive skin hardware is likely to spur theoretical and experimental work in many other disciplines that are far removed from robotics.
1.1. MACHINES IN UNSTRUCTURED ENVIRONMENTS
Todayâ„¢s machine automation is almost exclusively limited to the structured environment of the factory floor. The rest of the world, with perhaps 99% of all tasks that involve motion and could in principle be automated, goes unautomated. Think of the unstructured environments in agriculture, construction sites, offices, hospitals, etc. The majority of tasks that are of interest to us take place in unstructured environments, to which todayâ„¢s automation simply cannot be applied.
Automated moving machines can be divided into unattended those that can operate without continuous supervision by a human operator, and semi-attended, which are controlled by the operator in a remote (teleoperated) fashion. Today the use of both types of machines is limited exclusively to highly structured environments - a factory floor, a nuclear reactor, a space telescope. Such machines can operate successfully with relatively little and fairly localized sensing. Many existing machines could, in principle, be useful in an unstructured environment, if not for the fact that they would endanger people, surrounding objects, and themselves.
The same is true for remotely controlled machines. Unless the work cell is sanitized into a structured environment, no serious remote operation could be undertaken. Otherwise, at some instant the operator will overlook a small or occluded object, and an unfortunate collision will occur. And so the designers take precautions, either by sanitizing the environment, or by enforcing maddeningly slow operation with endless stops and checks. Much of the associated extra expense would not be necessary if the machines had enough sensing to cope with unpredictable objects around them.
The Way Out is All-Encompassing Sensing:
To operate in an unstructured environment, every point on the surface of a moving machine must be protected by this pointâ„¢s own local sensing.
1.2. SOCIETAL NEEDS AND CONCERNS OF SENSITIVE SKIN
1.2.1. HEALTH INDUSTRY
Sensitive skin will supplant sensing ability of the human skin in limb prosthetics and as a replacement of damaged human skin. It will augment human sensing in wearable clothing, by monitoring, processing, and wireless transfer of information about the well-being of the person wearing sensitive skin. This will advance the post-traumatic health care, care for disabled and elderly persons, and monitoring of military personnel on the battlefield.
1.2.2. ENVIRONMENT â€œ FRIENDLY TECHONOLOGY
For the first time in history, machines will be endowed with a capacity to be careful. By its very nature, sensitive skin will contribute in a dramatic way to the reversal of the well-known negative impact of machines on our environment, across a wide spectrum of natural and man-made settings.
We often hear about the role of computer revolution and office automation in the growth of economy and improved efficiency, which in turn affects the quality of life. Note the difference: while unstructured machine automation will have a similar effect on the economy, its use in service industry will have a direct impact on the quality of human life. Biology and medical science thrive to prolong human life; the unstructured machine automation will constitute a systematic effort by engineers to improve the quality of life.
1.2.3. DIFFICULTIES OF ACCEPTANCE
As with any fundamentally new and powerful technology, sensitive skin technology may evoke adverse psychological reactions, with a potential of diminishing its impact. Today we are psychologically unprepared for automatic moving machines operating in our midst. We are not sure we need them. We are uneasy about the idea of living side by side with a powerful unattended moving machine. It is difficult to imagine that one could stand next to a powerful moving machine and trust it enough to turn oneâ„¢s back to it, or expect it to step aside when passing. Do we not have more than enough invasion of machinery in our lives To need a very new product, one must first experience it.
2. SYSTEM CONCEPT
Figure-1 Sketch of interconnects between sensors, intelligence, and actuators
The system consists of a number of distributed sensor, actuator, and intelligence units, which are connected by some network of interconnects. The interconnects are necessary for providing power to the system as well as for communication. The sensors/actuators themselves may have intelligence associated with them, but there are other higher levels of intelligence to which they are connected.
The interconnects shown in the system might be electrical (conventional wires) or optical (fibers). The communication via the individual units might in some cases be wireless (implying also fiber-less) for some structures.
For delivering power, it was thought that the system probably would require physical interconnects (i.e. power delivered through fibers or wires), and that harnessing energy from the environment, such as via solar or RF pick-ups, would not be practical for most applications (especially for wireless systems). Therefore in all cases there would have to be a physical interconnect between the individual sensor / actuator / intelligence blocks, and so a major part of this report addresses issues associated with this physical level of interconnection.
Figure- 2. Potential applications of sensitive skin.
Four groups of research issues must be addressed in order to develop sensitive skin: Skin Materials, Sensing Devices, Signal and Data Processing, and Applications. Consider them one by one.
3. SKIN METERIALS
- Sensitive Skin material will hold embedded sensors and related signal processing hardware. It needs to be flexible enough for attaching it to the outer surfaces of machines with moving parts and flexible joints.
- The skin must stretch, shrink, and wrinkle the way human skin does, or to have other compensating features. Otherwise, some machine parts may become "exposed" due to the machine's moving parts, and have no associated sensing.
- Wiring must keep its integrity when Sensitive Skin is stretched or wrinkled. This requirement calls for novel wire materials, e.g. conductive elastomers or vessels carrying conductive liquid, or novel ways of wire design with traditional materials, such as helical, stretchable wires.
3.1. AERAS OF DISCUSSION
Three areas of potential discussion were considered:
1. What materials might be used for sensors, actuators, and intelligence (transistors) in such a system
2. How can we make an interconnection network that can flex and bend
3. How can we physically combine sensors/intelligence/actuators with the interconnect substrate
Figure -3. Semiconductor materials for sensitive skin
Fabricating sensitive skin is based on a new process of depositing polycrystalline CdSe (1.75 eV), CdS (2.4 eV), PbS (0.4 eV) , PbSe (0.24 eV) and CuS (semiconductor/ metal) films on flexible substrates at temperatures close to room temperature (eV here are electron-volts). Large area surfaces can be covered. Also, ternary and quaternary compounds as well as heterostructures can be deposited. Transparent conductors on flexible substrates (such as CuS), materials for sensors, with possible combination with higher mobility polycrystalline materials (such as laser annealed polycrystalline silicon), amorphous (such as a-Si), polycrystalline (such as CdS or CdSe), and deep submicron crystalline silicon technology (for fast data processing). We will also need sensors with multiple sensing capabilities, learning, once again, from the design of human or animal skin. These are new and exciting challenges for material science and device physics.
3.2. SUBSTRATE / INTERCONNECT ISSUES
3.2.1. STRECHING AND BENDING
A central issue for sensitive skin is that the skin be able to conform to surfaces of arbitrary shape, and be able to flex, bend, and stretch. Flexing, bending, and stretching are important not only for applications (e.g. covering moving arms and joints), but also for initial installation (like putting on clothes).
When a thin planar foil is deformed into developable surface such as a cylinder or a cone, the average strain in the foil is zero, and there exists a neutral plane within its bulk where the strain locally is zero. The strain on the surfaces scales as the thickness over the radius of curvature.
Therefore by making the substrate thin and /or placing interconnects at the neutral plane, bending to thin radii of curvature appears possible. However, deforming into arbitrary shapes (e.g. spheres), bending in multiple dimensions, and stretching require a finite strain, and hence may cause failure of the interconnects (e.g. if the strain is larger than 1%).
Three different models for the substrate/interconnect system evolved. Adding sensors/actuators/intelligence to the substrate will be discussed in the next major section.
3.2.2. SARAN WRAP MODEL / SOCCER BALL MODEL / PANTY -
Figure-5. Saran Wrap vs. Panty Hose Model vs. Soccer Ball Models
The Saran wrap model is an extension of current technology directions, and involves a continuous thin foil substrate with conventional interconnects on it (see Fig 5(a)). As discussed above, the key issues to overcome are the issues of flexing, bending, and stretching. All near-term demonstrations of sensitive skin are likely to be based on this model.
It would be highly desirable in the long run to have a sensitive skin which is extensible and conformal, i.e. like panty hose (Fig. 2(b)). In this case the extensibility or bending is not achieved by modifying the bulk material characteristics but rather by a system of fibers which themselves can bend in three dimensions (i.e. the individual fibers stretch far less than the fabric as a whole). A key concept of this model is that these fibers are the interconnect themselves (electrical wires or optical fibers). Thus the mechanical support and the interconnection functions are combined into a single system, the fabric itself. As in the Saran-wrap model, the sensors/ intelligence are added later to a universal fabric or substrate.
The soccer ball model is one based on relatively rigid tiles, which are connected by flexible interconnects. The interconnects would thus have to flex and stretch an extreme amount, because all of this action would be concentrated in the interconnects. Thus in this model a critical issue is the flexible/extensible interconnect, and the problem then reverts to the one discussed above â€œ with either the Saran wrap or panty hose models as solutions. Thus this model was not further discussed, and attention was focused on the Saran wrap and panty hose approaches.
3.3. ADDING SENSING / INTELLIGENCE / ACTUATION
Once a substrate/ interconnect fabric has been constructed, one must add the sensors/ intelligence to the fabric/ substrate. This can be done by:
3.3.1. A HYBRID APPROACH
The hybrid concept is similar to that used in printed circuit boards today, in which finished chips are attached to a network of wiring. Key issues are associated with the handling and placement of the sensor/intelligence units, so that a high density of reliable connections can be made at low cost. Especially attractive for this approach are recent advances in fluidic self-assembly based on surface mechanical or chemical forces.
Figure-6 .hybrid approach: attach prefabricated sensor intelligence units (e.g.
chips) to the substrate/fabric
3.3.2. AN INTEGRATED APPROACH
The success of integrated circuits has been in large part based on the ability to integrate more devices using thin film technology into a single product. Following this model, one would want to directly integrate the devices for the sensors, intelligence, etc directly onto the substrate /fabric with the interconnect. This approach may have a systems advantage over the hybrid approach in that one can locate the sensors /intelligence wherever one wants them, as opposed just to local areas represented by the attached chips. Furthermore, the high costs and reliability issues associated with hybrid assembly could be avoided.
Figure-7. Integrated approach: directly fabricate sensors / transistors / circuits
directly on the substrate.
This approach is very application specific and depends on materials compatibility issues, To achieve flexibility, etc., it may be necessary to fabricate hard islands of devices on a soft substrate. Rather than directly integrate all functions onto one substrate, one attractive approach would be to fabricate multiple thin film substrates with different functions, which could then be bonded together in a continuous fashion to achieve the integrated system.
Besides the compatibility issue, a critical issue for sensor/ intelligence integration is that of pattern definition. The ability to directly print either an etch mask or the electronic materials themselves, in patterned form might be an enabling technology which could lead to much lower cost products. Although it is unlikely that the line widths achievable with printing would approach those of conventional IC manufacturing, resulting in lower performance of the electronics or perhaps of the sensors manufactured with the technique, the lower performance might be sufficient for many applications.
3.3.3. DISTRIBUTED INTELLIGENCE APPROACH:
Up to this point, the sensing/intelligence/actuation function has been though of as separate from the interconnect function, as in Fig. 1. This led to a separate discussion of how to make the interconnect/substrate network and of how to attach/integrate the sensors/electronics. A critical long-term goal would be to integrate the intelligence into the interconnect network itself.
On a straightforward scale, this could mean using a network of optical fibers to locally sense some property (e.g. strain or temperature). In a long term, one needs more sophisticated intelligence. A very attractive long-term approach merging different concepts discussed above would be a fabric woven of fibers, where the fibers are not conventional fibers but rather very thin strips of devices (Fig. 8) and interconnect on flexible substrates, such as thin plastic or metal foils (Fig. 5). The electronics/sensors would first be fabricated (perhaps on 1 large 2-D area and then cut into fibers), and then woven into a fabric.
Figure-8. Distributed intelligence approach: embed Intelligence / sensing /
actuation in the interconnect /fibers themselves.
4. DEVICES FOR SENSITIVE SKIN
4.1. DEVICE CAPABILITIES SOUGHT FOR SENSITIVE SKIN
From the device point one might wish a Sensitive Skin to have some of the following capabilities:
Â¢ Flexible or deformable, Can be tiled or cut, This aspect ties in to cost and repair ability, High detectivity, On-skin switching and signal processing, Fault tolerances by distributing functions/computing, or protect processor units. Transmission by wire or optical fiber, or wireless: RF, UHF, free-space optical.
Â¢ Power by wire photovoltaics, RF, fuel cells, micro engines, or from energy harvesting - (skin-integrated mechanical power generators). Power storage in batteries. Or as fuel for fuel cells and micro engines.
Â¢ Sensitive Skin sensor components will be deployed in two dimensional arrays of sufficiently high density
Â¢ Smaller arrays may be of use as well: the key feature is that the skin should allow, by itself or with appropriate data processing, to identify with reasonable accuracy the points of the machine's body where the corresponding sensor readings take place.
Â¢ Self-sensing ability of the skin is highly desirable; this may include sensing of contamination, dust, chemical substances, temperature, radiation, as well as detection of failure of individual or multiple skin sensors and the ability to work around failed areas.
Â¢ The ability to measure distance to objects would be a great advantage for enabling dexterous motion of the machine that carries the skin.
Â¢ Ideally, sensors and their signal processing hardware would be spread within the array so as to allow cutting it to any shape (disc, rectangle, an arbitrary figure) without losing its sensing and control functionality. This suggests interesting studies in hardware architecture.
Â¢ Sensor arrays with special or unique properties are of much interest, for example a cleanable/washable skin for "dirty" tasks in nuclear / chemical waste site applications; radiation-hardened skin for nuclear reactor and space applications; and skins that can smell, taste, react to, or disregard ambient light.
4.2. LARGE-AREA ELECTRONICS IS COMING OF AGE
Sensitive Skin will be a form of large-area electronics, and a large-area electronics industry already does exist. Flat panel displays, including active matrix liquid crystal displays and plasma panel displays, are products of this industry. The medical X-ray sensor panels that are in pilot use likewise are large-area electronic products. These flat panel products use glass plates for substrate and encapsulation, and are rigid. Flexible, active circuit technology is just coming out of the research laboratory, like OLEDs on plastic foil, laser crystallized polysilicon on polyester, TFTs on polyamide, and OLEDs integrated with TFTs on steel foil. In other words, the basic technology for flexible skin electronics is coming together.
Figure-9. Flexible active electronics. (Penn State University.)
4.2.1. ORGANIC ELECTRONICS AND OPTOELECTRONICS ON
Organic thin film transistors (OTFT) are based on a new class of materials called conjugated polymers. Organic thin film transistors are considered as a competitive alternative to the traditional inorganic semiconductor based thin film transistors. In terms of performance, organic materials are not likely to catch the inorganic semiconductor based transistors, however, low cost, large area, and reel-to reel manufacturing can bring new opportunities where inorganic electronics cannot obtain.
The capability of plastic-based displays provides broad applications for industrial and product designers. The technical venture plans to create flexible organic-TFT technology, which has the potential to dramatically reduce the cost of display back planes while enabling the fabrication of lower cost flexible display devices.
Organic materials are poised as never before to transform the world of circuit and display technology. The future holds tremendous opportunity for the low cost and sometimes surprisingly high performance offered by organic electronic and optoelectronic devices. Using organic light-emitting devices (OLEDs), organic full-color displays may eventually replace liquid-crystal displays (LCDs). Such displays can be deposited on flexible plastic foils, eliminating the fragile and heavy glass substrates used in LCDs, and can emit bright light without the pronounced directionality inherent in LCD viewing, all with efficiencies higher than can be obtained with incandescent light bulbs.
Figure-10. Organic electronics on flexible substrates
4.2.2. THIN FILM MEMS ON FLEXIBLE SUBSTRATES
The fabrication of silicon electronics into sensitive skin backplanes can be integrated with silicon based sensor devices. Among these, silicon photodetectors are the most prominent. Silicon transistor/photosensor cells would follow the structure of amorphous silicon based photosensor arrays. An important recent development is thin film micro electromechanical (MEMS) devices on plastic substrates. These devices demonstrate that mechanical sensors (and actuators) can be built on the type of flexible substrate that sensitive skin requires.
4.2.3. NANOSTRUCTURES ON FLEXIBLE SUBSTRATES
The progress in microelectronics has been associated with scaling of the minimum feature size of integrated circuits. This trend described by the famous Moore's law is now running out of steam as this minimum feature size approaches the values where limitations related to non-ideal effects become important or even dominant. At the same time, the opposite trend of increasing the overall size of integrated circuits has emerged stimulated primarily by the development of flat panel displays. Emerging technology of nanostructures on flexible substrates promises to merge these opposing trends and lead to the development of ultra large area integrated circuits embedded into electrotextiles or into stretchable and flexible ''sensitive skin''.
A2B6 AND A4B6 SENSORS ON FLEXIBLE SUBSTRATES
In this section, we briefly review recently emerging technology of polycrystalline A2B6 and A4B6 compounds deposited on flexible substrates and even on cloth, at temperatures close to room temperature. These polycrystalline films, with grains oriented on average in the same direction, might be used for photosensors, as well as for proximity and tactile sensors.
Another application of these materials is for flexible solar cells for on-board power supply for sensitive skin and/or wearable electronics applications. This approach to fabricating sensitive skin is based on a new process of depositing polycrystalline CdSe (1.75 eV), CdS (2.4 eV), PbS (0.4 eV) , PbSe (0.24 eV) and CuS (semiconductor/ metal) films on flexible substrates at temperatures close to room temperature (eV here are electron-volts). Large area surfaces can be covered. Also, ternary and quaternary compounds as well as heterostructures can be deposited. The work is under way to develop all basic device building blocks and basic devicesâ€from ohmic contacts to pâ€œn junctions, heterojunctions, solar cells, and thin film. As an example of such a system prototype, shown in Fig. 6 is a one-dimensional photoconductive array.
Figure-11. One-dimensional photoconductive array fabricated on a flexible
Also under way is the work to develop semiconductor threads and semiconductor cloth. These semi conducting and metal films will serve as building blocks for thin-film technology, which will enable us to develop the sensitive skin arrays. Their properties are strongly affected by processing. For example, the dark resistance of the CdSe films can be reduced by more than five orders of magnitude using thermal annealing in the temperature range from 100oC to 200oC. The photosensitivity of PbS films can be increased by few orders of magnitude by annealing in the temperature range of 110â€œ140oC and optimized ambience. More recently, a new technique of increasing photosensitivity of CdS films processed at temperatures close to room temperatures has been proposed. These new material systems are ideally suited for sensitive skin applications, since these films are suitable for development of optical, thermal, piezoelectric and pyroelectric sensors.
4.3. MANUFACTURING OF LARGE-AREA SENSITIVE SKIN
The materials needed for the printing of sensor circuits include metallic conductors, insulators, semiconductors for transistors and light emitters, piezoelectric materials, etc. This approach to the printing of active circuits explores the territory that lies between ICâ„¢s and printed-wire boards. In effect, sensitive skin devices will contain active circuits monolithically integrated with their packaging. Completed thin-film circuits are at most a few micrometers thick. Therefore, the substrate and encapsulation constitute the bulk of the finished product. Reduction of their weight and thickness becomes important. When the substrate is reduced to a thickness where it becomes flexible, it also becomes usable in continuous, roll-to-roll paper-like production. The finished circuit then is a flexible foil, and using equally thin encapsulation will preserve this flexibility. Rugged thin-film circuits are a natural consequence of the mechanics of thin foil substrates. In devising printing techniques for fabricating sensitive skin, the questions of feature size and of overlay registration must be answered. The development of microelectronics has shown that the search for high pattern density is one of the main drivers of IC technology. Therefore, it is instructive to estimate the density of active devices that could be produced by using conventional printing techniques.
The physical limits of several printing techniques are considerably finer than the resolution and registration of conventional printing equipment. Laser writing can produce a resolution of the order of 1 micro meter. Nanoimprinting has demonstrated a resolution in the tens of nanometer range. The density of directly printed devices can be raised orders of magnitude above ~ 10000 per square centimeter.
4.3.1. DIRECT PRINTING
In order to fabricate novel devices that incorporate ink jet printed organic light emitting diodes and integrated active circuits based on printed organic logic components, it is highly desirable that all the other circuit components and connections can be printed with a compatible technology. These components and connections can include resistors, capacitors, diodes, inductors, sensors, transducers, and interconnects. Because the material properties and the material patterns are optimized best when done in separate steps, the application or modification of active material in IC fabrication is separated from its patterning. To directly print active circuits, one must devise materials that can be applied and patterned in a single step.
Figure-12. Direct patterning deposition Non-contact printing Minimum material
The printing process deposits aerosolized liquid particles as small as 20 nanometers in diameter using aerodynamic focusing. The droplet / particle beam can currently be focused down to a 25-micron diameter. Approximately one billion particles per second can be deposited, with accuracies on the order of 25 microns. Once the materials are deposited, they are usually post-treated to achieve densification and chemical decomposition to produce desired electrical and mechanical properties. This can be done either thermally or by a laser processing step depending upon the deposition material and substrate combination being used. A wide variety of other conductor materials have been formulated into inks that can be jetted and decompose cleanly into pure metal at low temperatures.
Superior MicroPowders has developed innovative inks, which build on its expertise in advanced particle technology and the development of suitable molecular precursors that can be converted to functional components. A typical ink consists of particles, a molecular precursor to the functional phase, vehicle, binder, and additives. Both mixtures of molecular precursors with and without particles can be formulated for inks. Particles can be microâ€œ or nanoâ€œsized particles.
4.3.2. LASER WRITING
In the electronics industry, the trend toward miniaturization of both components and subsystems has largely overlooked mesoscale passive devices (~10 Ã‚Âµm to 1 mm in size), mostly because of difficulties with their fabrication and performance. This is changing as the continuous drive for new electronic and sensor devices pushes current technologies to their limits. New processes are required to increase the density and reduce the size of mesoscale passive devices, while at the same time simplifying their manufacturing and accelerating their prototyping times.
To build mesoscale patterns and arrays on even the most delicate structures, engineers combine a standard laser-writing techniqueâ€laser-induced forward transferâ€with the vacuum-based MAPLE process. Material deposition begins when the beam of a high-repetition-rate, 355-nm ultraviolet (UV) laser is focused through a transparent support onto a 1- to 10-Ã‚Âµm matrix-based coating on its opposite side (see figure-13). The coating transfers to the receiving substrate and, with some thermal processing, forms an adherent film with electronic properties comparable to devices fabricated by typical thick-film approaches such as screen printing.
Figure-13(a) Laser direct writing
Figure-13. Laser direct writing (Fig 13)
The technique forms electronic circuit patterns with feature resolution smaller than 10 Ã‚Âµm by synchronously moving the ribbon to a fresh, unexposed region and then moving the receiving substrate approximately one beam diameter. The resulting individual mesoscopic bricks of electronic material, one per laser shot, are then assembled into the desired pattern. A rapid ribbon change from a metal to a dielectric and back to a metal allows building parallel-plate capacitors or other three-dimensional (3-D) structures. When the ribbon is removed, the MAPLE DW system has all the attributes of a laser micromachining system. Thus, engineers can etch grooves or vias in the substrate, preclean the surface, or even surface anneal or etch individual components to improve their performance or dimensional accuracy. MAPLE DW represents a paradigm shift for conventional electronic manufacturing and prototyping processes. The ability to generate components on demand any place on any substrate in a matter of minutes, instead of weeks, will provide engineers with a unique opportunity to bring new designs to life that are infeasible with today's manufacturing techniques.
Nanoimprint is an emerging lithographic technology that promises high-throughput patterning of nanostructures. Based on the mechanical embossing principle, nanoimprint technique can achieve pattern resolutions beyond the limitations set by the light diffractions or beam scatterings in other conventional techniques. This article reviews the basic principles of nanoimprint technology and some of the recent progress in this field. It also explores a few alternative approaches that are related to nanoimprint as well as additive approaches for patterning polymer structures. Nanoimprint technology can not only create resist patterns as in lithography but can also imprint functional device structures in polymers. This property is exploited in several non-traditional microelectronic applications in the areas of photonics and biotechnology.
The ability to replicate patterns at the micro- to the nanoscale is of crucial importance to the advance of micro- and nanotechnologies and the study of nanosciences. Critical issues such as resolution, reliability, speed, and overlay accuracy all need to be considered in developing new lithography methodologies. The primary driver for reliable and high-throughput nanolithography is the ability to make ever-shrinking transistors on an IC chip.
Figure-14. Nanoimprint lithography
The principle of nanoimprint lithography is quite simple. As shown in figure-14, NIL uses a hard mould that contains nanoscale features defined on its surface to emboss into polymer material cast on the wafer substrate under controlled temperature and pressure conditions, thereby creating a thickness contrast in the polymer material, which can be further transferred through the resist layer via anO2 plasma based anisotropic etching process. Nanoimprint lithography has the capability of patterning sub 10 nm features, yet it only entails simple equipment and easy processing. This is the key reason why NIL has attracted wide attention within only a few years after its inception.
5. SIGNAL PROCESSING
5.1. FAULT TOLERANCE
One of the most important characteristics of any sensitive skin system architecture should be the carefully planned incorporation of fault detection and tolerance. Faults are likely to occur in these systems from a number of different sources:
Sensor failure due to manufacturing defects or field conditions.
Network failure due to skin punctures, seams, or environmental noise.
Processor failure due to environmental stress.
While some of these failures are likely to be permanent and some transient, the overall system must be designed from the start to be fault tolerant. This observation has significant implications on the design of the entire system. Signal processing algorithms must be able to process data from an irregular array. This requirement already exists for flexible surfaces where spatial location can vary over time, even relative spacing on the surface.
The benefit of such a scheme is that the resulting structure remains regular, allowing the main algorithm to be unmodified. These approaches can introduce significant latency in response time, which could be a limiting factor in the face of transient faults. Network algorithms must be highly adaptive in order to route around congestion points and failed links.
The best performing algorithm naturally depends on the rate and characteristics of faults, though it is likely that a hybrid approach that incorporates multiple models will need to be developed. Higher-order functions, such as analysis and control applications, will also benefit from a hybrid and hierarchical structure that naturally presents multiple views of the system state and allows an overarching controller to select from the most reliable view.
History strongly suggests that these properties must be designed into each level of the system from the very beginning. Fault-tolerance is much like security in a computer system, and past attempts to address failure modes late in the design process have almost always failed to produce acceptable results.
5.2. DATA REDUCTION
A second key system characteristic that was discussed was the need for throwing away as much information as possible as early as possible. This approach has been used in order to achieve low power in wireless sensor systems. Custom VLSI ASICs are almost always more efficient at specific tasks than programmable processors. When used as front-end processing blocks for radios and sensors, they can be viewed as filters that thrown away most of the incident signal. By making these decisions as early as possible, the system is able to put more powerful components (e.g. processors) into lower-power sleep modes.
What are the core DSP tasks We feel that the core signal processing tasks are likely to begin operation right at the sensors themselves. Simple FIR or IIR filtering will be needed in order to reduce traffic on the primary networks and thus conserve battery power. Filtering will begin at the analog front end, but many opportunities exist for customized digital or programmed filtering as well. Similarly, Xerox has used wavelets and FFTs as a simple method for achieving dense data compression. These transforms are applied as close to the sensor as possible, with the sensors only reporting a few of the most significant coefficients. While complex and rich FFTs are likely to be too expensive to implement, simple and coarse analyses have proven very effective at dropping the false-alarm rate for interesting events to an acceptable level.
A whole set of higher-order DSP algorithms are likely to be used in these systems. Examples include:
Event detection and classification
Unfortunately, these algorithms are really classes of algorithms and it remains unclear how much system or architecture sharing can prove beneficial. For example, consider one system that classifies events over a 10 ms time period using data from a square meter of sensors, and a second that responds in 1 ms over 10 square meters. It is unlikely that one software architecture could distribute and analyze the appropriate data for both applications. It may occur that a coarse structuring model will emerge that can be reused across applications and levels of a hierarchical application. One simple model, involving overlapped gangs of sensors feeding processing modules, was discussed at the meeting. This approach also provides some spatial overlap, which helps in fault-tolerance. However, it remains to be seen just how far such a model can be refined and remain useful.
5.4. DATA PROCESSING
The signal-processing section spent some time discussing what signal processing meant in the context of sensitive skin. Clearly, signal processing means traditional signal processing (and more specifically DSP) with all of the associated transforms and methods for analyzing signals. However, in the context of sensitive skin, we believe that signal processing really includes into a deeper set of tasks.
As was discussed above, fault-tolerance must permeate all aspects of the system design from the very start. As a consequence, distributed and robust algorithms for communications and processing will be used through out the system. Similarly, as a consequence of dynamic system conditions, multiple analysis tools will need to be fused together to give a robust system view.
The properties of these fusions will vary over time as components fail and are repaired, or fall into the shadow of interference sources. It is possible that most of the work in signal processing will involve constructing an environment that lets engineers build high performance systems. This environment must present a software abstraction of the hardware resources that allows developers to quickly build highly optimized applications. Sensitive skin should be thought of as a sophisticated and high performance array for sensing and computing, not just a rich input channel. Much of the work in building effective signal processing systems for sensitive skin will need to go towards making an effective environment for applications, in addition to the effort spent developing the signal processing applications themselves.
6.1. HUMAN SKIN OR WEARABLE SKIN
Wearable sensor skins have started to appear in preliminary forms such as the Data Glove, which measures finger joint positions for human-computer interface (HCI). These wearable skins for HCI can be expanded to include body suits which not only measure joint angles, but could also measure and apply contact pressures, to give people a much higher dimensional and more natural interaction with computers. Obvious HCI applications are in training, education, and entertainment.
In the biomedical area, wearable sensitive skins can be used to restore sensory capability to people who have lost fine sensation in extremities (such as diabetics), or to people with spinal cord injuries. A relatively simple sensitive skin garment could be used to prevent pressure sores in bedridden or wheel chair bound people. A wearable sensitive skin would also be useful for overall physiological monitoring, such as frostbite detection. If the wearable sensitive skin can also include even a simple actuation capability, a very wide range of further biomedical applications becomes promising. For example simple distributed actuators could be used in applications such as thermoregulation, functional neuromuscular stimulation, smart compression for lymphatic system drainage, or controllable damping/stiffness for tremor reduction. Of course, the sensitive skin is not limited to the strain, vibration, and temperature senses of human skin. Proximity sensing would be a useful capability for the visually impaired. For military applications, sensors for laser, radar, chemicals, or puncture would be quite valuable.
By 2010, the "dream soldier will have sensors built into a skintight uniform." After 10 years, every piece of clothing will include some electronics,"
6.2. SENSITIVE SKINS FOR MACHINES
If machines are to work nimbly in cluttered environments or with humans, they need sensitive skins with proximity and contact sensors. These sensors would provide information so the machines could protect both themselves and people they work with. For human-computer interaction, robot companions could respond appropriately to human touch. Moving vehicles could have an intelligent skin, which allows easier navigation in tight spaces, for example maneuvering automobiles on crowded streets.
6.3. ENVIRONMENTAL SENSITIVE SKIN
Even fixed structures as simple as floors and walls could have improved functionality using a low-cost sensitive skin. For example, a floor with distributed pressure sensors could be used for tracking, or a safety measure to warn of slippery spots or report falls. In civil engineering, skins for buildings and bridges can warn of fatigue or impending failure. For human computer interaction, surfaces could respond to gestures and infer intent, such as changing a lighting level.
6.4. ACTUATED SENSITIVE SKIN
There is overlap between applications of passive sensitive skin and the whole area of active surfaces such as drag reduction in aero- and hydrodynamics. For example, active surface furniture such as chairs could increase comfort for people sitting for long periods of time. Active sensitive skin on walls could be used for sound and vibration canceling.
Sensitive skin is a large array of sensors embedded in a flexible, stretchable, and/or foldable substrate that might cover the surface of a moving machine. By endowing these machines with ability to sense their surroundings, sensitive skin will make it possible to have unsupervised machinery in unstructured, unpredictable surroundings. Sensitive skin will make the machines cautious and thus friendly to their environment. With these properties, sensitive skin will revolutionize important areas of service industry, make crucial contributions to human prosthetics, and augment human sensing when fashioned into clothing. Being transducers that produce and process information, sensitive skin devices will be generating and processing data flows in real time on a massive scale, which will lead to yet another leap in the information revolution. Sensitive skin presents a new paradigm in sensing and control. It is an enabling technology with far reaching applications, from medicine and biology to industry and defense. The state of the art in the areas that are basic to development of the skin technology shows that highly efficient devices should be feasible, meaning by this high density of sensors on the skin, and hierarchical and highly distributed real time sensor data processing. All this non withstanding the fact that the existing prototypes are clumsy, have low resolution, accuracy and reliability, and are not yet ready for commercialization. Serious research issues elaborated in this paper have to be resolved before sensitive skins can become a ubiquitous presence in our society. We hope the readers will view this paper as our first effort to map out the new territory, and as an invitation to join in the exploration.
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