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Nanotechnology and Nanosensors Final Project Bionic Eye Students Names 1. Hoffman Adam 2. Ohana Mor 3. Shlenkevich Dmitry 4. Shmuel Maayan

Nanotechnology and Nanosensors Final Project Bionic Eye · PDF fileNanotechnology and Nanosensors Final Project – Bionic Eye Students Names 1. Hoffman Adam 2. Ohana Mor 3. Shlenkevich

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Table of contents

1. Abstract ................................................................................................................... 2

2. Introduction ........................................................................................................... 2

3. Literature review ...................................................................................................... 3

4. Project description .................................................................................................... 7

4.1. Sensor fabrication ............................................................................................. 8

4.2. Sensor characterization .................................................................................... 7

4.3. Sensor application .......................................................................................... 10

5. Conclusions and recommendations ........................................................................ 11

6. References ............................................................................................................. 12

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1. Abstract

In this project, we learned about human sense of vision. People try to imitate this sense for

many decades. In this review, we will see several applications that made to imitate vision

sense. In addition, we will suggest new kind of vision sensor that will help people who lost

their vision. Our sensor will contain three major parts: 1. glasses with camera, 2. lens with

LED ant integrated circuit and, 3. retinal stimulator placed on the retina. The camera will

capture the picture then it will transmit a signal to the lens. From the lens, the signal

transmitted to the retinal stimulator by using LED. There the signal converted to an electrical

pulse and obtained in the brain.

In comparison with other applications, this sensor will be without physical interconnects. The

main problem is that the demanded technology is not developed enough by now, and a future

development is needed.

2. Introduction

In order to understand the purpose of the sensor we need to know how our eye is built. In

Figure 1 we can see the eye and retina structure. The retina converts light into neural

electrical signal. Electrical signal is transported by an optic nerve to a visual cortex of the

brain. There the signal is converted to a picture. The retina is composed of 126 million

photoreceptors, which provide a potential to neural cell layers (horizontal cells, bipolar cells,

amacrine cells, ganglion cells). In this layer, the electrical signal is converted to electrical

pulse and it is transferred with axons of the ganglion cells to a visual cortex of the brain.

Figure 1 - Eye scheme and the cells that build retina.

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Retinal Pigmentosa (RP):

Definition: A condition in which the retina is separated from the sclera as a result of a rupture

that occurs in it. In this situation the retina is isolated from its blood supply and its nutritional

source.

This disease is defined as a sight threatening condition with a frequency of about 1:10000.

Main risk factors: Pathological short-sightedness, previous intraocular surgery, trauma,

former event of retinal pigmentosa, family history.

Symptoms: Light flashes, a feeling of floating bodies in front of the eyes, visual field loss.

Diagnosis: The disease may be diagnosed by visual acuity test and comprehensive

examination of the retina in extended pupil condition with a magnifying glass. In case of

Vitreous Humour bleeding ultrasound test is required.

Treatment: The main treatment method is freezing the rupture in surgery, which leads to the

creation of an inflammatory process that later on creates a scar. This scar allows the

reconnection between the sclera and the separated retina.

A second treatment method is based on injection of gas into the eye, this gas spreads, settles

on the rupture location and closes it. This treatment is efficient only for ruptures that are

located in the upper half of the eye.

The third treatment method is Vitrectomy, which Vitreous Homour surgery for removing

some or all of it. This method is used only in case in which the previous methods are

inefficient.

3. Literature review

Retinal implant:

Retinal implants developed in order to restore partially vision to people who lost it due to

retina degeneration such as retinitis pigmentosa (RP). An example for a retinal implant

manufactured by a company "Second Sight". This prosthesis contains implant (Figure 3) and

external equipment. The retinal implant includes an antenna, an electronics and an electrode

array. The external equipment includes glasses with camera, an antenna and a video

processing unit. Device structure is shown in Figure 2. The picture is obtained by a camera on

the glasses then it is transmitted to the electronic device on the retinal implant. Electrical

signal is transmitted to the electrode array, which is placed on retina. The electrodes cause

electrical signal through the retina and it is transmitted to the brain with the neurological

nerve.

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Bionic Lens:

Another vision imitation device that was developed

is bionic contact lens as shown in Figure 4. These

lens contain LED, power-control circuit,

interconnects, antenna and lens. This device

transmits the light on the retina by using Fresnel

lens. It is impossible to generate images on the

retina by using original lens because of the

minimum focal distance of the human eye. The

solution for this problem is micro lens with a

diameter of 10 µm. These lenses have diffraction,

reflection properties and different focal lengths.

Fresnel lens has a distinctive focusing property and

it resolves the problem of focusing the image from the LED on the retina.

Nanometric solar cells:

A solar cell is a light driven battery - an apparatus whom must create the Direct Conversion of

light into electrical-energy and finally to electricity.

Electromagnetic radiation, the visible and near-infrared regions of the spectrum emitted from

the sun and absorbed by the solar cell. A photon will then excite a negatively charged electron

from the low energy state (valence band) to a higher energy state (the conduction band)

leaving behind a positively charged vacancy, called a hole. For this energy transfer to create

any usable energy, the photon must have energy greater than the band gap of the material, or

else the electron will immediately relax down and recombine with the hole and the energy

will be lost as heat.

Upon excitation above the band gap the photon creates an electron and a hole which are now

free to move throughout the semiconductor crystal. These acts as charge carriers, which

transport the energy to the electrical contacts, results a measurable external current. The

materials and structure of the solar cell are very important in light conversion process. A solar

Figure 2 – Retinal implant structure

Figure 4 - Lens with integrated

circuit and LED

Figure 3 - Retinal

Implant

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cell is made out of a semiconductor material which facilitates the creation and motion of

charge carriers. Current solar cells cannot convert all the incoming light into usable energy

because some of the light can escape. Additionally, sunlight comes in a variety of colors and

the cell might be more efficient at converting a specific part of the spectrum. If excited

electrons are not captured and redirected, they will spontaneously recombine with the created

holes, and the energy will be lost as heat or light.

Solar Cell operation and energetic efficiency depends on:

1. Absorption of light to create electron-hole pairs (carriers).

2. Direct band gap ~1.5 eV, absorbing a vast spectrum of visible light.

3. Diffusion of carriers.

4. Separation of electrons and holes.

5. Collection of carriers.

Electro-optical properties can be tuned by both material selection and quantum confinement,

advances in synthesis allow control over nanocrystal size and shape to optimize performance.

Solar cells may be formed using a pn junction, a Schottky barrier, or a metal insulator

semiconductor structure based on various semiconductor materials, such as crystalline silicon,

amorphous silicon, germanium, III-V compounds, quantum wells and quantum dots structure.

III-V compound semiconductor such as gallium arsenide and indium phosphide. Novel solar

concepts are proposed to further increase the power conversion efficiency using the low-

dimensional structures including hot carriers cell, tandem cell, multiple quantum wells

(MQW) cell and intermediate band solar cell. III-V quantum dot superlattice based solar cells

with promising potential.

Thin film and nanostructured solar cells, advantages:

1. Small thickness contributing to overall high absorption.

2. Extremely small diffusion length and high recombination velocity, donating large

efficiency on a small-scale structure.

3. Economical justified, low weight per power unit.

4. Allows use of all deposition techniques thus enabling new and highly sophisticated

designs donating a very large variety of nano-structures.

5. All types of junctions are possible to form in-situ integration made compatible with

other devices.

6. Tailor ability of various optoelectronic properties.

Nanoscale materials, defined as substances where at least one dimension is less than

approximately 100 nm. nanoscale in zero dimension (Quantum dots), one dimension (surface

films), two dimensions (strands or fibres), or three dimensions (particles). They can exist in

single, fused, aggregated or agglomerated forms with spherical, tubular, and irregular shapes.

Types of nanomaterials include nanotubes, dendrimers, quantum dots and fullerenes. The

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nanometer feature sizes of nanomaterials also have spatial confinement effect on the

materials, which bring the quantum effects. The energy band structure and charge carrier

density in the materials can be modified differently from their bulk and in turn will modify

the electronic and optical properties of the materials as well as reducing imperfections.

Nanomaterials and nanostructures hold promising potency to enhance the performance of

solar cells by improving both light trapping and photocarrier collection. As performance of a

PV device largely relies on both photon absorption and photocarrier collection. Therefore

both factors are optimized. Nevertheless, there requirements in optimizing optical absorption

and carrier collection can be in conflict. For example, in a planar structured solar cell thicker

materials are needed in order to achieve sufficient optical absorption; however, it will lower

carrier collection probability due to the increased minority carrier diffusion path length, and

vice versa. In fact, 3-D nanostructures not only improve light absorption utilizing the light

trapping effect, but also facilitate the photocarrier collection via orthogonalizing the

directions of light propagation and carrier collection.

As mentioned the surface interactions dominate nanoparticle behavior. For this reason, they

often have different characteristics and properties than the bulk of the same material.

Nanostructured layers in thin film solar cells offer three important advantages:

1. Multiple reflections - the effective optical path for absorption is much larger than the

actual film thickness.

2. Light generated electrons and holes need to travel over a much shorter path and thus

recombination losses are greatly reduced. (Thickness under 150 nm).

3. Energy band gap of various layers can be made to the desired design value by varying

the size of nano particles.

Polycrystalline thin-film solar cells such as CuInSe2 (CIS), Cu (In, Ga) Se2 (CIGS), and

CdTe are important for solar applications because of their high efficiency, long-term stable

performance and potential for low-cost production. Because of the high absorption coefficient

(about 1-150 cm), a thin layer of ~2 mm is sufficient to absorb the useful part of the spectrum.

However, the main drawback of this system is oxidation due to the metastable marcasite

structure that is detrimental to PV properties.

Nano structuring:

0D QDs also been extensively studied for PV applications. Featuring unique physical

properties enable new PV mechanism to potentially break current thermodynamic limit.

1D nanocrystaline materials such as semiconductor fibers or nanorods have are of much

considerable interests and their morphology control has been demonstrated.

2D nanowire fabricated using a wide variety of materials. Output efficiencies have been

steadily increased but a number of unresolved questions must be answered before such

materials can be used in commercial devices.

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3D nanocone structure with an aspect ratio height/diameter of approximately one provides an

optimal shape for light absorption enhancement.

CONCLUSION: The growing interest in applying nanoscale materials for solving the

problems in solar energy conversion technology can be enhanced by the introduction of new

materials such as quantum dots, multilayer of ultrathin nanocrystaline materials and the

availability of sufficient quantities of raw materials. The inexpensive purification or synthesis

of nanomaterials, deposition methods for the fabrication of thin film structures and easy

process control in order to achieve a large-area production within acceptable performance

tolerances and high lifetime expectancy are still the main challenges for the realization

(fabrication) of solar cells. Therefore in attaining the main objectives of photovoltaics, the

efficiency of solar cells should be improved without any compromise on the processing cost

of these devices. Nanotechnology incorporation into the films shows special promise in

enhancing the efficiency of solar energy conservation and also reducing the manufacturing

cost. Its efficiency can be improved by increasing the absorption efficiency of the light as

well as the overall radiation-to-electricity.

4. Project description

There are two types of retinal implants: epiretinal implants and subretinal implants. It is

important to understand the difference between them. Epiretinal implants are placed in the

internal surface of the retina and stimulate directly the ganglion cells. Subretinal implants

replaces the degenerated photoreceptors.

In this chapter, we will review the sensor that manufactured for epiretinal implants.

4.1. Sensor characterization

Why do we need a new design for retinal implant?

The disadvantage of retinal implant that was described in literature review, uses electrodes

that are connected by physical interconnects through the human eye to the external

equipment. This scheme includes complicated electrode construction on the human eye.

Therefore the patient should be under medical surveillance all the time. In addition, the

patient should be always with external equipment.

A new retinal implant and vision imitation sensor:

The sensor we suggest is a combination of two devices that were described before. This

device has similar construction to those devices, but it works in a different way. This sensor

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transfers energy wirelessly from the lens on the eye, to the retina electrode array. By using

this scheme there is no need in interconnects which connect the electrode array on the retina

and on eye electronic case with antenna as shown in Figure 3. This implant is more

complicated, but because there are no physical interconnections it is more friendly to the

patient. Therefore, there is no need in a medical surveillance all the time as for implant with

physical interconnects.

Sensor generic construction:

The general structure of retinal implant contain four main units: retina encoder, telemetry link

for power and data transmission, stimulator device and nanoscale solar cell.

The general structure of bionic lens contain four main units: LED, antenna, integrated circuit

ant lens.

How it works:

The retina encoder captures the picture and transmits data to the integrated circuit (IC) on the

lens by using telemetry transmitter and receiver (antenna on the lens). The IC converts the

obtained signal into light signal by using Light Emitting Diode (LED). The light projected on

the nanoscale solar cell, which is a power supply for stimulator device that attached to the

retina. The stimulator transmit electrical signal through the retina to neurological nerve and

then to the brain. By this way, the picture that obtained in the camera is transmitted to the

brain.

4.2. Sensor fabrication

Sensor fabrication can be divided into retinal implant and lens fabrication.

Retinal implant fabrication:

Device components such as image sensor, power\data receiver and stimulator are fabricated

by using standard CMOS technology that is used in industry. With this technology, the

components produced in a Top-down fabrication by using photolithography.

Some of the components must be flexible, such as the stimulator chip. These components are

made on thin polyimide (PI) film with embedded platinum microstructures. The flexible PI

foils serve as carrier insulation layers that hold the platinum/gold/iridium, based

microelectrodes/conductive lines/interconnection pads. The process flow of the flexible foil

fabrication is described in Figure 5.

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The flexibility of the PI thin film allows the contacts of the stimulator to adhere

smoothly to the retina. The chips are made from silicon, their connection to the

flexible foil is produced with special microflex interconnection (MFI) technology,

as described in Figure Figure 6. The connection occurs with gold ball which

serves as a rivet that mechanically and electrically connect the rigid silicon chip to

flexible PI thin foil. With this technology obtained flexible sensor which is

biocompatible.

Lens Fabrication:

The lens system consists of the following components: a light emitting diode, an antenna, an

integrated circuit for power harvesting and polymer substrate with electrical interconnects.

Blue micro LEDS: Produced by multistep process for epitaxial growth of InGaN/GaN buffer

layer on a 50mm diameter and 435𝜇𝑚 thick c-plane, sapphire substrate.

The process starts with the growth of 2𝜇𝑚 thick n-doped GaN layer at 1030 °𝑐 . Then starts

the main part that includes a growth of a multiple quantum well stack consist of five pairs of 3

nm thick InGaN and 20 nm thick GaN barriers that was grown on the n-doped GaN layer. A

50nm GaN capping layer was grown on the stack before the temperature was increased to 970

°𝑐 in order to grow 250 nm of p-GaN. Towards the end of the process, the LED structure was

annealed inside the MOVPE reactor for P-GaN activation.

The doping of p-type and n-type layers was achieved by using of Bis-

cyclopentadienylmagnesium (Cp2Mg) and silane (SiH4).

This blue micro LEDS is designed with peak intensity at ~475nm in order to be adequate to

illuminate the retina.

Figure 6 - MFI

technology

Figure 5 - Flexible microelectronic foil production flow

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Antenna: Composed mainly by 5mm radius metal loop that is designed in accordance to the

contact lens size and eye physiology. The antenna is used for RF energy harvesting without

damaging the pupil.

The frequency performance is determine by two factors:

1. Power received by the on-lens loop antenna.

2. Antenna to chip impedance matching.

CMOS integrated circuit: composed of a p-channel type MOS transistor which is formed on

an n-type silicon substrate and n-channel type MOS transistor which is formed on a P well

formed in the substrate and bipolar transistors which are electrically connected to each other

to form a kind of thyristor structure.

The power supply is applied to a source electrode of the p-channel type MOS transistor

through a part of the substrate, which presents a resistance. The resistance is electrically

connected to the bipolar transistor of the thyristor structure to thereby prevent the occurrence

of a latch-up phenomenon. In which a large current continuously flows through the bipolar

transistors and may destroy the CMOS integrated circuit.

This circuit is designed to perform various needs of the lens system like conversion of RF

power to DC supply voltage and supply energy to excite the micro-LED.

Finally all the components that are mentioned above are soldered together on the polyethylene

terephthalate substrate that was chosen because of its preferable properties like chemical

resistance and thermal stability.

Sensor application

Application of a nanosolar cell integrated with other in-situ systems may have great uses for

many industries. Due to the power crisis world-wide and the advance in nanotechnology and

nanofabrication the integrated in-situ sensor can be applied in many field of life:

1. Space applications – sensor environment where energetically corrosive environment

as proton and electron irradiation is critically shortening life time of traditional

devices, as well as the conservation of weight and energy to size ratio.

2. Information technology and microelectronics - Nano scale solar cells have proven

that their longterm chemical and mechanical stability and size dependent properties of

semiconducting and magnetic nanocrystals yielding are smaller and self-sustainable

can be solutions for even smaller standalone devices.

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3. Biomedical applications – all in-situ bio related processes may be able to relate to an

external reliable source, thus energy storing may be a critical issue due to insufficient

and discontinues solar energy.

5. Conclusions and recommendations

The device that we wrote about comes to help people who have lost their sight as a result of

retinal detachment. The device is based on a combination of two existing standards:

1. Install the electrode array that connects the eye and makes an appeal retina which causes an

electrical signal to the brain switch and the ability to see some of the objects.

2. The contact lens which has a processor and LED that can radiate light on the retina.

System components:

1. Glasses with Camera + image processing computer.

2. LED lens- like contact lenses, located on the patient's eye.

3. An install on the retina of the eye with nanoscale solar cell.

The role of the solar cell is to absorb photons from the LED and convert their energy into

electric energy and provide power array electrodes which cause passage of the signal to the

brain.

The camera-captured image is processed, and transfers to a signal which absorbed on the lens.

On the lens there is a processor which translates the signal that comes from the LED. The

photons that emit from the LED will be absorbed in the nano solar cells which are located on

the retina. Photon energy is translated into an electrical energy which passes within the retina.

Meaning, the camera-captured image on the glasses were translated into an electrical signal

and transmitted to the brain.

The novelty of the device is that there is no need in physical wires that connect the electrode

array, which is on the retina. Instead, the signal is transferred using photons. Thus, the device

is more friendly, and does not require often visits to doctors for surveillance.

Problematic issues- Currently carried out an experiment with LED devices that have only a

single pixel and required further development of lenses for building a LED with many pixels

so that it will be possiable to move it to different places on the retina. Furthermore, we do not

know about the efficacy of solar nano cells, and probably their further development is needed

for this purpose.

In order to improve the device we suggest adding future enhancements, such as improved

LED with a wider array of pixels or using more effective nano solar cells.

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6. References

1. http://en.wikipedia.org/wiki/Retinal_implant

2. http://en.wikipedia.org/wiki/Bionic_contact_lens

3. Jorg-Uwe Meyer, "Retina implant – a bioMEMs challenge", Fraunhofer Institute for

Biomedical Engineering, Ingbert, Germany.

4. A.R. Lingley, "A single-pixel wireless contact lens display", University of Washington,

USA.

5. http://electronics.howstuffworks.com/gadgets/other-gadgets/contact-lens-displays2.htm

6. http://www.kfupm.edu.sa/centers/CENT/AnalyticsReports/KFUPM-TFSC-Dec20.pdf

7. RECENT TRENDS ON NANOSTRUCTURES BASED SOLAR ENERGY

APPLICATIONS: A REVIEW, Suresh Sagadevan, Rev. Adv. Mater. Sci. 34 (2013) 44-

61.

8. Nanowire dye-sensitized solar cells MATT LAW, LORI E. GREENE.

9. Hybrid Solar Cells with Prescribed Nanoscale Morphologies Based on Hyperbranched

Semiconductor Nanocrystals. Gur, Ilan Fromer, Neil A.