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INV ITEDP A P E R

1 Visual Prosthesis2 Microelectronic implants that provide identification of simple objects and motion

3 detection for blind patients have been tested and evaluated; further development is

4 needed for face recognition and reading implants.

5 By James D. Weiland and Mark S. Humayun

6 ABSTRACT | Electronic visual prostheses have demonstrated

7 the ability to restore a rudimentary sense of vision to blind

8 individuals. This review paper will highlight past and recent

9 progress in this field as well as some technical challenges to

10 further advancement. Retinal implants have now been tested in

11 humans by four independent groups. Optic nerve and cortical

12 implants have been also been evaluated in humans. The first

13 implants have achieved remarkable results, including detection

14 of motion and distinguishing objects from a set. To improve on

15 these results, a number of research groups have performed

16 simulations that predict up to 1000 individual pixels may be

17 needed to restore significant functions such as face recognition

18 and reading. In order to achieve a device that can stimulate the

19 visual system in this many locations, issues of power con-

20 sumption and electronic packaging must be resolved.

21 KEYWORDS | Electrical stimulation; implantable medical pack-

22 aging; medical implants; neural prosthesis; retinal prosthesis

23 I . INTRODUCTION

24 Photoreceptors are the specialized neurons in the eye that25 convert photons into a neural signal (Fig. 1). The

26 photoreceptors are part of the retina, a multilayer neural

27 structure about 200 �m thick that lines the back of the eye.

28 Other cells in the retina process the signal from the

29 photoreceptors. Retinal ganglion cells send the processed

30 signal from the retina to the brain via the optic nerve.

31 Blindness can result when any part of this visual pathway is

32 damaged by injury or disease. Electronic visual prostheses33 are being developed that can be implanted in different

34 anatomical locations along the visual pathway (Fig. 2). While

35 the final implementation of the implant will depend on the

36 anatomy of the targeted area, visual prostheses have common

37requirements for a parallel neural interface and power

38efficiency that make certain engineering challenges impor-

39tant to all visual implants. This paper will review the causes

40of blindness, the microelectronic approaches to treating

41blindness, and technology needs to enable future implants.

42II . CAUSES OF BLINDNESS

43Blindness can result from damage to the optical pathway

44(cornea, aqueous humor, crystalline lens, and vitreous

45Fig. 3) that focuses light on the retina or damage to the

46visual neurons that sense light and send visual information

47to the brain. We will review only the neural diseases, but it is

48worth noting that cataracts (opacities in the crystalline lens)49are a major cause of blindness worldwide.1 An excellent

50review of the retina and vision can be found online.2

51Blindness has a significant impact on the economy.

52Recent studies have found that only 29% of severely

53visually impaired persons are gainfully employed, com-

54pared with a national average of 84% [1]. Persons with

55severe visual impairment earn 37% per year less than their

56able bodied counterparts [2]. The total economic impact57of vision loss in the United States is estimated at nearly

58$68 billion annually.3

59The two most common retinal degenerative diseases that

60result in blindness secondary to photoreceptor loss are age-

61related macular degeneration (AMD) and retinitis pigmen-

62tosa (RP). RP is generally more severe, and its symptoms

63appear earlier in life, but AMD is more prevalent. In the

64United States, there are approximately 700 000 new AMD65patients; each year, 70 000 of these patients will become

66legally blind, with many more suffering significant vision

67loss [3]. AMD results from a slow degeneration of the

68photoreceptor cells of the retina, ultimately culminating in

69photoreceptor cell death. This is sometimes accompanied by

Manuscript received August 14, 2007; revised January 14, 2008.

The authors are with Doheny Eye Institute, University of Southern California,

Los Angeles, CA 90033 USA (e-mail: [email protected]; [email protected]).

Digital Object Identifier: 10.1109/JPROC.2008.922589

1See World Health Organization, www.who.int/mediacentre/factsheets/fs282/en/.

2www.webvision.med.utah.edu/.3See http://www.silverbook.org/visionloss.

Vol. 96, No. 7, July 2008 | Proceedings of the IEEE 10018-9219/$25.00 �2008 IEEE

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fFig. 1. Cross-section of the retina. The photoreceptors (rods and cones) convert photons into neurochemical energy, which is relayed through

the retina. The different layers of the retina processing the image signal. The output of the retina comes from the retinal ganglion cells,

whose axons gather at the optic disc to form the optic nerve. In retinitis pigmentosa and age-related macular degeneration,

the photoreceptors are degenerated but the other layers of the retina remain. (Image courtesy of Webvision, webvision.med.utah.edu.)

Fig. 2. Human visual system. The optic nerve transmits retinal information to the lateral geniculate, which relays the information to the primary

visual cortex (striate cortex or V1). (Image courtesy of Webvision, webvision.med.utah.edu.)

Weiland and Humayun: Visual Prosthesis

2 Proceedings of the IEEE | Vol. 96, No. 7, July 2008

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f70 the formation of new blood vessels. Individuals afflicted

71 with AMD will start to have distorted central vision and

72 eventually will lose most vision in the central 30� of visual73 field, rendering them legally blind (less than 20/200 vision).

74 RP is a collective name for a number of genetic defects that

75 also result in photoreceptor loss [4]. More than 100 genetic

76 defects have been identified that cause the different forms of

77 RP. The overall incidence of RP is 1 in 3500 live births. In

78 general, RP strikes the rod photoreceptor cells first,

79 resulting in poor night vision and loss of peripheral vision.

80 Eventually, cone photoreceptors, which mediate color and81 daytime vision, are lost, leading to complete blindness.

82 Neither AMD nor RP is presently curable through surgery

83 or treatment, but there are some treatments that can slow

84 the progression of AMD [5].

85 Diseases that damage the optic nerve include diabetic

86 retinopathy and glaucoma.4 In diabetic retinopathy, retinal

87 blood vessel abnormalities can prevent nourishment from

88 reaching neural cells in the retina, leading to ganglion cell89 and optic nerve damage. Glaucoma often includes high

90 intraocular pressure as a symptom. In the past, it was thought

91 that high eye pressure was damaging to the retina and led to

92 ganglion cell and optic nerve loss, but more recently it has

93 been found that even individuals with normal eye pressure

94 can have optic nerve damage from glaucoma [6].

95 A. Visual Prostheses96 A visual prosthesis can create a sense of vision by

97 electrically activating neural cells in the visual system. The

98 prosthesis must convert images from a camera into pat-

99 terns of electrical stimulation applied to the tissue by an

100 implanted neural stimulator. Visual prostheses are de-

101 lineated based on the anatomical location of the stimu-

102lating electrode array. Implants in humans have been103tested in the retina, visual cortex, and optic nerve.

104Recent clinical trials of retinal implants have included

105epiretinal implants [7]–[10], a passive subretinal device [11],

106and an active subretinal device [12] (see Fig. 3). Passive

107devices rely on incident light for power, whereas active

108devices have an external power source. All of the devices

109currently being tested are manufactured by companies,

110which have the required quality systems and manufacturing111skills to produce robust, medical-grade implants. The trials

112reviewed below will be referred to by the company that

113produced the implant.

114The first clinical trial of a permanently implanted

115retinal prosthesis was initiated by Optobionics, Inc., in

1162000. The device was a passive microphotodiode array

117with 3500 elements. Subjects in the passive subretinal trial

118do not report pixelized vision, as might be expected if each119photodiode were acting as a photoreceptor [11]. However,

120some of the subjects have reported improved visual

121function away from the implant site, suggesting that the

122presence of the implant alone, or coupled with low-level

123electrical stimulation, induced a Bneurotrophic effect[124that improved the health of the retina and consequently

125improved visual function.

126A prototype epiretinal implant with 16 electrodes is127being tested by Second Sight Medical Products, Inc.

128(Sylmar, CA). This trial began in 2002 and has enrolled

129six individuals with bare light perception secondary to RP.

130Test subjects can use spatial information from the

131stimulator to detect motion and locate objects [9].

132Subjects demonstrated their ability to distinguish between

133three common objects (plate, cup, and knife) at levels

134statistically above chance. Subjects have also demonstrat-135ed that they can discern the direction of motion of a bar

136passed in front of the camera. Recent reports involve

137detection of the orientation of a black and white grating

138pattern [13]. A subject was able to detect these gratings at

139increasing spatial frequency up to the theoretical limit

140predicted by the electrode spacing on the retina.

141Perceptual thresholds are in general low, compared to

142the earlier short-term implants [8]. Some subjects have143shown a majority of electrodes with a perceptual

144threshold below 50 �A (1 ms pulse), with a range of

14524–702 �A (1 ms pulse) reported across three subjects. A

146second clinical trial of an epiretinal implant, built by

147Intelligent Medical Implants AG (Zurich, Switzerland),

148has recently begun. This implant features a microfabri-

149cated electrode array of 49 platinum electrodes on a

150polyimide substrate. Subjects can see phosphenes and151crude shapes that correlate to the applied stimulus [10],

152but, since this a relatively recent clinical trial, no

153threshold data or visual task performance with subjects’

154using a camera has been reported.

155An active subretinal device developed by Retina

156Implant GmbH (Reutlingen, Germany) began clinical

157trials in 2006. An integrated circuit (IC) was implanted4See www.nei.nih.gov/health/.

Fig. 3. Cross-section of a human eye. Light is focused by the cornea

and lens on the retina. The retina lines the posterior of the eye.

(Image from http://www.nei.nih.gov/health/cataract/

cataract_facts.asp.)


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158 underneath the retina. The IC had 1500 microphotodiodes,159 which served to modulate current pulses based on the

160 amount of light incident on the photodiode [14]. This

161 group had not yet developed telemetry electronics, but

162 instead connected to the subretinal IC using a micro-

163 fabricated cable that passed out of the orbit and crossed the

164 skin behind the ear. The cable supplied command and

165 configuration signals as well as power to the IC. Because

166 such a cable arrangement is a potential infection source,167 the implant was limited to 30 days, although one subject

168 continued beyond this point. This implant also included

169 16 Bdirect stimulation[ electrodes, which were not

170 activated by the subretinal IC but were directly connected

171 to the cable and were activated by external equipment.

172 Due to packaging limitations, the subretinal IC did not

173 function in all of the implants. Direct stimulation elec-

174 trodes were able to elicit responses in all subjects [12].175 This device uses constant voltage stimulation and the

176 stimulus levels needed to generate perceptions have been

177 reported as 1–2.5 V for 3 ms. The active subretinal trial is

178 relatively recent, and few details of the clinical investiga-

179 tions are published.

180 Experimental artificial vision systems have also been

181 employed to electrical stimulate other anatomical locations

182in the visual system. The first experimental work towards a183visual prosthesis began with electrical stimulation of the

184visual cortex using a grid of large surface electrodes. This

185has progressed to microelectrode arrays that penetrate

186deep into the cortex. A brief summary of the important

187findings in visual cortex stimulation is given below.

188The seminal experiment in this field (and possibly in

189the field of neural prosthesis) was performed by G. Brindley

190in 1968 [15]. Brindley implanted an 80-electrode device on191the visual cortical surface of a 52-year-old blind woman.

192Each electrode was connected by a wire to a radio receiver

193screwed to the outer bony surface. An oscillator coil was

194placed above a given receiver in order to activate the

195receiver via radio frequency and stimulate the cortex via

196the induced electrical current. With this system, the

197patient was able to see phosphenes in 40 different

198positions of the visual field, demonstrating that at least199half of the implanted electrodes were functional. This

200experiment showed that an implanted electrical stimu-

201lation device could restore some degree of vision.

202However, because surface electrodes were used, large

203stimulus currents were needed in order to produce the

204sensation of light, and the phosphenes produced were

205quite large. A second group using surface stimulation in a

Fig. 4. Retinal prosthesis concept. A retinal prosthesis will capture an image with a video camera. The image data will be processed and

wirelessly transmitted to the implanted stimulator, which will stimulate the retina in a pattern. (Image courtesy of Annual Review of

Biomedical Engineering, vol. 7, pp. 361–401, 2005.)



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206 visual prosthesis has shown similar visual acuity [16] but207 also reported seizures resulting from overstimulation [17].

208 Intracortical electrodes were developed in order to

209 reduce the amount of electrical stimulus current, thus

210 avoiding seizures and producing smaller phosphenes. An

211 array of stimulating microelectrodes was inserted into the

212 occipital cortex of patients who were being treated for

213 excision of epileptic foci [18]. Phospenes were elicited with

214 both surface and intracortical electrodes, but intracortical215 microstimulation required 10–100 times less current than

216 surface electrodes. The same group later performed a long-

217 term implant in the visual cortex of a test subject with

218 glaucoma who had been blind for more than ten years. The

219 38 electrode system was implanted for a period of four

220 months. The subject was able to perceive phosphenes at a

221 predictable and reproducible location of the visual space. It

222 was also demonstrated that several microelectrodes used in223 combination could evoke the perception of patterns. This

224 project was centered at the National Institutes of Health and

225 was discontinued in the 1990s.

226 Recent efforts in visual cortex prosthesis have evalu-

227 ated efficacy of visual cortex stimulation in a nonhuman

228 primate (NHP) model [19]. A stimulating array was

229 implanted in the visual cortex of a monkey. The monkey

230 was trained to reach to a location in the visual field that231 was illuminated by a light. This area was mapped using

232 electrical activity recorded on the electrodes. It was then

233 demonstrated that electrical stimulation on that electrode

234 produced the sensation of a visual percept in the same

235 area, and the monkey was able to perform the same visual

236 saccade task with both light and electrical stimulation.

237 Optic nerve stimulation has been demonstrated in one

238 human test subject [20]. A stimulator connected to a cuff239 electrode with four contacts was implanted in an RP

240 patient. The cuff electrode encircled the optic nerve, which

241 is about 1–2 mm in diameter. Since the electrodes are on

242 the outside of a very densely packed nerve (1.2 million

243 fibers within the nerve), focal stimulation and detailed

244 perception are difficult to achieve. Stimulation through a

245 single electrode sometimes produces multiple percepts

246 throughout the visual field. However, the test subject has247 adopted a strategy of scanning a head-worn camera to

248 achieve remarkable results in pattern recognition. A similar

249 scanning strategy was used by test subject in the epiretinal

250 implant trial.

251 A variety of other visual prosthesis implant configura-

252 tions are in development but have not yet been implanted

253 in humans. Extraocular implants have been developed that

254 stimulate the retina through the sclera in animal models255 [21]. Initial testing of electrical stimulation in the lateral

256 geniculate nucleus has demonstrated spatial sensitivity in

257 this visual center [22]. Retinal implants have shown the

258 most promise in clinical trials to date but are limited

259 because they require at least part of the retina to be

260 functional, so there remains a need for implants proximal

261 to the retina. In any visual prosthesis, to improve the

262quality of the image, a greater number of electrodes will be263needed to provide more information at higher spatial

264resolution. However, as will be discussed below, even

265more complexity will be required to restore significant

266visual function.

267The requirements for a high-resolution retinal pros-

268thesis should follow from the needs and desires of blind

269individuals who will benefit from the device. Our

270interactions with these patients have indicated that the271visual functions that are most important to the blind

272include mobility without a cane, face recognition, and

273reading. A number of simulation studies have estimated

274visual task performance for prostheses of varying acuity.

275These studies are typically performed in normally sighted

276individuals who don an apparatus that limits their vision.

277Cha et al. developed a system to project a defined number

278of pixels on the fovea of a normally sighted person (1–2� of279visual field) [22]. Foveal projection was used because this

280group was developing a visual cortex prosthesis; the foveal

281representation in the primary visual cortex was the target

282of their implant. The pixelized vision simulation system

283consisted of a video camera and monitor worn on the head.

284The monitor was masked by a perforated film to create

285pixels over a visual angle of 1.7� or less (different masks

286were used for different angles). Standard visual acuity287testing demonstrated that 20/30 vision could be achieved

288using 625 pixels in the 1.7 central degrees of the visual

289field [22]. Mobility testing using 625 pixels showed that

290test subjects could easily navigate a maze [23]. Reading

291experiments also using this system with 625 pixels

292determined that reading speeds of 100 words per minute

293with fixed text and 170 words per minute with scrolling

294text could be achieved [23].295Simulations of prosthetic vision with a retinal implant

296have yielded similar results. These simulations differ from

297the earlier work because the activated area of the retina was

298up to 17� of the visual field, which is similar to the coverage

299of some prototype retinal prostheses [2]. Dagnelie et al. have

300studied pixelized vision using a modified low vision301enhancement system (LVES) to pixelize vision and stabilize

302the image on the retina (image stabilization is done through303eye tracking) [23]. Normally sighted subjects with pixelized

304vision were able to recognize faces at rates that were

305significantly above chance with as few as 10 � 10 pixels

306(60% correct versus 25% chance). When 32 � 32 electrodes

307were used, the recognition scores improved to over 80%.

308Pixel dropout of 70% led to worse scores that were

309equivalent to guessing (dropout means the pixel is turned

310off, analogous to a nonfunctional electrode). Using a similar311system (Fig. 3), Hayes et al. studied performance in a

312number of different visual tasks including reading [24].

313Reading speeds of 15 words per minute were possible,

314even with only a 16 � 16 pixelized view. While this is

315below normal reading speed, it does approach a level of

316utility that may be acceptable to a blind individual.

317Mobility testing with a visual prosthetic simulator suggests



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318 that a 6 � 10 array of pixels can be used for navigation319 through an office environment [25].

320 B. Technology Needs for Advanced Visual Prostheses321 New technology is needed to realize an advanced

322 device that effectively stimulates the visual system in

323 600–1000 locations. A visual prosthesis consists of an

324 imaging system to acquire and process video. Power and

325 data will be wirelessly transmitted to an implant by the326 external system. The implant will receive data/power and

327 convert to power and program the implant. The implant

328 stimulator chip will convert digital data to an analog output

329 which will be delivered to the nerves via microelectrodes.

330 An implanted imager is a prominent variant of this system

331 architecture. This would increase the complexity of the

332 implant but remove data telemetry. Intraocular imaging

333 components have been proposed for both subretinal [14]334 and epiretinal [26] implants. All of the components of a

335 retinal prosthesis will require significant engineering

336 research in order to optimize them for a high-resolution

337 implant. In the remainder of this paper, we will focus on

338 engineering challenges in the areas of implant power and

339 packaging.

340 Efficient use of power is a significant issue for a

341 1000-electrode visual prosthesis. This is related to three342 factors: 1) small electrodes are needed to create focused

343 areas of excitation, 2) the relatively high resistivity of

344 biological media, and 3) the output requirements for

345 neural stimulation.

346 The first two factors listed above will determine the

347 load for the current driver. To achieve a high-resolution

348 visual perception, small areas of the retina must be

349 activated. This will require small electrodes. One approach350 to determining the best electrode size is to draw an

351 analogy to natural vision. For example, an individual with

352 20/20 vision can resolve differences of 1/60 of one degree

353 of visual angle, which translates to 5 �m on the retina.

354 Thus, it has been argued that to restore vision to this level,

355 one would need 5-�m-diameter electrodes. Using this

356 argument, Palanker et al. have estimated that a 100 �m

357 electrode could provide vision equivalent to 20/400 [27].358 A second way to determine the best electrode size is to rely

359 on simulations of prosthetic vision, which predict that

360 1000 electrodes are needed to perform desired visual tasks,

361 and then optimize the electrode size, total area covered by

362 the electrodes (the visual angle covered), and number of

363 electrodes. An analysis of this type has been reported

364 previously [28] and suggests that with 1000 electrodes

365 in the macula (center 20� of vision), 100-�m-diameter366 electrodes are acceptable. This diameter electrode will

367 result in a tissue resistance of approximately 30 K� [29]

368 due to the relatively high resistivity of neural tissue

369 (versus metallic conductors).

370 The output requirements for the current drivers are

371 another factor in determining power consumption. Con-

372 stant current pulses are used to activate nerves for several

373reasons. First, it is important to control the amount of374charge applied to an electrode for safety reasons [30], [31].

375Secondly, no net charge can accumulate on the electrode

376to prevent corrosion and gas formation [32]. Finally, a

377constant current results in a constant electric field in

378tissue, AQ1although near a disk electrodes, current distribution

379changes rapidly with time. Human clinical trials have

380shown that in some subjects, a majority of electrodes had a

381stimulus threshold below 50 �A (1 ms/phase, biphasic382pulse). Other subjects, whose stimulating electrodes were

383further away from the retina, had higher thresholds [8].

384Although it will not be discussed further here, an

385imperative for continued improvement of retinal prosthe-

386sis is a surgical technique to consistently position the

387electrodes near the retina.

388The power requirements for a retinal implant can be

389calculated based on the information presented above,390although some assumptions must be made. We have very

391good data on electrode impedance, plus most implants

392have the ability to record impedance as diagnostic data.

393We also have precise perceptual threshold data, although

394this is variable amongst electrodes in a single subject and

395between subjects. For the purposes of this analysis, we

396will assume that we have a 30 K� load and a 100 �A

397output. The instantaneous power at a single electrode is3980.3 mW. This would result in a power requirement of

399300 mW if all channels were continuously active, but

400neural stimulation is done with pulses of current. If the

401electrode is on for 2 ms (1 ms for each phase of a

402biphasic pulse) and the stimulus is applied at 60 pulses/s

403(which has been shown to result in fused percepts in

404acute stimulation trials [33]), then this is a duty cycle of

40512% and the stimulation power requirement is 36 mW.406This analysis assumes that all electrodes are active, but the

407number of electrodes active and the level of activity will be

408dependent on the images incident on the camera which

409will reduce the stimulation power requirement. However,

410in addition to stimulation power, the implant microelec-

411tronics will consume power during operation. For

412example, converting an inductively coupled ac signal into

413dc power will consume energy. It is difficult to predict414exact numbers for power consumption for an implant

415with 1000 electrodes, but it is clear that efficient micro-

416electronic design will be needed to minimize implant

417power.

418Electronic packaging technology must be developed to

419protect the active components from the corrosive saline

420environment in the eye. The package should be virtually

421impervious (or hermetic) to water or ion ingress and422should protect the electronics for at least ten years. At the

423same time, the package must have electrical feedthroughs

424(vias) that electrically connect the circuitry to the

425electrodes interfacing with nerve cells (Fig. 5). However,

426this technology should not add significant size to the

427electronics since the total implant size needs to be small to

428safely fit in the body. This is particularly important for a



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f429 retinal implant, which ideally should fit inside the orbit.430 Also, the various electronic components must be efficient-

431 ly packaged in order to minimize implant size. This last

432 requirement is complicated by the large storage capacitor

433 and large inductors typically used for inductive power

434 transfer.

435 Two bioelectronic packaging schemes have been

436 proposed and developed for medical implants. First is a

437 discrete shell or capsule in which an electronics module is438 placed. The electronics module consists of ICs and off-

439 chip components such as capacitors, diodes, and induc-

440 tors that are difficult to put on chip (Fig. 5). In this

441 scheme, a physical gap exists between the chip and the

442 package. This gap usually is filled with an inert gas or is a

443 vacuum. Discrete packages add size to the overall unit

444 because the package is typically larger than the chip and

445 sometimes involves a complicated assembly process, but it446 allows for flexibility in fabricating the individual

447 components in their own process before assembly.

448 Examples of neural stimulators in clinical use with this

449 type of packaging are BIONs and cochlear implants.

450 BIONs use cylindrical glass capsules with feedthroughs to

451 an electrode on either end of the implant. Cochlear

452 implants typically use ceramic or metal cases and have at

453most 32 feedthroughs. Feedthrough number and density454are a significant impediment to achieving a 1000-channel

455visual prosthesis.

456The second type of biomedical implant packaging relies

457on using a thin film as coating material. This approach is

458more technically difficult to achieve because the coating

459process must conformally coat the entire electronics

460module. If all electronics function can be realized on a

461single system-on-chip, then the coating problem is easier462since there are few contours and crevices to coat (Fig. 5).

463But if there are multiple chips bonded together or large

464discrete components bonded on chips, then small gaps in

465this bond areas are difficult to coat completely. A second

466difficulty in coating is making pinhole free coatings. Even

467small holes will allow saline access to active conductors,

468leading to corrosion. Both organic and inorganic coatings

469have been developed. Organic materials include films470such as epoxies, silicones, and polymers including poly-

471imides, polyurethanes, and parylene, which can typically

472be deposited at lower temperatures than inorganic mate-

473rials. Stieglitz et al. [34] used a parylene coating to protect

474a wireless retinal prosthesis system implanted in a re-

475search animal. The device was not chronically activated,

476but the system was periodically powered to check for

Fig. 5. Packaging schemes for neural implants. (Top) A discrete case is typically used to protect electronic components, but this adds size to

the implant. (Bottom) If a hermetic coating can be developed and if all electronic functions can be placed on a single integrated circuit,

then device size can be reduced. PCBVprinted circuit board, ASICVapplication-specific integrated circuit, RFVradio frequency.



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477 functionality. After 14 months of implantation, the system478 could still activate the retina and produce a cortical

479 response. However, the system was not continuously

480 powered during this period, and it is generally accepted

481 that biased lines will drive corrosion. Polymers are in

482 general considered not adequate for protection of active

483 circuitry for ten years. Water vapor penetration is too

484 rapid. Ceramics and glass have better water barrier pro-

485 perties. These materials include films such as silicon ni-486 tride, silicon carbide, polycrystalline diamond, and metal

487 thin films. Semiconductor materials like silicon, silicon

488 dioxide, or silicon nitride are very popular because they

489 are resistant to many corrosive environments. Hetke et al.490 have demonstrated more than one year of stability of a

491 ribbon cable passivated by a stack of silicon dioxide/silicon

492 nitride stress-compensated dielectrics [35]. Xiao, et al.493 have developed a promising coating based on ultranano494 crystalline diamond and have shown some evidence of

495 stability and biocompatibily [36]. However, the fact that no

496 commercially available neural implant uses an organic or

497 an inorganic coating as a hermetic barrier suggests that this

498 approach is still in the development stage. Thin-film

499coatings continue to be pursued because such a technology500can potentially result in a packaged device that is virtually

501the same size as the chip.

502III . SUMMARY

503Visual prostheses have potential to treat intractable

504medical conditions. While the visual perceptions that are

505created may not have the fine detail of natural vision, there506is ample evidence that individuals can learn to use reduced

507input to perform simple tasks. Subjects in these trials can

508identify simple objects and detect motion. From an

509engineering perspective, one of the more encouraging

510results of these early trials is the low stimulus thresholds,

511which suggest that a larger number of smaller electrodes

512can be used. These early examples are motivating

513engineering research that produces novel technology to514support advanced visual prostheses, which will require

515hundreds of electrodes independently stimulating the

516retina. Among the most significant engineering challenges

517are efficient delivery of implant power and hermetic

518packaging of electronics. h

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ABOUT T HE AUTHO RS

519 James D. Weiland received the B.S. degree, the

520 M.S. and Ph.D. degrees in biomedical engineering,

521 and the M.S. degree in electrical engineering from

522 the University of Michigan, Ann Arbor, in 1988,

523 1993, 1997, and 1995, respectively.

524 He spent four years in industry with Pratt &

525 Whitney Aircraft Engines. He joined the Wilmer

526 Ophthalmological Institute, The Johns Hopkins

527 University, Baltimore, MD, in 1997 as a Postdoc-

528 toral Fellow. In 1999, he became an Assistant

529 Professor of ophthalmology at Johns Hopkins. He became an Assistant

530 Professor at the Doheny Eye Institute, University of Southern California,

531 Los Angeles, in 2001. Currently, he is an Associate Professor of

532 ophthalmology and biomedical engineering, University of Southern

533 California. His research interests include retinal prostheses, neural

534 prostheses, electrode technology, visual evoked responses, and implant-

535 able electrical systems.

536 Prof. Weiland is a member of Sigma Xi. He is a member of the IEEE

537 Engineering in Medicine and Biology Society, the Biomedical Engineering

538 Society, and the Association for Research in Vision and Ophthalmology.

539Mark S. Humayun received the B.S. degree from

540Georgetown University, Washington, DC, in 1984,

541the M.D. degree from Duke University, Durham,

542NC, in 1989, and the Ph.D. degree from the

543University of North Carolina, Chapel Hill, in 1994.

544He is a Professor of ophthalmology, biomed-

545ical engineering, and cell and neurobiology at

546the Doheny Eye Institute, Keck School of Med-

547icine, University of Southern California (USC),

548Los Angeles. He is Director of the National Science

549Foundation BioMimetic MicroElectronic Systems Engineering Research

550Center and the Department of Energy Artificial Retina Project. He

551completed his residency in ophthalmology with Duke Eye Center and

552fellowships in both vitreoretinal and retinovascular surgery at The

553Johns Hopkins Hospital, Baltimore, MD. He stayed on as Faculty at

554Johns Hopkins, where he became an Associate Professor before joining

555USC in 2001. He has been a key member on a number of National

556Academies panels. He has authored more than 120 peer-review

557scientific papers and chapters. He has been invited to participate as

558a Guest Speaker in more than 20 countries. His work on the intraocular

559retinal prosthesis (Bartificial vision[) has been featured prominently in

560more than 500 newspapers and television programs, throughout the

561United States and abroad. He has received 11 patents with numerous

562patents pending. His work has spawned three companies to date.

563Dr. Humayun is a member of 11 academic organizations including the

564IEEE Engineering in Medicine and Biology Society, the Biomedical Engi-

565neering Society, the Association for Research in Vision and Ophthalmol-

566ogy, the American Society of Retinal Specialists, the Retina Society, the

567American Ophthalmological Society, the American Academy of Ophthal-

568mology, and Biomedical Engineering in Medicine and Biology. He was

569voted as one of the Best Doctors in America and has received numerous

570research awards. He was named Innovator of the Year in 2005 by R&D

571Magazine for his outstanding contributions to engineering.



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