Abstract:
A visual prosthesis includes an artificial muscle configured to deform in response to a focusing signal. The artificial muscle is coupled to at least a portion of an optical system for changing a focal point thereof.

Description:
FIELD OF INVENTION 
     This invention relates to a vision prosthesis, and in particular, to actuators for assisting in vision accommodation. 
     BACKGROUND 
     In the course of daily life, one typically regards objects located at different distances from the eye. To selectively focus on such objects, the focal length of the eye&#39;s lens must change. In a healthy eye, this is achieved through the contraction of a ciliary muscle that is mechanically coupled to the lens. To the extent that the ciliary muscle contracts, it deforms the lens. This deformation changes the focal length of the lens. By selectively deforming the lens in this manner, it becomes possible to focus on objects that are at different distances from the eye. This process of selectively focusing on objects at different distances by deforming the lens is referred to as “accommodation.” 
     As a person ages, the lens gradually loses its plasticity. As a result, it becomes increasingly difficult to deform the lens sufficiently to focus on objects at different distances of regard. To compensate for this loss of function, it is necessary to provide different optical corrections for focusing on objects at different distances. 
     One approach to applying different optical corrections is to carry different pairs of glasses and to swap glasses as the need arises. For example, one might carry reading glasses for reading and a separate pair of distance glasses for driving. This is inconvenient both because of the need to carry more than one pair of glasses and because of the need to swap glasses frequently. 
     Bifocal lenses assist accommodation by integrating two different optical corrections onto different portions of the same lens. The lower part of the bifocal lens is ground to provide a correction suitable for reading or other close-up work, while the remainder of the lens is ground to provide a correction for distance vision. To regard an object, a wearer of a bifocal lens need only maneuver the head so that rays extending between the object-of-regard and the pupil pass through that portion of the bifocal lens having an optical correction appropriate for the range to that object. 
     The concept of a bifocal lens, in which different optical corrections are integrated into the same lens, has been generalized to include trifocal lenses, in which three different optical corrections are integrated into the same lens, and continuous gradient lenses in which a continuum of optical corrections are integrated into the same lens. However, just as in the case of bifocal lenses, optical correction for different ranges of distance using these multifocal lenses relies extensively on relative motion between the pupil and the lens. 
     Once a lens is implanted in the eye, the lens and the pupil move together as a unit. Thus, no matter how the patient&#39;s head is tilted, rays extending between the object-of-regard and the pupil cannot be made to pass through a selected portion of the implanted lens. As a result, multifocal lenses are generally unsuitable for intraocular implantation. Once the lens is implanted into the eye, there can no longer be relative motion between the lens and the pupil. 
     A lens suitable for intraocular implantation is therefore generally restricted to being a single focus lens. Such a lens can provide optical correction for only a single range of distances. A patient who has had such a lens implanted into the eye may therefore have to continue wearing glasses to provide optical corrections for those distances that are not accommodated by the intraocular lens. 
     SUMMARY 
     In one aspect, the invention includes a visual prosthesis having an artificial muscle configured to deform in response to a focusing signal. The artificial muscle is coupled to at least a portion of an optical system for changing a focal point thereof. An optional range finder can be included to provide a focusing signal to an object of regard. 
     In some embodiments, the artificial muscle is coupled to a natural lens. However, the muscle can also be coupled to an artificial lens. The artificial muscle can be any of a variety of electrically responsive materials. For example, in some embodiments, the artificial muscle includes an electro-active polymer. The artificial muscle can alter the refractive properties of the lens in various ways. For example, in some embodiments, the artificial muscle is configured to cause translation of at least a portion of the optical system in response to the focusing signal. In other embodiments the artificial muscle is configured to deform at least a portion of the optical system in response to the focusing signal. 
     Deformation of the lens can be achieved by directly or indirectly applying pressure on the lens. For example, in some embodiments, the artificial muscle includes an expandable ring disposed on a periphery of the lens. In others, a plate is coupled to the artificial muscle. The plate is configured to press against at least a portion of the optical system in response to the focusing signal. The plate can be flat plate or a plate having a peripheral portion that contacts a first portion of the optical system and a central portion that defines an expansion cavity between the plate and a second portion of the optical system. 
     Other embodiments include those in which movement of fluid into or out of the lens causes a change in refractive properties of the lens. For example, the optical system can be a reservoir and a lens in fluid communication with the reservoir. In such embodiments, the artificial muscle can be configured to cause fluid to move between the reservoir and the lens. This can be achieved by positioning the artificial muscle so that it can squeeze the reservoir, thereby pumping fluid from the reservoir to the lens. Alternatively, the artificial muscle is disposed to exert pressure against the lens, thereby pumping fluid from the lens to the reservoir. The artificial muscle can also be integral with the reservoir. In such embodiments, when the artificial muscle contracts, the reservoir squeezes fluid into the lens, thereby changing its refractive properties. 
     In other embodiments of the visual prosthesis, the optical system includes an artificial muscle that is integral with a lens surface of a lens in the optical system. In these embodiments, contraction of the artificial muscle causes a change in optical properties of the lens. An optional biasing element can be provided for urging the lens surface to deform in a preferred direction. 
     Additional embodiments include those in which there is local control over the refractive properties of the lens. For example, in some embodiments, the pillars of artificial muscle extend across a lens in the optical system. Each of the pillars is individually addressable. When one of these pillars contracts, it causes deformation of a local portion of the lens. 
     In other embodiments, the optical system includes a lens having lenslets. In this case, individually addressable artificial muscle elements are each configured to deform the surface of a corresponding lenslet. 
     Embodiments of the visual prosthesis include those in which the lens is an intraocular lens, or a contact lens, or a lens from a pair of eyeglasses. 
     These and other features and advantages of the invention will be apparent from the following detailed description and the accompanying figures, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIGS. 1 and 2  show artificial muscles configured to deform a lens; 
         FIGS. 3 and 4  show artificial muscles coupled to translate a plate toward a lens; 
         FIGS. 5-7 ,  FIG. 13 , and  FIG. 14  show artificial muscles configured to pump fluid between a reservoir and a lens; 
         FIGS. 8-10  show artificial muscles configured to translate a lens; 
         FIG. 11  shows an artificial muscle made with two different electro-active polymers; 
         FIG. 12  shows a lens having an artificial muscle integral with a lens surface thereof; 
         FIG. 15  shows a lens having internal artificial muscles configured to locally deform its surface; 
         FIGS. 16-17  show exemplary layouts for the locations of artificial muscles in the lens shown in  FIG. 15 ; 
         FIGS. 18-19  are cross-section and planar views respectively of a lens having lenslets that can be selectively deformed by artificial muscles; 
         FIG. 20  is a close-up view of a lenslet from  FIG. 18 ; and 
         FIG. 21  is a block diagram of a vision prosthesis. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 21  shows a block diagram of a vision prosthesis  100  having a lens  120  whose index of refraction can be made to vary in response to a focusing signal provided to the lens  120  by an actuator  140 . The lens  120  has refractive properties that vary in response to an applied electric field. The actuator  140  includes structures that change shape in response to applied electrical signals. These structures are in mechanical communication with the lens  120 . 
     Throughout this specification, the terms “lens” and “intraocular lens” refer to the prosthetic lens that is part of the vision prosthesis  100 . The lens that is an anatomical structure within the eye is referred to as the “natural lens.” 
     The nature of the focusing signal provided by the actuator  140  controls the extent to which the refractive properties of the lens are changed. The actuator  140  generates a focusing signal in response to instructions from a controller  160  in communication with the actuator  140 . 
     The controller  160  is typically a microcontroller having instructions encoded therein. These instructions can be implemented as software or firmware. However, the instructions can also be encoded directly in hardware in, for example, an application-specific integrated circuit. The instructions provided to the microcontroller include instructions for receiving, from a range finder  180 , data indicative of the distance to an object-of-regard, and instructions for processing that data to obtain a focusing signal. The focusing signal alters the lens refractive properties to focus an image of the object-of-regard on the retina. 
     The rangefinder  180  typically includes a transducer  190  for detecting a stimulus from which a range to an object can be inferred. The signal generated by the transducer  190  often requires amplification before it is of sufficient power to provide to the controller  160 . Additionally, the signal may require some preliminary signal conditioning. Accordingly, in addition to a transducer  190 , the rangefinder  180  includes an amplifier  210  to amplify the signal, an A/D converter  230  to sample the resultant amplified signal, and a digital signal processor  250  to receive the sampled signal. The output of the digital signal processor  250  is provided to the controller  160 . 
     A power source  200  supplies power to the controller  160 , the range finder  180 , and the actuator  140 . A single power source  200  can provide power to all three components. However, the vision prosthesis  100  can also include a separate power source  200  for any combination of those components that require power. 
     A vision prosthesis thus includes an optical element whose refractive properties can be selectively changed by an actuator in response to a focusing signal. The focusing signal is provided by a controller that determines, on the basis of various cues, how far away an object of regard is. Examples of visual prostheses are described in Azar, U.S. Pat. No. 6,638,304, the contents of which are herein incorporated by reference. 
     One configuration for an actuator, shown in  FIG. 1  includes first and second EAP (“electro-active polymer”) rings  10 ,  12  resting on peripheral portions of opposed surfaces  16 ,  18  of a lens  14 . The lens  14  can be an artificial lens, or the patient&#39;s natural crystalline lens. In response to a focusing signal provided by a controller  19 , the EAP rings  10 ,  12  deform. This, in turn, causes the lens  14  to deform. 
     As is well known, a muscle is an anatomical structure that contracts in response to an electrical signal, typically carried by a nerve. The EAP structures described herein can thus be viewed as “artificial muscles” that responds to electrical signals provided by a controller  19 . The controller  19 , in turn, decides what electrical signals to provide on the basis of a feed back signal. This feedback signal is derived from cues as to how far way an object-of-regard is. 
     Artificial muscles can be used to change the shape of a lens in other ways. For example, the lens shown in  FIG. 2  uses a single EAP ring  12 . In this case an inner rim of the EAP ring  12  rests on a haptic  20  of a lens  14 , and an outer rim of the EAP ring  12  rests on a stationary surface  22 . In response to a focusing signal provided by a controller  19 , the ring  12  expands. In so doing, the outer rim braces the ring  12  against the stationary surface  22  and the inner rim presses against the lens  14 , causing it to bulge outward. 
     Another embodiment, shown in  FIG. 3 , features a transparent flat plate  24  sandwiched between a lens  14  and an EAP ring  12 . In this embodiment, a focusing signal provided by a controller  19  causes the EAP ring  12  to expand, thereby causing the plate  24  to press against, and to thereby flatten, the lens  14 . 
     Pressing a plate  24  against the lens  14  can also cause the lens  14  to bulge outwards. For example, the embodiment shown in  FIG. 4  features a plate  24  having a peripheral portion  26  that contacts a peripheral portion of a lens  14 , and a central portion  28  that bulges outward, away from the lens  14 . The lens  14  and the central portion of the plate  24  together define an expansion cavity  30 . The peripheral portion  26  of the plate  24  is attached to an inner rim of an EAP ring  12 . The outer rim of the EAP ring  12  is attached to a stationary surface  22 . In response to a focusing signal provided by a controller  19 , the EAP ring  12  expands, thereby forcing the plate  24  toward the lens  14 . In this case, the lens  14  bulges outward into the expansion cavity  30 . 
     Additional embodiments feature a reservoir in fluid communication with a lens. In these embodiments, movement of a clear fluid from the reservoir and into the lens causes the lens to bulge outward. Conversely, movement of the fluid from the lens into the reservoir tends to flatten the lens. Various configurations of artificial muscles are available for driving motion of fluid between the reservoir and the lens. 
     For example, in  FIG. 5 , an EAP ring  12  surrounds a reservoir  32  in fluid communication with a lens  14  through a neck  34 . In response to a focusing signal provided by a controller  19 , the EAP ring  12  expands, thereby squeezing the reservoir  32 . This pumps fluid from the reservoir  32  and into the lens  14 . When the focusing signal is removed, the reservoir  32  expands, drawing fluid out of the lens  14  and back into itself. 
     In another embodiment, shown in  FIG. 6 , a transparent jacket  36  surrounds the lens  14 , but not the reservoir  32 . A pair of EAP rings  10 ,  12  is disposed on opposed outer surfaces of the jacket  36 . In this embodiment, a focusing signal provided by a controller  19  causes the EAP rings  10 ,  12  to expand. This exerts a pressure against the jacket  36 . The jacket  36  transmits the pressure to the lens  14 , thereby pumping fluid from the lens  14 , through a neck  34 , and into the reservoir  32 . 
       FIG. 7  shows an embodiment in which the reservoir  32  itself is made of an EAP  12 . When a voltage is applied across flexible electrodes  38 ,  40  on opposite sides of the EAP reservoir  32 , the reservoir  32  changes its shape so as to squeeze fluid within the reservoir  32  through a neck  34  and into a lens  14 . Removing the voltage causes the reservoir  32  to relax and expand, thereby drawing fluid out of the lens  14  and into the reservoir  32 . In this embodiment, the reservoir  32  functions essentially as a single-chamber artificial heart. 
     In the embodiments discussed thus far, artificial muscles are used to cause a shape change in a lens. However, there also exist embodiments in which an artificial muscle causes translation, rather then deformation, of a lens. 
     For example, in  FIG. 8 , first and second EAP rings  10 ,  12  have inner rims  42  attached to a periphery of a lens  14  and outer rims  44  anchored to a stationary surface  22 . The outer rims  44  of the EAP rings  10 ,  12  are longitudinally displaced from the inner ring  42  thereof. As a result, when voltages are selectively applied to the EAP rings  10 ,  12 , the lens  14  translates longitudinally. For example, a voltage that causes contraction of the first ring  10  and relaxation of the second ring  12  will translate the lens  14  forward, while the converse will translate the lens  14  backward. 
     A disadvantage of the arrangement shown in  FIG. 8  is that a great deal of translation is often necessary to effect a significant change in the patient&#39;s vision. This disadvantage is addressed by the embodiment of  FIG. 9 , in which a translating lens  14  like that shown in  FIG. 8  is mounted within a frame  46  that includes one or more stationary optical elements  48 . 
     A variety of ways are available for configuring artificial muscles to move a lens  14  in addition to those already shown in  FIGS. 8 and 9 . For example, in  FIG. 10 , the haptic  50  of the lens  14  includes a genue  52  that buckles in a preferred direction. An outer rim of the haptic  50  is coupled to an inner rim of an EAP ring  10 , the outer rim of which is fixed to a stationary surface  22 . In this embodiment, expansion of the EAP ring  10  causes the genue  52  to buckle, thereby shifting the lens  14  in the axial direction. 
     Another configuration for an EAP ring  10 , shown in  FIG. 11 , makes use of the same principle as a bimetallic strip in a thermostat. In this configuration, the EAP ring  10  has two different layers  54 ,  56 , each made of a different type EAP. In response to a voltage, both EAP layers  54 ,  56  will expand by different amounts, thereby causing the EAP ring  10  to bend in a preferred direction. 
     In another embodiment, shown in  FIG. 12 , the lens  14  can be made of an optically transmissive EAP  10 . In this case, the front and rear surfaces of the lens  14  can be coated with flexible, optically transmissive electrodes  58 ,  60 . In response to a voltage applied between the electrodes  58 ,  60 , the lens  14  changes shape. 
     A variation of the embodiment shown in  FIG. 7  is one in which it is the lens  14  rather than the reservoir  32  that is made an EAP. Referring to  FIG. 13 , a reservoir  32  is again placed in fluid communication with a lens  14  though a neck  34 . At least one surface  10  of the lens  14  is made of an EAP that expands in response to voltage applied between a pair of electrodes  58 ,  60 . 
     Another embodiment, shown in  FIG. 14 , includes the structures shown in the embodiment of  FIG. 13  but with the addition of a biasing structure  62  to urge deformation of the EAP surfaces in a preferred direction. The biasing structure  62  can include springs, as shown, or foam blocks that extend from a midline of the lens  14  toward the EAP lens surface  10 . 
     In the embodiments presented thus far, artificial muscles alter the shape of the entire lens. However, in other embodiments, artificial muscles can be used to locally alter the shapes of selected portions of a lens  14  and to do so in different ways. For example, in  FIG. 16  individually addressable pillars  10  made of an EAP are distributed throughout a lens  14  in a grid. The grid can be a rectangular grid (as shown in  FIG. 16 ) or a grid of concentric circles (as shown in  FIG. 17 ). Each pillar  10  extends across the lens  14 , with the ends of the pillars  10  being attached to opposed surfaces  64 ,  66  thereof. In response to a focusing signal, a particular pillar  10  will change its length. This, in turn, will change the thickness of the lens  14  in a region local to that pillar  10 . Since the pillars  10  are individually addressable, the controller  19  can vary the shape of the lens  14  in an essentially arbitrary way. 
     Another way to provide localized control over the shape of a lens  14  is to make the lens  14  from a honeycomb of lenslets  68  as shown in  FIGS. 18 and 19 . Each lenslet  68  has a flexible surface  78  that is coupled to a rigid surface  22  by an individually addressable EAP ring  10 . 
     In response to an applied voltage, the EAP material in the ring  10  expands, thereby reducing the ring&#39;s inner diameter. This causes the upper surface  78  to flatten, which in turn locally changes the curvature of the lens  14 . 
     A variety of EAP materials can be used in the embodiments described herein. One class of materials includes piezoelectric materials. These materials offer the advantage of high actuation pressures (ranging from 7 to 70 kilopascals meter 3 /kg). However, piezoelectric materials undergo only a limited strain of, which in many cases is less than 1%. 
     Another class of EAPs includes ionic EAPs, such as polymer gels, ionomeric polymer-metal composites, conductive polymers, and carbon nanotubes. These materials undergo strain even at low voltages (less than or equal to approximately 9 volts). A disadvantage of ionic EAPs is that they are best kept wet, and hence sealed within a flexible coating. 
     Another class of EAPs for use in a vision prosthesis includes electronic EAPs, such as ferroelectric polymers, electrets, dielectric elastomers, and electrostictive graft elastomers. These materials require high voltages for actuation. However they can deliver considerable force in a short time. Unlike the ionic EAPs, these materials can function without a protective coating and require only a minimal current to maintain their position. 
     Vision prosthesis that include EAP actuators can be used in a variety of applications. These include intraocular lenses, contact lenses, and spectacle lenses. These applications are fully described in U.S. Pat. No. 6,638,304.