Patent Publication Number: US-2015088253-A1

Title: Systems and Methods for Power Management of Implantable Ophthalmic Devices

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims the benefit of: U.S. Provisional Application No. 61/621,193 filed Apr. 6, 2012 and titled “An Application Specific Integrated Circuit (ASIC) For Use In Intraocular Implants”; U.S. Provisional Application No. 61/637,564 filed Apr. 24, 2012 and titled “Electronic Control System for an Intraocular Implant”; and U.S. Provisional Application No. 61/638,016 filed Apr. 25, 2012 and titled “Rechargeable Batteries for Intraocular Implants.” Each of the above-referenced applications is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     There are two major conditions that affect an individual&#39;s ability to focus on near and intermediate distance objects: presbyopia and pseudophakia. Presbyopia is the loss of accommodation of the crystalline lens of the human eye that often accompanies aging. In a presbyopic individual, this loss of accommodation first results in an inability to focus on near distance objects and later results in an inability to focus on intermediate distance objects. It is estimated that there are approximately 90 million to 100 million presbyopes in the United States. Worldwide, it is estimated that there are approximately 1.6 billion presbyopes. 
     The standard tools for correcting presbyopia are reading glasses, multifocal ophthalmic lenses, and contact lenses fit to provide monovision. Reading glasses have a single optical power for correcting near distance focusing problems. A multifocal lens is a lens that has more than one focal length (i.e., optical power) for correcting focusing problems across a range of distances. Multifocal optics are used in eyeglasses, contact lenses, and intra-ocular lenses (IOLs). Multifocal ophthalmic lenses work by means of a division of the lens&#39;s area into regions of different optical powers. Multifocal lenses may be comprised of continuous surfaces that create continuous optical power as in a Progressive Addition Lens (PAL). Alternatively, multifocal lenses may be comprised of discontinuous surfaces that create discontinuous optical power as in bifocals or trifocals. Contact lenses fit to provide monovision are two contact lenses having different optical powers. One contact lens is for correcting mostly far distance focusing problems and the other contact lens is for correcting mostly near distance focusing problems. 
     Pseudophakia is the replacement of the crystalline lens of the eye with an IOL, usually following surgical removal of the crystalline lens during cataract surgery. For all practical purposes, an individual will get cataracts if he or she lives long enough. Furthermore, most individuals with cataracts will have a cataract operation at some point in their lives. It is estimated that approximately 1.2 million cataract surgeries are performed annually in the United States. In a pseudophakic individual, the absence of the crystalline lens causes a complete loss of accommodation that results in an inability to focus on either near or intermediate distance objects. 
     Conventional IOLs are monofocal, spherical lenses that provide focused retinal images for far objects (e.g., objects over two meters away). Generally, the focal length (or optical power) of a spherical IOL is chosen based on viewing a far object that subtends a small angle (e.g., about seven degrees) at the fovea. Unfortunately, because monofocal IOLs have a fixed focal length, they are not capable of mimicking or replacing the eye&#39;s natural accommodation response. Fortunately, ophthalmic devices with electro-active elements, such as liquid crystal cells, can be used to provide variable optical power as a substitute for the accommodation of an damaged or removed crystalline lens. For example, electro-active elements can be used as shutters that provide dynamically variable optical power as disclosed in U.S. Pat. No. 7,926,940 to Blum et al., which is incorporated herein by reference in its entirety. 
     SUMMARY 
     Embodiments of the disclosed technology include an implantable device, such as an implantable ophthalmic device suitable for treating aphakia or pseudophakia. The device can include a first rechargeable battery and a processor operably coupled to the first rechargeable battery. The processor can be configured to charge the first rechargeable battery for a first time interval using a first constant current. The processor can also be configured to charge the first rechargeable battery for a second time interval using a second constant current less than the first constant current. The processor can also be configured to charge the first rechargeable battery for a third time interval using a constant voltage. 
     In some implementations, the first rechargeable battery is a solid-state lithium battery or a lithium-ion battery. In some implementations, the first rechargeable battery has a volume of less than five cubic millimeters. In some implementations, the processor is configured to determine an end of the first time interval when a voltage of the first rechargeable battery exceeds a first threshold voltage. In some implementations, the processor is configured to determine an end of the second time interval when the voltage of the first rechargeable battery exceeds a second threshold voltage. In some implementations, the second constant current is substantially equal to half the first constant current. For example, the first constant current can be from about 20 to about 40 μA. 
     In some implementations, the processor can also include a power conversion module. The power conversion module can be configured to receive power from a power source external to the implantable device and convert the power to the first constant current, the second constant current, and the constant voltage. For example, the power source can be a radio-frequency source or a light source. 
     In some implementations, the device can include a second rechargeable battery operably coupled to the processor. The processor can be configured to charge the second rechargeable battery for a fourth time interval using a third constant current. The processor can be configured to charge the second rechargeable battery for a fifth time interval using a fourth constant current less than the third constant current. The processor can also be configured to charge the second rechargeable battery for a sixth time interval using a second constant voltage. In some implementations, the device can also include an electro-active element operably coupled to the processor. The electro-active element can be configured to modulate at least one optical characteristic of the implantable device. 
     Another aspect of the disclosed technology relates to a method of charging a battery. The method can include charging the rechargeable battery for a first time interval using a first constant current. The method can include determining that a voltage of the rechargeable battery exceeds a first threshold value. The method can include charging the rechargeable battery for a second time interval using a second constant current less than the first constant current. The method can include determining that the voltage of the rechargeable battery exceeds a second threshold value. The method can also include charging the rechargeable battery for a third time interval using a constant voltage. 
     Another aspect of the disclose technology relates to an intraocular optic. The optic can include an electro-active element configured to vary an optical characteristic. The optic can include a sensor configured to generate a sensor signal within less than about 100 milliseconds in response to sensing a change in light level or a physiological response. The optic can include a first control circuit, operably coupled to the sensor, configured to sample the sensor signal and to generate an actuation signal within 100 milliseconds of sampling the sensor signal in response to the sensor signal. The optic can also include a second control circuit operably coupled to the first control circuit and to the electro-active element. The second control circuit can be configured to receive the actuation signal. The second control circuit can be configured to transition from a low-power state to a high-power state and actuate the electro-active element within about 5 milliseconds of receiving the actuation signal so as to vary the optical characteristic of the intraocular optic in response to the actuation signal. The second control circuit can also be configured to transition from the high-power state to the low-power state within about 5 milliseconds of actuating the electro-active element so as to minimize current leakage from the second control circuit. 
     In some implementations, the first control circuit is configured to sample the sensor signal at a period of about 200 milliseconds to about 310 milliseconds. In some other implementations, the first control circuit is configured to sample the sensor signal aperiodically. 
     Another aspect of the disclosed technology relates to a method of altering an optical characteristic of an intraocular optic in response to a change in light level or a physiological response. The method can include sensing the change in light level or the physiological response. The method can include generating a sensor signal within about 100 milliseconds of sensing the change in light level or a physiological response. The method can include sampling the sensor signal with a first control circuit. The method can include generating an actuation signal, with the first control circuit, within 100 milliseconds of sampling the sensor signal. The method can include actuating the intraocular optic based on the actuation signal so as to minimize current leakage from the second control circuit. The method can include receiving the actuation signal at a second control circuit. The method can include transitioning the second control circuit from a low-power state to a high-power state in response to the actuation signal. The method can include actuating an electro-active element with the second control circuit so as alter the characteristic of the intraocular optic within about 5 milliseconds of receiving the actuation signal The method can also include transitioning the second control circuit from the high-power state to the low-power state within about 5 milliseconds of actuating the electro-active element so as to minimize current leakage from the second control circuit. 
     Another aspect of the disclosed technology relates to an implantable device. The implantable device can include a first rechargeable battery having a first voltage, a second rechargeable battery having a second voltage, and a processor operably coupled to the first rechargeable battery and the second rechargeable battery. The processor can be configured to determine that the first voltage has fallen below the second voltage. The processor can be configured to select the second rechargeable battery to discharge in response to the determination that the first voltage has fallen below the second voltage. The processor can be configured to determine that the second voltage has fallen below the first voltage. The processor can be configured to select the first rechargeable battery to discharge in response to the determination that the first voltage has fallen below the second voltage. In some implementations, the processor is configured to perform these steps iteratively. 
     In some implementations, the first rechargeable battery or the second rechargeable battery includes at least one of a solid-state lithium battery and a lithium-ion battery. The first rechargeable battery or the second rechargeable battery can have a volume of less than five cubic millimeters. In some implementations, the processor is further configured to determine that the first voltage has fallen below a first threshold, determine that the second voltage had fallen below a second threshold, and cause a reduction in power flow from the first rechargeable battery and the second rechargeable battery in response to the determination that the first voltage has fallen below the first threshold and the determination that the second voltage had fallen below the second threshold. 
     In some implementations, the device also includes an electro-active element operably coupled to the processor, the first rechargeable battery, and the second rechargeable battery. The electro-active element can be configured to vary an optical characteristic of the implantable device when powered by at least one of the first rechargeable battery and the second rechargeable battery. 
     Another aspect of the disclosed technology relates to an intraocular optic. The intraocular optic includes a sensor configured to sense at least one of a light level and a physiological response. The intraocular optic also includes an electro-active element to vary at least one optical characteristic of the intraocular implant. The intraocular optic also includes a first control circuit, operably coupled to the sensor, configured to sample the sensor signal and to generate an actuation signal within 100 milliseconds of sampling the sensor signal in response to the sensor signal. The intraocular optic also includes a second control circuit, operably coupled to the first control circuit and to the electro-active element. 
     The second control circuit can be configured to receive the actuation signal. The second control circuit can be configured to transition from a low-power state to a high-power state and actuate the electro-active element so as to vary the at least one optical characteristic of the intraocular optic in response to the actuation signal. The second control circuit can be configured to transition from the high-power state to the low-power state of actuating the electro-active element so as to minimize current leakage from the second control circuit. 
     The intraocular optic can also include at least one rechargeable battery, operably coupled to the first control circuit and the second control circuit. The at least one rechargeable battery can be configured to provide power to the second control circuit when the second control circuit is in the high-power state and to be recharged by a first constant current provided by the first control circuit over a first time interval, a second constant current less than the first constant current provided by the first control circuit over a second time interval after the first time interval, and a constant voltage provided by the first control circuit over a third time interval after the second time interval. 
     In some implementations, the at least one rechargeable battery includes a first rechargeable battery having a first voltage and a second rechargeable battery having a second voltage. The first control circuit can further be configured to provide power to the second control circuit by determining that the first voltage has fallen below the second voltage, selecting the second rechargeable battery to discharge in response to the determination that the first voltage has fallen below the second voltage, determining that the second voltage has fallen below the first voltage, and selecting the first rechargeable battery to discharge in response to the determination that the first voltage has fallen below the second voltage. In some implementations, the first control circuit can be configured to iterate these steps. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the following drawings and the detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosed technology and together with the description serve to explain principles of the disclosed technology. 
         FIG. 1A  is a perspective view of an intraocular lens (IOL), according to an illustrative implementation. 
         FIG. 1B  is an exploded view of the IOL shown in  FIG. 1A , according to an illustrative implementation. 
         FIG. 2  illustrates first and second ASICs suitable for use in an implantable device such as the IOL of  FIG. 1A , according to an illustrative implementation. 
         FIG. 3  is a state transition diagram for the first and second ASICs shown in  FIG. 2 , according to an illustrative implementation. 
         FIG. 4  is a timing diagram showing the signal processing characteristics of an implantable device such as the IOL of  FIG. 1A , according to an illustrative implementation. 
         FIG. 5A  is diagram of a lithium ion rechargeable battery suitable for use in an implantable device such as the IOL of  FIG. 1A , according to an illustrative implementation. 
         FIG. 5B  is a diagram of a rechargeable solid-state battery suitable for use in an implantable device such as the IOL of  FIG. 1A , according to an illustrative implementation. 
         FIG. 6  is circuit diagram showing a battery charging circuit suitable for use in an implantable device such as the IOL of  FIG. 1A , according to an illustrative implementation. 
         FIG. 7  is a graph showing the charging characteristics of a battery such as the lithium ion battery of  FIG. 5A  or the solid-state battery of  FIG. 5B , according to an illustrative implementation. 
         FIG. 8  is a flow diagram of a process for charging a rechargeable battery, according to an illustrative implementation. 
         FIG. 9  is a graph showing the discharging characteristics of the two batteries shown in  FIG. 2 , according to an illustrative implementation. 
         FIG. 10  is a flow diagram of a process for discharging two rechargeable batteries substantially simultaneously, according to an illustrative implementation. 
     
    
    
     DETAILED DESCRIPTION 
     Presently preferred embodiments of the invention are illustrated in the drawings. An effort has been made to use the same or like reference numbers to refer to the same or like parts. 
     Electronic Control Systems for Implantable Ophthalmic Devices 
     This invention generally relates to power management of implantable devices, such as implantable ophthalmic devices.  FIG. 1A  shows an exemplary implantable ophthalmic device  100 , such as an intraocular lens (IOL), for use in dynamically correcting or adjusting a patient&#39;s vision. The device  100  includes a power supply—in this case, a rechargeable battery  140 —coupled to a first application-specific integrated circuit (ASIC)  130   a  and a second ASIC  130   b . The battery  140  provides current at a relatively low voltage, e.g., about 4 V or less, to both ASICs  130   a  and  130   b . The first ASIC  130   a  is coupled to an electro-active element  160  that operates at a relatively high voltage, e.g., about 5 V to about 11 V. And the second ASIC  130   b  operates at a lower voltage (e.g., about 5 V or less) to monitor environmental and/or physiological conditions for indications of accommodative triggers and to control the first ASIC  130   a.    
     The electro-active element  160  provides a dynamically variable optical power and/or depth of field that adds to the (optional) static optical power provided by the device&#39;s curved surface. For example, the electro-active element  160  can act as a variable diameter aperture that opens and closes in response to accommodative triggers to increase or decrease the depth of field. The device  100  may also include a sensor  180 , such as a photodetector or ion sensor, for detecting the eye&#39;s accommodative response and an antenna  190  for receiving radio-frequency power or data communication. The electronics can be embedded or otherwise hermetically sealed inside the device  100  itself, which may be molded of glass, resin, plastic, or any other suitable material. 
       FIG. 1B  is an exploded view of the implantable ophthalmic device  100  shown in  FIG. 1A . The device includes cavities  110  and feedthroughs  112  that are hermetically sealed to prevent leakage of foreign material from the device  100  into the eye. As defined herein, a hermetically sealed cavity or feedthrough is a cavity or feedthrough that passes an American Society for Testing and Materials (ASTM) E493/E493M-11 helium leak test with a leak rate of less than 5.0×10 −12  Pa m 3 s −1 . In some embodiments, the amount of helium that leaks through a hermetic seal during a helium leak is undetectable, i.e., it is lower than the normal atmospheric concentration of helium. 
     The assembly  100  includes electronic components—in this case, ASICs  130  that have different functional blocks and may be populated with additional electronic components—disposed within the cavities  110  in an intermediate wafer  104 . The ASICs  130  can be populated with subcomponents using thermo-compression bonding via TiAgNiAu pads material with mechanical tolerances of ±10 μm in all three dimensions. The assembly may also include AgPb capacitors (not shown), such as 01005 SMD surface-mount capacitors, that are bonded to a printed circuit board (PCB) (not shown) with anisotropic conductive adhesives with a lateral alignment tolerance of ±50 μm. In preferred embodiments, the total height from the surface of the PCB to the top of the capacitor is about 255±10 μm. 
     The cavities  110  are defined by sealing apertures in the intermediate wafer  104  between a bottom wafer  102  and a top wafer  106 , which can be bonded together using laser fusion bonding, pressure bonding, and/or anodic bonding. Other elements, such as the electro-active cell  160  and an obscuration  162 , which comprises an opaque layer that absorbs more than 90% of incident light, may be affixed to or sealed between the wafers  102 ,  104 , and  106 , which can be made of borosilicate glass (e.g., Borofloat® 33 or D263™), pure silica (SiO 2 ), fused silica, or any other suitable material. 
     The ASICs  130  are electrically connected to batteries  140  via the feedthroughs  112  that run through the top wafer  106 . The batteries  140 , which may be rechargeable, include cells  141  held apart by a separator  144  and covered in a casing  142  that provides leakage protection for up to 25 years or more. A battery casing isolation ring  146  insulates the cells  141  from the rest of the assembly  100 , and a battery insert plate  148  hold the battery  140  and its components in place with respect to the top wafer  106 . 
     The assembly  100  also includes an inductive antenna coil  150  and a photovoltaic cell  170  that can be used to recharge the batteries  140 . The coil  150  and the photovoltaic cell  170  can also be used for wireless communication with external processors, e.g., to update and/or extract information store in memory on one or both of the ASICs  130 . The photovoltaic cell  170  can also be used to detect accommodative triggers, changes in pupil diameter, and/or other physiological or environmental indications with an average sensitivity of about 0.48 nA/lux mm2. In some embodiments, the assembly  100  includes two TiAu—PIN—ZnO photovoltaic cells: a first cell with diameter of about 1.175-1.225 mm and a second cell with dimensions of about 0.1 mm×1.8 mm. In some examples, the coil  150  has about fifteen windings arranged about a perimeter of 5.7 mm×2.6 mm. 
     The coil  150  and photovoltaic cell  170  are also be in electrical communication with the ASICs  130  via the feedthroughs  112 . For instance, a battery charger (not shown) in one of the ASICs  130  may control the recharging process as described in PCT/US2011/040896 to Fehr et al., which is incorporated herein by reference in its entirety. Similarly, a processor in one of the ASICs  130  may receive signals from the photovoltaic cell  170  representing the pupil diameter as also described in PCT/US2011/040896 to Fehr et al. The processor may also control the diameter of an aperture defined by the electro-active cell  160  in response to signals from the photovoltaic cell  170 , e.g., as described in U.S. Pat. No. 7,926,940 to Blum et al., which is also incorporated herein by reference in its entirety. 
     The implantable ophthalmic device  100  shown in  FIG. 1B  is illustrative only. In some implementations, the implantable ophthalmic device  100  may include more or fewer components than are shown in  FIG. 1B . The arrangements of the components can also be different in various implementations. For example, the coil  150  can be wrapped around the batteries  140 . Winding the coil  150  around The batteries  140  provide good mechanical stability for the coil  150 , but may impose constraints on how the implant is assembled (e.g., batteries  140  before the coil  150 ). The batteries  140  may also interfere with inductive coupling between the coil  150  and external electromagnetic sources (antennas). 
     The coil  150  can also be wound around a separate support  152 . In some cases, an optic, such as an aspheric lens or a spherical lens, may be integrated into the support  152 . For example, a portion of the support&#39;s outer surface may be curved or patterned to refract or diffract incident light. Using a separate support  152  also increases the flexibility of the manufacturing process by obviating any need to install certain components (e.g., batteries  140 ) before the coil  150 . It also makes it possible to optimize the coil&#39;s coupling efficiency by allowing the coil  150  to follow a path away from potential sources of interference. However, using the separate support  152  may increase the manufacturing complexity and total mass of the implantable ophthalmic device. 
     Alternatively, the coil  150  may be self-sustaining, i.e., it may not require any additional support. Like other coils, self-sustaining coils should be positioned within acceptable mechanical tolerances, and may be held in place with respect to the wafers using an adhesive. Care should be taken to prevent self-sustaining coils from deforming during encapsulation of the electronics assembly  100  in acrylic, resin, or other media. 
     The coil  150  can also be sealed within a cavity to eliminate the need for feedthroughs between the coil  150  and the ASICs  130 . In this example, the coil  150  is embedded inside a 0.3 mm thick glass “disc” with two electrical connection on one side of the “disc”. Because the coil  150  is hermetically sealed within the cavity, non-biocompatible material can be used for the coil wires (e.g., copper instead of gold) and for the insulation layer. Sealing the coil  150  within a cavity also eliminates the need to use biocompatible conductive materials to connect the coil  150  to components within the cavity. 
       FIG. 2  illustrates the first and second ASICs  130   a  and  130   b  of the IOL  100  of  FIG. 1A , according to an illustrative implementation. The first ASIC  130   a  includes a radio-frequency (RF) frontend  202  with a magnetic antenna  180  for power and data management. In some implementations, the magnetic antenna  180  can receive power from an external radio-frequency source, which can be used by a battery charge and power management module  204  to charge the batteries  140 . The battery charge and power management module  204  also discharges the batteries  140  to power the first and second ASICs  130   a  and  130   b  as described in greater detail below. 
     The battery charge and power management module  204  is also coupled to a diffractive optical element (DOE) driver  210  in the first ASIC  130   a  that actuates a diffractive optical element (DOE)  260 , which may correspond to the electroactive element  160  of  FIG. 1A . As explained below, the first ASIC  130   a  remains in a low-power state (also known as a sleep or inactive state) unless the DOE  260  is being actuated, in which case the first ASIC  130   a  enters a high-power or active state. When in the active state, the first ASIC  130   a  activates a charge pump  208  coupled to the battery charge and power management module  204  generates a high voltage signal for actuating the DOE  260  (e.g., opening or closing an aperture defined by the DOE  260 ). Once actuation is complete, the first ASIC  130   a  returns to the low-power state to reduce leakage current from the charge pump  208  or the battery charge and power management module  204 . 
     The first ASIC  130   a  also includes an electrically erasable programmable read-only memory (EEPROM) module  206  for storing system parameters. The first ASIC  130   a  can also include a local data flow controller module  212  that can be configured to control data transmission between the various components of the first ASIC  130   a . An oscillator  214  in the first ASIC  130   a  provides a timing signal to synchronize communication between the components of the both the first ASIC  130   a  and the second ASIC  130   a.    
     In some implementations, the first ASIC  130   a  can also include a low-dropout (LDO) regulator  216  and a bandgap reference (BGR) circuit  218 . The LDO regulator  216  is a DC linear voltage regulator that converts unregulated battery voltage into a regulated power supply voltage. The BGR circuit  218  is a voltage reference circuit that emits a reference voltage (e.g., 1.25 V) that is does not vary much, if at all, with temperature. In other words, the BGR circuit&#39;s reference voltage remains stable despite changes in temperature. The reference voltage from the BGR circuit  218  is input into other ASIC blocks, including the power management block  204 , which comprises a comparator (not shown) that compares the battery voltage to the reference voltage to determine when the batteries  140  should charged, discharged, etc., as described in greater detail below. 
     The second ASIC  130   b  is coupled to one or more photodetectors  210 . The photodetectors  210  can determine an ambient light level in the environment surrounding the eye. The ambient light level determined by the photodetectors  210  can be converted to a digital signal by an analog to digital converter  222 . The resulting digital signals can be used by the second ASIC  130   b  to control the operation of the first ASIC  130   a . For example, the second ASIC  130   b  can include a logic module  220  for implementing an actuation algorithm based on the ambient light level. The results of the algorithm can then be communicated to the first ASIC  130   a , which can actuate the DOE accordingly. The second ASIC  130   b  can also include a random access memory (RAM) module  224 , which can be configured to store information such as ambient light levels determined by the photodetectors  210 , digital outputs from the ADC module  222 , and parameters to be used by the logic module  220 . The first ASIC  130   a  and the second ASIC  130   b  can each include a respective inter-chip interface module  226  to facilitate communication between the first ASIC  130   a  and the second ASIC  130   b.    
     Operation of an Implantable Ophthalmic Device 
       FIG. 3  is a state transition diagram for the ASICs shown in  FIG. 2 . The first and second ASICs can have four main power conditions corresponding to different device states, all of which are listed below in TABLE 1. When the system is off, the low-voltage ASIC (e.g., second ASIC  130   b ) is in an unpowered idle mode  305 , and the high-voltage ASIC (e.g., first ASIC  130   a ) is in a sleep (shutdown) state  310 . Under normal operating conditions, e.g., when the user is going about his or her day, the system operates in autonomous therapeutic function mode to provided automatic accommodation upon detection of accommodative responses. The high-voltage ASIC switches to its operational mode  315  and the low-voltage ASIC remains in idle mode when the device is operating in autonomous therapeutic function mode. The device can also be charged and/or communicate wirelessly with external readers while continuing to provide autonomous therapeutic function for the patient. When charging and providing autonomous therapeutic function, the low-voltage ASIC switches to an externally (e.g., inductively) powered state and the high-voltage ASIC remains in its operational mode. The device may also be charged and/or communicate wirelessly without providing autonomous therapeutic function, in which case the high-voltage ASIC shuts down to reduce power consumption and current leakage. 
     In each case, the low-voltage ASIC can change the state of the high-voltage ASIC by issuing an “interrupt” signal (spi_vdd) to the high-voltage ASIC via an interchip data interface. If high-voltage ASIC is in a power-down state  310 , the low-voltage ASIC initiates a power-on of the high-voltage ASIC, setting it to a temporary on state  360 , and sets the interchip data interface into a command receive state. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 ASIC Powering Conditions 
               
            
           
           
               
               
               
            
               
                   
                 High-Voltage 
                   
               
               
                 Low-Voltage ASIC 
                 ASIC 
                 Device State 
               
               
                   
               
               
                 IDLE (unpowered) 
                 Shutdown 
                 System Off 
               
               
                 IDLE (unpowered) 
                 Operation 
                 Autonomous Therapeutic Function 
               
               
                 RF powered 
                 Operation 
                 Charging or communication in 
               
               
                 (blank states in 
                   
                 progress, therapeutic function 
               
               
                 FIG. 3) 
                   
                 running 
               
               
                 RF powered 
                 Shutdown 
                 Charging or communication in 
               
               
                 (blank states in  
                   
                 progress, therapeutic function 
               
               
                 FIG. 3) 
                   
                 disabled 
               
               
                   
               
            
           
         
       
     
     As shown in  FIG. 3 , the low-voltage ASIC may transition from idle state  305  to an operational state  320  through application of an RF carrier signal to an RF front-end resonant circuit in the ophthalmic device. For example, the patient may use a remote control to actuate or upload new data to the ophthalmic device. Alternatively, the patient may charge the ophthalmic device with a charging unit. 
     When the RF front-end resonant circuit detects an rf carrier signal, it sends a signal to a control logic section block on the low-voltage ASIC. At the beginning of the application of an RF field, the control logic section block may be unaware of whether the RF field is being applied for communication and/or battery charging, or both. The logic section block checks the RF signal to determine whether to enter communication mode or battery charging mode. At the same time, a local memory (EEPROM) boot sequence is initiated to transfer the relevant control bits required on the low-voltage ASIC to local data latches. These bits may include trim bits for the rf tuning or control bits for battery charging. 
     If the logic section block determines that it should enter communication mode, it either begins data communication with the remote control (state  345 ), processes commands from the remote control (state  350 ), and stores/retrieves information from local memory (state  355 ). If the logic section block determines that it should enter charging mode, it begins constant current charging (state  325 ), then boots the EEPROM (state  330 ) and switches to constant voltage charging once the battery reaches a predetermined charge level as described above (state  335 ). Once communication or charging is finished, the patient removes the remote control or the charging unit, and the low-voltage ASIC returns to its idle state  305 . 
       FIG. 4  is a timing diagram showing the signal processing characteristics of an implantable ophthalmic device such as the one shown in  FIG. 1A . The timing diagram shows two cycles of a periodic process that can be implemented by the IOL. Each cycle lasts for a sample period  405  denoted by T sample . In some implementations, T sample  is in the range of about 200 ms to about 310 ms. This sample period duration enables the implantable ophthalmic device to detect and respond to accommodative triggers quickly enough to mimic accommodation in a healthy eye. 
     Each cycle begins with a pair of sequential photodetector polling periods  410  and  412 , each of which is about 0 ms to about 40 ms (e.g., 5 ms, 10 ms, 20 ms, 30 ms, or any other value less than 40 ms). During the first polling period  410 , control logic in the low-voltage ASIC polls, integrates, or samples an analog electrical signal, such as a photocurrent, charge packet, or change in voltage, from a first photodetector. An ADC in the low-voltage ASIC converts this analog signal into a digital signal representative of a light level detected by the first photodetector, and the digital signal is latched during a first ADC latching period  414 . The electrical signal output by the first photodetector is then converted to a digital signal using an analog-to-digital converter (e.g., ADC  220  in  FIG. 2 ). A second photodetector converts ambient light to an analog signal during the second polling period  412 , which is then converted to a digital signal and latched by the ADC during a second latching period  414 . The polling and latching periods occur during an acquisition period  415  that lasts for a time t acq , which may be less than about 100 ms (e.g., 10 ms, 20 ms, 30 ms, 40 ms, 50 ms, 60 ms, 70 ms, 80 ms, 90 ms, or any other value between 0 and 100 ms). 
     The logic module in the low-voltage ASIC processes the digital signals during a processing period  420  that begins after the second latching period  414 . During processing, the logic module may compare the digital signals to values stored in a look-up table in the memory. If the comparison indicates that the ambient light levels have changed in a way indicative of the presence of an accommodative trigger, the low-voltage ASIC generates an actuation signal, which can be used to control an electroactive element such as the DOE  260  of  FIG. 2 . In some implementations, the processing period  420  lasts less than about 100 ms (e.g., 10 ms, 20 ms, 30 ms, 40 ms, 50 ms, 60 ms, 70 ms, 80 ms, 90 ms, or any other value between 0 and 100 ms). The actuation signal is transmitted to the high-voltage ASIC (e.g., first ASIC  130   a  in  FIG. 2 ), which responds by transitioning from a sleep state to an active state, actuating the DOE  260 , and returning to the sleep state during a control latching period  425  of about 5 ms or less (e.g., about 1 ms, 2 ms, 3 ms, or 4 ms). Because the high-voltage ASIC is only briefly in an active state (e.g., for a duty cycle of under about 3% given the total cycle period  405 ), the high-voltage ASIC consumes less power and has a lower leakage current. After an optional wait period  430  following DOE  260  actuation, the cycle can begins again. 
     The exact length of the control latching period  425  depends at least in part on the degree and direction of actuation experience by the DOE  260 . For instance, it may take the DOE  260  more time to transition from a fully transmissive state (e.g., 90% transmissive) to a fully opaque state (e.g., 0% transmissive) than from a partially transmissive state (e.g., 60% transmissive) to a partially opaque state (e.g., 10% transmissive). Similarly, the DOE  260  may exhibit hysteresis: for instance, it may take longer to transition from an opaque state to a transmissive state than vice versa. The number, arrangement, and location of the actuated pixels in the DOE  260  may also affect the length of the control latching period  425 . 
     Rechargeable Batteries for Implantable Ophthalmic Devices 
       FIG. 5A  shows a lithium ion rechargeable battery  500  suitable for use in an implantable device such as the implantable ophthalmic device  100  of  FIGS. 1A and 1B . For example, the battery  500  can correspond to the either of the batteries  140  shown in  FIG. 2 . The battery  500  includes a casing  510 , which can be made from gold or any other suitable material. The battery  500  also includes an anode  520  and a cathode  530 , which can correspond to the battery cells  141  of  FIG. 2 . The anode  520  and cathode  530  are separated by two separators  540 . The battery  500  can be sealed onto the wafer  106 , for example by gold laser welding. In some implementations, the battery  500  can have a capacity of 160 μAh and a lifetime of more than 6000 charging cycles. 
       FIG. 5B  shows a thin-film, solid-state battery  550  that is also suitable for use in an implantable device such as the implantable ophthalmic device  100  of  FIGS. 1A and 1B . The battery  550  can be used as either or both of the batteries  140  shown in  FIG. 2 . In some implementations, the battery  550  can be used together with the lithium ion rechargeable battery  500  of  FIG. 5A . The battery  550  is built on a substrate  560  which can be formed from mica. The substrate  560  can have a thickness of about 25 microns. An electrical contact  565  can be formed atop the substrate  560 . In some implementations, the contact  565  is formed from platinum and has a thickness of about 0.5 microns. A cathode layer  570  can be formed on top of the electrical contact  565 . In some implementations, the cathode  570  can be made from LiCoO 2  and can have a thickness of about 30 microns. An electrolyte layer  575  can be in contact with the cathode  570 . In some implementations, the electrolyte layer  575  can be made from LiPON and can have a thickness of about three microns. The electrolyte layer  575  separates the cathode  570  from an anode  580 . The anode  580  can be made from lithium and can have a thickness in the range of about 18 microns. A second contact layer  585  can be formed on top of the anode  580 . The second contact can be formed from platinum, for example, and can be about 0.5 microns thick. 
     In some implementations, the entire battery  550  can have a thickness of about 80 microns and an electrical storage capacity of about 11 μAh/mm 3 . Because the battery  550  is so thin, it can be flexible enough to bend, e.g., for implantation through a small incision in the body. As understood by those of ordinary skill in the art, smaller incisions tend to heal more rapidly and usually accompanied by less swelling than large incisions. As a result, implantations performed with smaller incisions tend to be associated with shorter recovery times, lower complication rates, and less discomfort. 
     In addition, it may also be safer than other batteries for implantable devices. For example, the battery  550  can be implanted into the eye of a patient as part of an IOL. Because the battery  550  is a solid-state device, there is little to no risk of out gassing or liquid leaks, which means that there is lower risk of eye damage due to a defective or damaged battery. 
     Battery Charging Circuitry and Processes for Implantable Ophthalmic Devices 
       FIG. 6  is circuit diagram showing a battery charging circuit  660  suitable for charging batteries use in an implantable device such as the implantable ophthalmic device  100  shown in  FIGS. 1A ,  1 B, and  2 . The battery charger  660  may draw power inductively via an RF antenna, which supplies current to the batteries, for example, via a rectification circuit. The RF antenna can correspond to the RF antenna  190  of  FIG. 1A . The battery charging circuit  660  includes one or more trimming blocks  661 , each of which includes a tuning capacitor  662  coupled in series with both a switch  664  and a load capacitor  668 . As shown in  FIG. 6 , the switch  664  and load capacitor  668  are in parallel. Closing the switch  664  connects the tuning capacitor  662  to a load  666 , increasing the impedance to provide better power flow from an external power supply to the rectification circuit. The trimming blocks  661  can be activated or de-activated as desired to optimize power flow. Once the battery charging circuit  660  is set appropriately, a magnetic field induces current flow in the device. The rectification circuit can then harvest a DC voltage for charging the batteries. 
       FIG. 7  is a graph that illustrates a charging process (recharging profile) for a rechargeable battery such as the lithium ion battery  500  of  FIG. 5A  or the solid-state battery  550  of  FIG. 5B  using the charging circuitry  660  in  FIG. 6 . The graph shows a battery voltage level  702  and a battery current level  704  over three time intervals: a first time interval  710  (t CCQ ), a second time interval  720  (t CCS ), and a third time interval  730  (t CV ). Applying a larger current during the first time interval  710  and a smaller current during the second time interval  720  can result in a faster overall charging process for the battery. 
     In some implementations, the current  704  can be applied to the battery by a control module such as the battery charge and power management module  204  of  FIG. 2 . As shown in the graph, the battery voltage begins from V start  at time zero (i.e., the beginning of the first time interval  710 ). A constant current denoted by I CCQ  is applied to the battery throughout the first time interval  710 , causing the voltage  702  of the battery to increase linearly until it reaches a predetermined battery charge termination voltage labeled V BAT,EOC  at the end of the first time interval  710 . The battery charge termination voltage may be a function of the battery and can be detected, for example, by the battery charge and power management module  204  of  FIG. 2 . The battery charge and power management module  204  determines that the time interval  710  has ended upon sensing the that the battery voltage has reached the battery charge termination voltage. 
     The current  704  is then reduced to the level denoted by I CCS  at the beginning of the second time interval  720 , and is held constant throughout the second time interval  720 . In some cases, this second current level I CCS  is about half the constant current level I CCQ  applied during the first time interval  710 . The reduction in current causes the battery voltage  702  to drop at the beginning of the time interval  720 , but the voltage increases linearly throughout the time interval  720  until it again reaches the charge termination voltage. In some implementations, the rate of increase of the battery voltage  702  is proportional to the level of applied current  704 . Thus, during the second time interval  720 , the voltage  702  increases at a slower rate due to the decreased current  704 . 
     Upon sensing that the battery voltage  702  has reached the charge termination voltage, the battery control module determines that the second time interval  720  has ended. In response, the battery control module causes the current  704  to decreased until it approaches a level denoted as I stop  at the end of the time interval  730 . The battery control module maintains the battery voltage  702  at the charge termination voltage throughout the third time interval  730  by changing the applied current. 
       FIG. 8  is a flow diagram of a process  800  for charging a rechargeable battery, according to an illustrative implementation. The process  800  includes charging the battery for a first time interval with a first constant current (step  805 ). In some implementations, the first time interval can correspond to the time interval  710  of  FIG. 7 , and the first constant current can correspond to the current level I CCQ . During the first time interval, the battery voltage can increase linearly in response to the first constant current applied. The process  800  can also include determining that a voltage of the battery exceeds a first threshold value (step  810 ). For example, the first threshold value can be equal to the charge termination voltage V BAT,EOC  as shown in  FIG. 7 . The first time interval can end when the battery voltage meets or exceeds the first threshold value. 
     The process  800  can include charging the battery for a second time interval with a second constant current (Step  815 ). In some implementations, the second time interval can correspond to the time interval  720  of  FIG. 7 . The second current can be less than the first constant current. For instance, the second current may be about 40-60% of the first current (e.g., 45%, 50%, 55%, or any other percentage from 40% to 60%). In some implementations, the second constant current can correspond to the current level I CCS . During the second time interval, the battery voltage can increase linearly in response to the second constant current applied. The rate of increase in voltage can be greater than or less than the rate of increase in voltage experienced by the battery during the first time interval. 
     The process  800  can also include determining that a voltage of the battery exceeds a second threshold value (step  820 ) with a control module in an ASIC. In some implementations, the second threshold value can be equal to the first threshold value. For example, the second threshold value can be equal to the charge termination voltage V BAT,EOC  as shown in  FIG. 7 . The second time interval can end when the battery voltage meets or exceeds the second threshold value. 
     The process  800  can also include charging the battery for a third time interval with a constant voltage (Step  825 ). For example, the third time interval can correspond to the time interval  730  shown in  FIG. 7 , and the constant voltage can be equal to V BAT,EOC . In order to achieve the constant voltage, the current applied to the battery can be decreased, e.g., so as to maintain the constant voltage level. As the charge stored by the battery increases, less current is required to maintain the constant voltage. In some implementations, the current applied to the battery can be decreased exponentially, as shown in  FIG. 7 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Battery Management Parameters 
               
            
           
           
               
               
               
               
               
               
            
               
                 Symbol 
                 Parameter 
                 min 
                 nom 
                 max 
                 Unit 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 V start   
                 Battery charge start voltage @ 70% DOD 
                 — 
                 3.500 
                 — 
                 mV 
               
               
                 I CCS   
                 Battery charge current for standard charging 
                 20 
                 25 
                 30 
                 μA 
               
               
                   
                 (per battery; regular use) 
               
               
                 I CCQ   
                 Battery charge current for quick charging 
                 40 
                 50 
                 60 
                 μA 
               
               
                   
                 (per battery; exceptional use for clinical trials) 
               
               
                 t CCQ   
                 Constant current mode battery charge time for 
                 — 
                 1 
                 1.2 
                 hr 
               
               
                   
                 quick charging (timeout controlled) 
               
               
                 t CCS   
                 Constant current mode battery charge time for 
                 — 
                 5 
                 6 
                 hr 
               
               
                   
                 standard charging (timeout controlled) 
               
               
                 t CV   
                 Time for constant voltage mode; used to settle the 
                 — 
                 2 
                 2.4 
                 hr 
               
               
                   
                 charge in the battery (timeout controlled) 
               
               
                 I stop   
                 Residual battery charge current when CV charge 
                   
                   
                   
                 μA 
               
               
                   
                 mode terminates. 
               
               
                 V BAT,EOC   
                 Battery charge termination voltage 
                 4100 
                 4150 
                 4200 
                 mV 
               
               
                 V SD   
                 Battery shutdown voltage 
                 3450 
                 3500 
                 3550 
                 mV 
               
               
                 V BAT,EOD   
                 Allowed battery discharge voltage before 
                 2500 
                 — 
                 — 
                 mV 
               
               
                   
                 permanent damage 
               
               
                   
               
            
           
         
       
     
     TABLE 1 lists exemplary charging times, currents, and voltages for use in the charging process illustrated in  FIGS. 7 and 8 . As understood by those skilled in the art, the exact choice of currents and times for defining the recharging profile depends on the battery technology and the recharge strategy. For example, some batteries can be charged at constant voltage. The exact recharging time, recharging percentage of the maximum battery capacity, and battery life may be adjusted to achieve the desired performance. In some cases, the battery charge start voltage V start  may be raised or lowered depending on the battery&#39;s state of charge (SOC) or depth of discharge (DOD). (The SOC represents a battery&#39;s available charge; the DOD represents the amount charge expended by the battery.) 
     Dual-Battery Discharging 
       FIG. 9  is a graph showing the discharge of the two batteries  140  as controlled by the battery charge and power management module  204  shown in  FIG. 2 . The voltage of the first battery is shown by a first line  910  while the voltage of the second battery is shown by the line  920 . As depicted in the graph, the voltage of both batteries begins at a voltage denoted by V10 at time 0. In some implementations, the voltage V10 can correspond to a charge termination voltage, such as about 4200 mV. The exact charge termination voltage may depend on the battery technology and the battery&#39;s depth of discharge. The charge termination voltage can be measured using a reference voltage, e.g., as provided by the BGR circuit  218  in the low-voltage ASIC  130   b  ( FIG. 2 ). 
     In operation, the power management module  204  discharges the first battery linearly over a first time interval, while the second battery remains at a constant voltage. For example, the first battery can be discharged to actuate an electroactive element as discussed above in connection with  FIGS. 1A and 1B , while the second battery remains unused. Upon sensing that the voltage of the first battery has decreased by a first increment to a first predetermined lower voltage level (V9 at time 1), the power management module stops discharging the first battery and starts discharging the second battery while the first battery remains at a constant voltage. 
     In response to sensing that the second battery has reached the first predetermined lower voltage level V9, the power management module stops discharging the second battery and starts discharging the first battery to a second predetermined voltage level V8 while the second battery remains at a constant voltage (V9). The power management module repeats this process iteratively through a series of predetermined voltage levels (V10 through V0) so that the two batteries are discharged substantially simultaneously. In some examples, these predetermined voltage levels are spaced evenly, e.g., at increments of 100 mV. In operation, it may take hours to days for the first and second batteries to reach the ultimate discharge level V0. 
     The discharging scheme shown in  FIG. 9  can help to increase the useful life of an implantable device. For example, the device can be designed such that one battery is sufficient to power the device. Alternately discharging two batteries according to the process shown in  FIG. 9  can therefore extend the time between charging cycles. Less frequent charging cycles can also lead to increased battery life over time. Using two or more batteries also mitigates the risk of battery failure; if one battery fails, the other may continue to operate, prolonging the implantable device&#39;s useful life. 
       FIG. 10  is a flow diagram of a process  1000  for discharging two rechargeable batteries substantially simultaneously, according to an illustrative implementation. The process  1000  includes determining, with a power management module (e.g., module  204  in  FIG. 2 ) that a voltage of a first battery has fallen below a voltage of a second battery (Step  1005 ). For example, a voltage sensing circuit and a comparator can be used to determine the relative voltage levels of the first and second batteries. The process  1000  can also include selecting the second battery to discharge in response to determining that the voltage level of the first battery has fallen below the voltage level of the second battery (Step  1010 ). Electrical power extracted from the second battery as it is discharged can be used to actuate an electroactive element, such as the DOE  260  shown in  FIG. 2 . While the second battery is discharged (Step  1010 ), the first battery can be held at a constant voltage, for example by electrically isolating it from the DOE. 
     The process  1000  includes determining that the voltage of the second battery has fallen below the voltage of the first battery (Step  1015 ). Because the first battery is held at a constant voltage while the second battery is discharged in Step  1010 , the voltage of the second battery will eventually reach a level below the voltage of the first battery. Voltage levels of both batteries can be continuously or periodically monitored and compared in order to make the determination. The process  1000  can also include selecting the first battery to discharge in response to determining that the voltage level of the second battery has fallen below the voltage level of the first battery (Step  1020 ). The first battery can be discharged to power the DOE that was previously powered by the second battery, while the second battery can be turned off so that it maintains a substantially constant voltage. 
     In some implementations, the steps of the process  1000  can be performed iteratively. In this way, the first and second batteries can be discharged substantially simultaneously, although only one battery is discharged at any given time. The process  1000  can help to extend the life of a device in which the first and second batteries are used. Because each battery is used for only about half the time that the device is powered on, charging cycles for the batteries are required less frequently and the expected life of the device is increased. 
     CONCLUSION 
     The herein described subject matter sometimes illustrates different components contained within, or connected with, different other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “operably connected”, or “operably coupled”, to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being “operably couplable”, to each other to achieve the desired functionality. Specific examples of operably couplable include but are not limited to physically mateable and/or physically interacting components and/or wirelessly interactable and/or wirelessly interacting components and/or logically interacting and/or logically interactable components. 
     With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. 
     It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. 
     However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). 
     Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). 
     It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.” 
     The foregoing description of illustrative embodiments has been presented for purposes of illustration and of description. It is not intended to be exhaustive or limiting with respect to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosed embodiments. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.