Biomimetics - Biologically Inspired Technologies - Yoseph Bar Cohen Episode 2 Part 1 doc

ed.,Electroactive Polymer EAP Actuators as Artificial Muscles — Reality, Potential and lenges, ISBN 0-8194-4054-X, SPIE Press, San Diego, California, Vol.. 1–671.Bar-Cohen Y., Electroact

10.6 MILESTONE FOR THE FIELDImproved collaboration among developers, users, and sponsors as well as increased resources with

a growing number of investigators have led to rapid progress in the field In the last two yearsseveral milestones have been made in this field:

(a) In December 2002, the first commercial product emerged in the form of a Fish-Robot (Eamex,Japan) An example of this Fish-Robot can be seen at http://eap.jpl.nasa.gov where there is also alink to a video showing Fish-Robots swimming in a fish tank These robots swim without batteries

or motor and use EAP materials that simply bend on stimulation For power they use inductive coilsthat are energized from the top and bottom of the fish tank

In 1999, in an effort to promote worldwide development towards the realization of the potential

of EAP materials an arm wrestling challenge was posed to the engineers worldwide (Bar-Cohen,

Figure 10.11 (See color insert following page 302) A new class of multi-limbed robots called Limbed Excursion Mobile Utility Robot (LEMUR) is under development at JPL (Courtesy of Brett Kennedy, JPL.)

Figure 10.12 Artists’ drawing of the solid state IPMC-based aircraft concept (Courtesy of Anthony Colozza, Ohio Aerospace Institute, Cleveland, Ohio, U.S.A.)

2001) Success in developing such an arm will lead to the possible use of EAP to replace damagedhuman muscles, that is making ‘‘bionic human.’’ A remarkable contribution of the EAP field would

be to see, one day, a handicapped person jogging to the grocery store using this technology

A graphic rendering of this challenge that was posed by the author is illustrated in Figure 10.13.The intent of posing this challenge was to use the human arm as a baseline for the implementation

of the advances in the development of EAP materials Success in wrestling against humans willenable biomimetic capabilities that are currently considered impossible It would allow applyingEAP materials to improve many aspects of our life where some of the possibilities include smartimplants and prosthetics (also known as cyborgs), active clothing (de Rossi et al., 1997), realisticbiologically inspired robots as well as fabricating products with unmatched capabilities Recentadvances in understanding the behavior of EAP materials and the improvement of their efficiencyled to the historical first competition held in March 2005 In this competition, three robotic armsparticipated and the human opponent was a 17-year-old female student The three arms were made

by Environmental Robots Incorporated (ERI), New Mexico; Swiss Federal Laboratories forMaterials Testing and Research, EMPA, Dubendorf, Switzerland; and three senior students fromthe Engineering Science and Mechanics Department, Virginia Tech

1 The arm that was made by Environmental Robots Incorporated (ERI), New Mexico held for 26 secagainst the 17-year-old student This wrestling arm (see Figure 10.14) had the size of an averagehuman arm and it was made of polypropylene and Derlin This arm was driven by two groups ofartificial muscle One group consisted of dielectric elastomeric resilient type that was used tomaintain an equilibrium force and the other was composed of ionic polymer–metal composites(IPMC) type strips that flex to increase or decrease the main resilient force

2 The Materials Testing and Research, EMPA, Dubendorf, Switzerland, arm (see Figure 10.15) heldfor 4 sec before losing This arm was driven by the dielectric elastomer type using multi-layeredscrolled actuators that were organized in four groups A photo of one of the group lifting two5-gallon water containers (about 20-kg) is shown in Figure 10.16 Using electronic control, theseactuators were operated similar to human muscles, where two of these groups acted as protagonistsand the other two operated as antagonists The arm had an outer shell made of fiberglass that wasused as a shield for the electric section The arm structure was made of composite sandwichconsisting of fiberglass and carbon fibers

Figure 10.13 An artistic interpretation of the Grand Challenge for the development of EAP-actuated robotics.

3 The arm that was made by the three senior students from the Engineering Science and MechanicsDepartment, Virginia Tech (see Figure 10.17) managed to last 3 sec As an EAP actuator theyconstructed batches of polyacrylonitrile (PAN) gel fibers that were designed to operate as artificialmuscles This EAP material was shown experimentally to produce close to 200% linear strain and

Figure 10.14 The ERI arm wrestling with the 17-year-old human opponent, Panna Felsen This arm has the size

of an average human arm and it managed to last for 26 sec against Panna.

Figure 10.15 The arm that was made by the Swiss Company, EMPA, is shown wrestling with Panna Felsen The rubber glove that the Panna is using provided her electrical insulation for protection.

pulling strength that is higher than human muscles (Schreyer et al., 2000) To encase the fibers andchemicals that make up their EAP actuator, they designed an electrochemical cell For the skeleton

of the arm they used a structure that is made of composite material and, for support, this structurewas connected to an aluminum base

This competition has been a very important milestone for the field and helped accomplish the goals

of this challenge, namely:

1 promote advances towards making EAP actuators that are superior to the performance of humanmuscles;

2 increase the worldwide visibility and recognition of EAP materials;

3 attract interest among potential sponsors and users;

4 lead to general public awareness since it is hoped that they will be the end users and beneficiaries inmany areas including medical, commercial, and other fields

Figure 10.16 One of the groups of EAP actuators made by EMPA lifting two 5-gallon water containers.

10.7 SUMMARY AND OUTLOOKFor many years, EAP received relatively little attention due to their limited actuation capability andthe small number of available materials In the last 15 years, a series of new EAP materials haveemerged that exhibit large displacement in response to electrical stimulation The capability ofthese new materials is making them attractive as actuators for their operational similarity tobiological muscles, particularly their resilience, damage tolerance, and ability to induce largeactuation strains (stretching, contracting, or bending) The application of these materials asactuators to drive various manipulation, mobility, and robotic devices involves multi-disciplinesincluding materials, chemistry, electromechanics, computers, and electronics Even though theforce of actuation of existing EAP materials and their robustness require further improvement, therehas already been a series of reported successes in the development of EAP-actuated mechanisms.Successful devices that have been reported include a fish-robot, audio speakers, catheter-steeringelement, miniature manipulator and gripper, active diaphragm, and dust wiper The field of EAPhas enormous potential in many application areas, and, judging from the range of inquiries that theauthor has received since his start in this field in 1995, it seems that almost any aspect of our livescan potentially be impacted Some of the considered applications are still far from being practical,and it is important to tailor the requirements to the level that current materials can address UsingEAP to replace existing actuators may be a difficult challenge and therefore it is highly desirable

to identify niche applications where EAP materials would not need to compete with existingtechnologies

Space applications are among the most demanding in terms of the harshness of the operatingconditions, requiring a high level of robustness and durability Making biomimetic capability usingEAP material will potentially allow NASA to conduct missions in other planets using robots thatemulate human operation ahead of a landing of human For an emerging technology, the require-ments and challenges associated with making hardware for space flight are very difficult toovercome However, since such applications usually involve producing only small batches, theycan provide an important avenue for introducing and experimenting with new actuators and

Figure 10.17 (See color insert following page 302) The Virginia Tech students’ arm being prepared for the match against Panna Felsen, the 17-year-old student from San Diego.

devices This is in contrast to commercial applications, for which issues of mass production,consumer demand, and cost per unit can be critical to the transfer of technology to practical use.Some of the challenges that are facing the users of EAP materials in expanding their potentialapplications to space include their capability to respond at low or high temperatures.Space applications are of great need for materials that can operate at single digit degrees ofKelvin or at temperatures as high as hundreds of Celsius as on Venus Another challenge to EAP

is the development of large scale EAP in the form of films, fibers, and others The requireddimensions can be as large as several meters or kilometers and in such dimensions they can beused to produce large gossamer structures such as antennas, solar sails, and various large opticalcomponents

In order to exploit the highest benefits from EAP, multidisciplinary international cooperativeefforts need to grow further among scientists, engineers, and other experts (e.g., medical doctors,etc.) Experts in chemistry, materials science, electromechanics or robotics, computer science,electronics, etc., need to advance the understanding of the material behavior, as well as developEAP materials with enhanced performance, processing techniques, and applications Effectivefeedback sensors and control algorithms are needed to address the unique and challenging aspects

of EAP actuators If EAP-driven artificial muscles can be implanted into a human body, thistechnology can make a tremendously positive impact on many human lives

This field of EAP is far from mature and progress is expected to change the field in futureyears Recent technology advances led to the development of three EAP-actuated robotic armsthat wrestled with a 17-year-old female student who was the human opponent in the competitionheld on March 7, 2005 Even though the 17-year-old student won against the three armsthe competition helped increase the visibility of the field worldwide and the recognition ofits potential While more work is needed to reach the level of winning against humans it isinevitable that this would happen just like the chess game between the champion and theBig Blue IBM computer (http://www.geocities.com/siliconValley/lab/7378/comphis.htm) Ini-tially, the challenge is to win a wrestling match against a human (any human) using a simpleshape arm with minimum functionality However, the ultimate goal is to win against the strongesthuman using as close as possible a resemblance of the shape and performance of the human arm.Once such a robotic arm wins against humans, it would become clear that EAP performance hasreached a level where devices can be designed and produced with the many physical functions ofhumans with far superior capability Such a success is one of the ultimate goals of the field ofbiomimetics

ACKNOWLEDGMENTSome of the research reported in this chapter was conducted at the Jet Propulsion Laboratory (JPL),California Institute of Technology under a contract with National Aeronautics and Space Admin-istration (NASA)

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11 Biologically Inspired Optical Systems Robert Szema and Luke P Lee

CONTENTS

11.1 Introduction 291

11.2 Camera Eyes 292

11.2.1 A Fluidic Adaptive Lens 292

11.2.2 An Artificial Cephalopod Eye 293

11.2.3 A Foveated Imaging System 294

11.3 Compound Eyes 296

11.3.1 Appositional Compound Eyes 297

11.3.2 Superpositional Compound Eyes 298

11.3.3 Hybrid Appositional or Superpositional Compound Eyes 303

11.4 Other Biomimetic Approaches 304

11.4.1 Brittlestar Eyes 304

11.4.2 Melanophila Acuminate Beetle 305

11.5 Conclusion 305

References 307

Website 308

11.1 INTRODUCTION

Of the five senses, the mechanism of sight is perhaps the most diverse in the animal kingdom There exist at least eight generalized types of optical systems with numerous variations within each classification This is to be expected, as each animal-eye is tailored to the specific needs of its owner From the defense-oriented pinhole clam eye to the night-adapted owl eye, nature has provided a plethora of examples to study and emulate

The ability to reproduce biological optical systems using man-made materials has applications

in navigation systems, specialized detectors, and in surveillance cameras Of late, there has been particular interest within the military which has provided much of the funding towards research in this field Advancements in materials science and manufacturing technologies have shown to be invaluable in the construction of biomimetic optics

Biomimetic optics is a relatively new and expanding field, although it can be argued that older technologies such as photographic cameras already mimic biology by having analogous structures (i.e., glass lens to biological lens, film to retina, etc.) However, this chapter is devoted to those optical devices, which, by their design, seek to imitate living organisms The examples that follow

291

offer a survey of the ever-increasing attempts to reconstruct biological eyes They are roughlydivided by the general classifications of the eyes they seek to replicate; that is, they are separatedinto biomimetic camera (single lens) eyes, compound eyes, and others.

11.2 CAMERA EYESCertainly the most familiar example, the human eye is but one of many forms of camera-type eyesand generally relies on a single lens to focus images onto a retina for image acquisition The lensmaterial properties, structure, and focusing mechanism vary from organism to organism Forexample, some amphibian species have eyes which accommodate by moving a lens closer orfarther to the retina By contrast, the human eye adjusts the curvature of the lens itself to accomplishthe same task Birds have the added benefit of being able to reshape the cornea as well as the lens foraccommodation Some of the various camera eye designs are shown below (Figure 11.1).These natural eyes have provided the inspiration for a number of optical systems with specificcapability requirements These include different approaches for adaptive optics, efficient imageprocessing, and size-constrained wide-angle views

11.2.1 A Fluidic Adaptive Lens

Some forms of camera eyes, oftentimes in amphibious animals, use hydraulics to adjust their focallengths A chamber behind the lens is filled or emptied with fluid depending on the desired focus(Figure 11.2) One example of this is the whale eye, where this design allows for good vision both

in and out of the water In addition, the fluid also compensates for increased pressure at deeperaquatic environments

Lens Ciliary muscle

Retina

Mammal Eye

Brucke's muscle Brucke's muscle

Fish Eye

Bird Eye Reptile Eye

Protractor lentis

Amphibian Eye

Figure 11.1 (See color insert following page 302) Various types of camera-type eyes The arrangement of the mammalian ciliary muscle allows for passive changes in lens thickness Brucke’s muscles attached to bony ossicles in reptiles and birds, on the other hand, actively change the lens thickness Birds have an additional muscle, Crampton’s muscle, which can alter the shape of the cornea The protractor lentis in some amphibian eyes moves a fixed-shape lens closer to or farther from the retina for accommodation.

By using similar principles, Zhang et al (2003) at the University of California at San Diego havecreated an adaptive fluidic lens The lens itself is made of an inexpensive polymer, polydimethyl-siloxane (PDMS), processed using soft lithography to include a fluid chamber and injection port.The 60-mm thick PDMS membrane is then bonded to a glass substrate using oxygen plasmabonding technology By filling the chamber, Zhang et al were able to demonstrate a focal lengthrange from 41 to 172 mm, with corresponding numerical aperture values of 0.24 to 0.058 Thehighest recorded resolution was 25.39 lp/mm in both horizontal and perpendicular directions Theseresults are shown in Figure 11.3.

This unique design allows for a variable focal length system in a rather compact and robustarrangement It should be noted, however, that it may be possible to improve upon their design bystudying nature further It is well known that a homogenous spherical lens will suffer from sphericalaberration, when the peripheral light rays are refracted more than the axial ones In the biologicalworld, this problem has been managed in two ways The first, and most appropriate to this design, is

to have a nonspherical profile such that the periphery of the lens is flatter than the center Byfollowing nature’s example again, Zhang et al may achieve even better results

11.2.2 An Artificial Cephalopod Eye

As alluded to in the previous section, there is a second method nature has used to deal with sphericalaberration (Land, 1988) This involves the use of a ball lens with a spherically-symmetric refractiveindex gradient that decreases from the center outwards (Figure 11.4) This is a particularlyappropriate adaptation to the watery habitat of cephalopods, such as octopi and squid Such anenvironment necessitates that the entire focusing power of the eye lie within the lens itself, as bothsides of the cornea consist of essentially the same medium In this arrangement, a spherical lensprovides the shortest possible focal length The result is a wide field of view from a relativelycompact apparatus

The theory that cephalopods use a spherical lens with a refractive index gradient was initiallypostulated in the latter half of the 1800s, first by Maxwell and later by Matthiessen (Land, 1988).Indeed, it was Matthiessen who determined that the ratio of the focal length to the lens radius isapproximately 2.5 (‘‘Matthiessen’s ratio’’) in animals with lenses of this design A precise math-ematical description of the gradient was not established until 1944 (Luneberg, 1944), followed by anumerical solution in 1953 (Fletcher, 1953) Still, it was not until 33 years later that Koike et al.created an artificial ball lens with the required index of refraction gradient (Koike et al., 1986).Besides the lens, construction of an artificial cephalopod eye involves a critical design issue.The retinas of many animals including cephalopods are curved structures, whereas man-madephotodetector arrays are flat This has much to do with the way electronics are manufactured ingeneral, on flat semiconductor surfaces Hung et al (2004) have overcome this limitation by

Variable fluid

Figure 11.2 Biological fluidic adaptive lens schematic The thickened sclera allows the eye to withstand pressures

at increased diving depths.

fabricating a unique array where individual photodetectors are connected by flexible structures ontop of a PDMS polymer substrate These S-flexures are a key requirement in the development of anartificial cephalopod eye, as each pixel remains connected to those around it while allowing aflexible, curve retina (Figure 11.5).

While research is ongoing, it is apparent that the development of this type of optical system will

allow for a much wider field of view (180 to 2008) than conventional cameras At the same time, the

device will maintain a compact arrangement, allowing for space-efficient implementation invarious applications

11.2.3 A Foveated Imaging System

A final example of a camera-type system borrows more from the strategy of certain living organismsthan from the design A commonly observed trait in animals is the ability to scan a scene in order toincrease their field of view Many animals, including humans, have a much higher resolution in the

y = − 0.4859x + 7.9069

R2 = 0.9797 5

center of their field of view than towards the periphery This is known as foveated imaging andallows for a relatively wide angle of view with the option of detailed resolution by scanning.Optical engineers are often faced with a similar challenge of increasing the field of view whilemaintaining resolution Traditional methods include decreasing the entrance pupil size (whichincreases the f/# at the expense of resolving power and illumination) and adding optical elements(increasing the complexity, size, and weight of the system) Martinez et al (2001) have devised anartificial foveated viewing system as a unique solution.

Their design uses liquid crystal spatial light modulators (SLM), which are used to manipulateoptical wavefronts Voltages applied to liquid crystals alter the index of refraction such thataberrations are corrected However, only aberrations from a limited range of angles can becorrected at one time (Figure 11.6) Rays of light from different field angles are not corrected,and the result is a region of high resolution surrounded by areas of low resolution

By appropriately varying the SLM, the optical system effectively scans with a narrow field ofview of high resolution while maintaining peripheral vision much like the human eye An additionalbenefit of this optical system is a decreased bandwidth requirement for transmitting digital images,

as only a portion of the entire field of view has high resolution The low resolution areas may serve apurpose as well; they may be used as an initial assessment of whether an area warrants highresolution probing

The appeal of insect compound eyes may be due, in part, to their being so different than our own

On the surface they also appear to be more diverse and complex with anywhere from a singleommatidium (individual eye unit) in the ant speciesPomera punctatissima to over 10,000 per eye insome species of dragonflies Again, the various manifestations of compound eyes are customized tothe needs of their users In general, compound eyes are broadly divided into two categories,superpositional and appositional

The individual facets of appositional compound eyes are optically isolated, and each of themprovides part of a scene The result is a series of images slightly offset from one another (see Figure11.7) The advantage of this arrangement is that the images are processed in parallel, leading to

Incident light

Unfocused areas

Focused area Liquid crystal SLM

Image plane

Figure 11.6 (a) A foveated imaging system where incident light rays are directed to a single imaging plane The SLM changes the index of refraction to focus light from a specific direction (b) Sample image from such a system (From Martinez, T., Wick, D., and Restaino, S Optics Express 2001: 8(10), 555–560 With permission.)