Ординатура / Офтальмология / Английские материалы / Artificial Sight Basic Research, Biomedical Engineering, and Clinical Advances_Humayun, Weiland, Chader_2007

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Figure 7.1. System diagram of a chronic high-resolution retinal prosthesis.

a video-processing unit for transforming the video data to stimulation data for the electrical pulses delivered to the retina. This data is sent through a wireless link to avoid any penetrating wires that would cause discomfort, open up the possibility of infection to the patient, and require frequent care after the implantation. The stimulation data is received at the implant unit by a data receiver. This data acts as an input to a stimulator that generates the electrical pulses to the retina. A flexible cable transmits the electrical outputs to an electrode array that interfaces the outputs of the stimulator to the retina. All the electronics inside the eye receive power wirelessly, through a power receiver unit. The electronics in the external unit operate from a battery. In an ideal case, the stimulator, data recovery, and power recovery units will be integrated into a single integrated circuit (IC) leading to a system- on-chip (SoC). All the electronics have to be shielded from the biological fluid inside the eye, and vice versa. This requires a hermetic, biocompatible package that protects the electronics. Also, all the implant components inside the eye such as coil, cable should be biocompatible for realizing a chronic implant. The device should function safely and reliably for many years to avoid frequent surgeries which would be needed if there are catastrophic failures. The reliability of electronics refers to the breakdown of any device due to constant stress over a long period of time. The breakdown of devices can occur if the package fails and direct contact is made with biological fluid.

The quality of vision restored through the retinal prosthesis is highly dependent on the resolution of the image, which translates to the number of stimulation sites on the retina. Simulations of prosthetic vision suggest that 600–1000 electrodes will be required to restore visual function to

a level that would allow reading vision, independent mobility, and facial recognition [7–10]. A retinal prosthesis with 1000 stimulation outputs is currently under development [11–14]. Many challenges exist in the design of the electronics in order to realize such a chronic high-resolution retinal prosthesis. This article will examine the challenges, and describe ways to overcome these challenges, especially in design and technology areas. We hope that the challenges will also serve as motivation for the researchers in the field to develop new solutions. We observe three basic requirements as the challenges for design of electronics for a chronic high-resolution retinal prosthesis.

1.Increase flexibility – This refers to the amount and variety of functions of the device. More functionality is needed for the patients to improve the quality of the vision, and for the physician to monitor the implant after the implantation.

2.Minimize power – This refers to the power efficiency of the device. In other words, it refers to the amount of power consumed during the operation. Less power consumption translates to less heat dissipation from the electronics and longer battery life, both of which are desirable in any electronic prosthetic device.

3.Reduce size – This refers to the footprint of the device. Miniaturizing the electronics will ease the surgery procedure and will occupy a small volume inside the eye.

As it often happens in engineering design, the three requirements form three corners of a triangle as shown in Figure 7.2. A great deal of flexibility needs to be built in the prosthetic devices for several reasons. The exact requirements for the prosthesis (e.g. stimulation threshold) vary between different patients. Furthermore, these requirements can vary even after the implantation and willl need to be tuned for optimal performance. This requires the device to be programmable externally by both the patient and the physician. Also in a chronic implant, periodic monitoring of the implant is essential, especially in the initial period after the implantation. For the monitoring to be efficient, there needs to be test patterns built in, which the physician can activate using an external interface. This flexibility requires additional circuits to be built which consumes area and power during periodic operation along with standby power consumption. Simply stated, a flexible system, which is naturally complex, results in larger size and consumes more power, compared to a less flexible device. In a wireless system, increasing the size of the receiving element usually reduces the effort (power) to receive the wireless signal. So, a receiving system consuming large area requires smaller power than the system consuming small area. The future sections will describe several challenges related to the three corners of the triangle. The inevitable tradeoffs that have to be made while overcoming one of the challenges, while increasing the other, will also be discussed.

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Figure 7.2. Challenges for a chronic high-resolution retinal prosthesis.

External Video Processing Unit

From the first look at Figure 7.1, it might seem ideal that the retinal prosthesis consisting of electronics would have all components fully implanted such that communication with any external devices is minimized or even eliminated. But migrating some of the functionality from the implant to the external unit is advantageous in many ways.

•It reduces the area and power consumption of the electronics that would be part of the implant unit.

•It allows for upgrades of functionality by programmable hardware or software, or even by attaching additional hardware to the external unit.

•It allows the image information to be processed and compressed so that the forward telemetry data rate can be reduced.

•It is possible to enhance the quality of the prosthetic vision which requires significant power.

It is well known that the human retina is not a mere receptor of photonic information, but performs significant image processing due to its layered neural network structure. Since retinal prosthesis can stimulate only a limited number of neurons, it is crucial to enhance this limited perception by means of image processing. These enhancement techniques include edge detection and enhancement, zoom, contrast and brightness adjustment. An external image processing also provides the users the option of tuning their individual devices based on their visual experience after the implantation. The external unit can also supplement the visual sensation provided by the retinal prosthesis with audio information to enhance the quality of the vision. Such a system is described in Ref. [15]. A digital camera mounted on the patient’s eyeglasses or head captures images. The contents of the captured image are determined through real-time image-processing algorithms. Following the image processing, a sentence is constructed describing the object’s attributes. This method can be used to train the system with the images of critical objects that the patient

frequently comes across, so that it can enhance the performance of identification of those objects used in day-to-day life. The challenge faced in designing the external video-processing unit is to reduce the size and weight so as to make it wearable. The unit should also have an easily accessible user interface that can allow the patient to perform the necessary image enhancement adjustments. This user interface can be a visual, tactile, or audio or a combination of these. The interface between the output of this sensing system and the hardware that performs the function should be highly reliable since any malfunction could confuse the patient. An error detection system that can reset the system to the standard mode or prompt the patient to see assistance should be built in. The external unit can also be used by the physician during diagnosis to apply test patterns through software system in the clinic. Thus the unit should also be able to interface with known software.

Large Stimulation Voltage

The typical biphasic pulse used for stimulating the retina is shown in Figure 7.3. The time interval between the two phases is called the interphase interval. Usually the cathodic phase is the leading one which depolarizes the cell membrane and elicits a neural response. This is followed by the anodic phase which is used for balancing the first phase so that no net charge accumulates at the electrode site. The interphase interval separates the pulses slightly so that the second pulse does not reverse the physiological effect of the first pulse. The amount of charge generated at the stimulation site, which should be above the threshold for creating a response from the stimulation, is equal to the product of the amplitude of the current pulse and the pulse width. The rate at which the stimulation pulses are delivered is called the stimulation rate and is expressed in pulses per second. Experiments have shown that non-flickering perception can be achieved with stimulation rates of 40 to 50 Hz [3]. The biphasic pulse can be generated by using one or two supply voltages. In Figure 7.4a, two supply voltages are used. Two switches control the flow of current by turning ‘on’ the required path in a mutually exclusive fashion. In Figure 7.4b, only one supply voltage is used. A careful observation will show that while in Figure 7.4a, only one connection lead is needed from the electrode site to the biphasic current generator, two connection leads are needed from the electrode site as in Figure 7.4b.

In a high-resolution device with 1000 electrodes, Figs. 7.4a and 7.4b need 1001 and 2000 connection leads, respectively, from the electrodes to the stimulator IC. The additional one electrode is needed for the common ground connection. Since most retinal prostheses are aiming to increase the number of electrodes and thus the resolution, Figure 7.4a is highly preferred. Also, considering that it is a great challenge to place 1000 electrodes, it is important to convert every interconnection lead from the electrodes on the retina to the stimulator IC to a stimulation site. But using two supply voltages (usually equal) presents a challenge of accommodating a large voltage in the chip. For example, if the individual supply voltage is 5 V, then the maximum voltage in the chip is 10 V. In reality, this

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Figure 7.3. Biphasic stimulus pulse.

voltage is even higher since additional headroom of 1 to 2 V is consumed by the power recovery circuits to convert the AC voltage to the required regulated DC levels. In semiconductor device technology, operating supply voltage levels are scaled down along with reduction in device dimensions. This is necessary to avoid the breakdown of the gate oxide and the impact ionization that occurs at the drain due to high fields if voltage levels are not reduced. Table 7.1 shows the feature sizes, and the number of metal layers (used for interconnecting circuits) available for some process technologies from different vendors. This data was derived from MOSIS, an IC fabrication service [16]. As it can be seen from the table, except for some special processes, most CMOS processes operate at less than 5 V. In general, smaller feature size and more metal layers are available for lower operating voltages. Smaller feature size helps in reducing the area of the circuits (also referred to as active area) and more metal layers, the interconnect

Figure 7.4. Biphasic stimulus generation using (a) one supply voltage (b) two supply voltages.

demultiplexing scheme is used, the electrodes which do not require simultaneous stimulation should be grouped together.

Another challenge for high-density retinal prosthesis is the requirement of a large data rate for forward telemetry. The data rate depends on the number of electrodes, number of bits per stimulation, and stimulation rate. The stimulation data consists of information on cathodic and anodic pulse amplitudes, pulse widths, whether anodic or cathodic is the leading pulse, and interphase interval. Also additional information for connecting the electrodes to the common ground potential to remove any residual charge (this is required for ensuring tissue safety) need to be transmitted. If we consider 20 bits for stimulation with a stimulation rate of 50 Hz, for 1000 electrodes, a data rate of 1 Mbps is required. This calculation assumes that it is not necessary to address each electrode separately. In this way no address bits are required. But if the flexibility of arbitrary stimulation is required, each electrode needs to be addressable with a 10-bit address. This increases the data rate by 50%, to 1.5 Mbps. If additional data such as error detection and configuration data are included, then the required data rate can go up to 2 Mbps. An increase in data rate is usually accompanied with increase in power consumption of the data transmitter and/or receiver. The challenge of achieving high data rate will be addressed in the coming sections. At this point it is not clear how much flexibility would be needed for a 1000 electrode retinal prosthesis in terms of stimulation sequence, delay between two arbitrary stimulations. So for a test device, the implant controller has to be designed to provide the maximum flexibility to enable the physician to conduct various experiments on different parameters.

Powering of the Retinal Implant

Both the external unit and the implant unit need electrical power for their operation. For the external unit, battery is a natural choice for the energy source and it should be rechargeable. Currently, lithium-ion battery is the most widely used rechargeable battery due to its high energy density 0 2 Watt hours/cm3 and long shelf-time (> 10 years) [18]. With the size of a cell phone battery, the capacity is about 5 Watt hours. If 20 hours of continuous operation is required, the maximum power it can support is 0.25 Watts. This includes the power dissipated by the camera, external unit, and the implant unit. If more is needed, obviously the size (and weight) of the battery has to be increased. The overall power dissipation of the system has to be maintained at a low level to implement an external unit which is wearable. In order to minimize the battery size, the implant powered by the external battery should consume low power and the power transfer from the external battery to the implant should be highly efficient.

The retinal implant needs power for the stimulator to stimulate the tissue. In addition, other circuits such as the voltage regulation, data telemetry, and implant controller also need power for operation. Also, for continuous real-time imaging, a continuous supply of power is necessary. The amount of power needed depends

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on the stimulation current requirements. At this time, the optimal stimulation parameters (amplitude, frequency, and pattern) are not clearly known. The total power dissipation of the implant with 1000 electrodes is estimated to be less than 100 mW. In order to provide the full-scale functionality, the system should be able to transmit the maximum power. In addition, it would be ideal if the power transmission adaptively adjusts the transmitted power by sensing the required implant power.

Powering the retinal implant through an implantable battery is not an option due to the stringent size limitation and relatively large power requirement. With 100 mW power capacity and continuous operation for 20 hours, the minimum size of the lithium-ion battery would be around 10 cm3, which is larger than the size of an eyeball. In addition, the number of rechargeable cycles is also limited (about 500), which makes it impossible for the requirement of more than 10year lifetime. This shows that an implantable battery is not feasible for chronic high-resolution retinal prosthesis, unless the power requirement is dramatically reduced through technology advancements.

A conductive tethering wire connecting a battery in the external unit to the implant unit is another straightforward way to provide power. But wire penetrating through the skin may require continued medical supervision to guard against any infection. In retinal prosthesis, rapid eye movement can break any penetrating wires passing through the sclera (tissue envelope covering the eyeball except the cornea). In addition to the technical difficulties, it also poses inconvenience to the patients. Considering the difficulties of dealing with a percutaneous wire in an implantable device, especially for long-term implantation, wireless methods of power delivery are preferred. Inductively coupled coil pair, infrared power link, and thermal-to-electric conversion are some of the methods of obtaining power wirelessly. Of the wireless methods of power transfer, inductive link is the widely used one. It can continuously provide relatively large amounts of power (up to several Watts) with reasonable efficiency. The challenges associated with an efficient inductive power transmission design are addressed in the next section.

Wireless Power Transmission

Coil, the Key Component for High Power Efficiency

A coil is formed when a conducting wire is wound into one or several loops [Figure 7.6a]. The magnetic field lines generated by each loop of the coil combine with the lines generated by other loops to produce a concentrated field at the center of the coil. When another (receiving) coil is placed close to the transmitting coil, the magnetic field generated by the transmitting coil passes through the windings of the receiving coil. Now, these two coils are magnetically (inductively) coupled and energy transfer between the two coils is possible.

For a coil, there are several important properties such as self-inductance (L), resistance (R), and self-resonating frequency 0. For a given size of a coil, the

Figure 7.6. Coil (a) conceptual inductive coupling between two coils (b) turns and strands of a coil.

inductance is roughly proportional to the square of the number of turns (N ) [19]. The effective resistance of the coil is a function of the coil geometry and increases with the frequency of the electrical current passing through it [20]. The selfresonating frequency is determined by the coil inductance and the capacitances between the windings of the coil. In general, the coil has to be operated at a frequency below its self-resonating frequency. Beyond the self-resonating frequency, the coil loses its inductive property and acts like a capacitor.

A coil is generally characterized by its quality factor, Q, which is defined as:

In Eq. (1) is the angular frequency of the current, L is the inductance, and R is the resistance of the coil. For high efficiency power transfer, the Q of the coils should be maximized. From Eq. (1), the quality factor can be increased by decreasing the resistance of the coil, increasing the inductance, or increasing the frequency. To decrease the resistance, a phenomenon called skin effect should be considered. Skin effect refers to the phenomenon of increase in the resistance of a conductor when AC current passes through it. The current tends to concentrate on the surface of the cross section of the conductor (also called current crowding), thus increasing the equivalent resistance. Higher the frequency, more severe is the skin effect. The wire diameter of the coil should be chosen according to the frequency of the electrical current to minimize the skin effect. One effective way to reduce the skin effect is to construct the coil with several insulated wires (strands) twisted together (e.g. Litz wire) to increase the equivalent cross section conducting area while keeping the individual wire small. However, the maximum number of strands is limited by the physical size of the coil. For a given size, the number of strands trades off with the number of turns since the total number of wires that a coil can hold is fixed. Increasing the number of strands per turn will reduce the number of turns, and thus the inductance is reduced. The number of strands needs to be optimized for a given design to