Abstract:
An apparatus includes a substrate and a plurality of biological material stimulators positioned on the substrate. Each biological material stimulator forms a fluctuating magnetic field capable of inducing a current in biological material.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is based on and claims the benefit of U.S. Provisional patent applications Ser. No. 62/148,053, filed Apr. 15, 2015 and Ser. No. 62/148,465, filed Apr. 16, 2015, the content of which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    By applying a voltage or a changing magnetic field to a nerve cell, it is possible to cause the nerve cell to “fire” during which the nerve cell depolarizes and then repolarizes. 
         [0003]    In external magnetic stimulation, a strong alternating magnetic field is generated external to the body and is directed into the body. Within the body, the fluctuating magnetic field creates a current along the nerve cells that cause the cells to fire. 
         [0004]    Such external systems require strong magnetic fields in order to penetrate into the body. However, as the magnetic fields increase in strength, the area affected by the magnetic fields also increases resulting in low resolution stimulus of the nerve cells. As a result, it is difficult to direct the external magnetic field to only a select number of nerve cells. 
         [0005]    In deep brain stimulation, an electrode is inserted into the body and an electric field is generated in the target area to be stimulated. This electric field causes nerve cells to fire. 
       SUMMARY 
       [0006]    An apparatus includes a substrate and a plurality of biological material stimulators positioned on the substrate. Each biological material stimulator forms a fluctuating magnetic field capable of inducing a current in biological material. 
         [0007]    In accordance with a further embodiment, a method includes introducing an array of biological material stimulators mounted on a substrate into a living body and applying signals to the array of biological material stimulators to cause the biological material stimulators to generate a fluctuating magnetic field that induces a current in biological material surrounding the biological material stimulator. 
         [0008]    A neuron stimulator includes a substrate having an outer surface and a magnetic nanowire extending transverse to the outer surface such that a top of the nanowire is above the substrate. The magnetic nanowire has at least two magnetic domains with at least one moveable domain wall between the magnetic domains. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram of a nanostimulation/nanosensing system in accordance with one embodiment. 
           [0010]      FIG. 2  is a perspective view of the implanted device of  FIG. 1  next to a neuron. 
           [0011]      FIG. 3  is a perspective view of the implanted device of  FIG. 1  in a bent configuration. 
           [0012]      FIG. 4  is a sectional side view of a nanowire stimulator in accordance with one embodiment. 
           [0013]      FIG. 5  is a sectional side view of a nanowire bridge stimulator in accordance with one embodiment. 
           [0014]      FIG. 6  is a perspective view of the nanowire bridge stimulator of  FIG. 5 . 
           [0015]      FIG. 7  is a section side view of a nanowire half loop stimulator in accordance with one embodiment. 
           [0016]      FIG. 8  is a perspective view of the nanowire half loop stimulator of  FIG. 7 . 
           [0017]      FIG. 9  is a sectional side view of a Magnetic Tunnel Junction (MTJ) nanostimulator in accordance with one embodiment. 
           [0018]      FIG. 10  is a section side view of a Magnetic Tunnel Junction (MTJ) nanosensor in accordance with one embodiment. 
           [0019]      FIG. 11  is a top view of the MTJ nanosensor of  FIG. 10 . 
           [0020]      FIG. 12  is a top view of a Wheatstone Bridge nanosensor in accordance with one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Embodiments described below provide arrays of magnetic nanostructures that generate and/or use magnetic fields for biomedical applications. For example, some magnetic nanostructures are be used to modulate (stimulate) biological material such as cells, tissue and organs while other magnetic nanostructures used to sense cellular activity. The magnetic nanostructures can include structures such as nanowires, magnetic tunnel junctions or giant magnetoresistance elements, for example. For embodiments that use nanowires, domain walls are introduced into a nanowire and are moved through the nanowire thereby creating a fluctuating magnetic field external to the nanowire. In accordance with some embodiments, the nanowires are formed as vertical half-loops or bridges that extend away from a substrate so as to place the nanowires in closer contact with the cells that they are stimulating. In accordance with some embodiments, half loops and bridges of differing heights are used to improve contact with the biological material. For embodiments that use magnetic tunnel junctions or giant magnetoresistance elements as the nanostructures, the magnetic tunnel junction or the giant magnetoresistance elements are raised relative to the substrate to provide better contact between the elements and the cells they are to stimulate. The magnetic domain of the top-most magnetic layer in the magnetic tunnel junction or the giant magnetoresistance element is then rotated based on an input signal to form a low frequency magnetic field around the magnetic tunnel junction or giant magnetoresistance element. The magnetic field generated by the magnetic nanostructures can be controlled by either an electrical signal or by a light signal. 
         [0022]    In accordance with some embodiments, the magnetic nanostructures are mounted on a flexible substrate such that the substrate and the magnetic nanostructures can be wrapped around biological material to thereby better focus the stimulation signals onto a particular area in the body, such as a particular area in the brain. 
         [0023]      FIG. 1  is a schematic diagram of a nanostimulation/nanosensor system  100  including an implanted device  102  and an external device  104 . Implanted device  102  is located within a living body  105  and external device  104  is located in an environment  107  exterior to living body  105  and separated from living body  105  by an outer surface  106  of the living body. In accordance with one embodiment, implanted device  102  is inserted into living body  105  through an incision in outer surface  106  and then the incision is sealed such that there is no wired connection between implanted device  102  and external device  104 . 
         [0024]    Implanted device  102  includes a flexible substrate  110  having a plurality of nanostimulators and nanosensors mounted thereon. In accordance with one embodiment, the plurality of nanostimulators and nanosensors are arranged in an array. In  FIG. 1 , there are four columns  112 ,  114 ,  116  and  118  of nanostimulators, such as nanostimulators  120 ,  122 ,  124 ,  126  and  128 . There are also four columns of nanosensors  130 ,  132 ,  134  and  136  that include nanosensors, such as nanosensors  138 ,  140 ,  142 ,  144  and  146 . 
         [0025]    Each nanostimulator generates a fluctuating magnetic field when active that interacts with biological material. Although these interactions can include interfering with the normal operation of the biological material, changing the physical structure of the biological material, changing the operation of the biological material and stimulating the biological material to generate a response from the biological material, for example, the interactions are referred to generically as stimulating the biological materials and the devices used to generate the fluctuating magnetic fields are referred to generically as stimulators. In the descriptions herein, the reference to stimulating should be read to include all interactions between the magnetic field generating devices and the biological tissue and the references to stimulators and nanostimulators should be read to include all implanted devices that generate a fluctuating magnetic field that interacts with biological tissue. 
         [0026]    In addition to the nanostimulators and nanosensors, a stimulator controller  150 , a sensor controller  154 , a memory  158 , a communication circuit  160  and a power circuit  170  are mounted on flexible substrate  110 . The nanostimulators are connected to stimulator controller  150  by a stimulation bus  152  on flexible substrate  110  to allow controller  150  to activate individual nanostimulators at any one time and to activate multiple nanostimulators at the same time. The stimulator nanostructures such as the nanowire, mtj and giant magnetoresistance nanostructure can be controlled by applying electrical signals to stimulation bus  152  or by applying light signals to stimulation bus  152 . In embodiments where light is used to control the stimulators, stimulation bus  152  takes the form of optic wave guides. 
         [0027]    Similarly, the nanosensors are connected to sensor controller  154  by a sense control bus  156  that allows the sensor controller  154  to receive a sensed value from each of the nanosensors. Sensor controller  154  can store the sensed values in memory  158 . In addition, sensor controller  154  can transmit the sensed values or the values stored in memory  158  wirelessly using communication circuit  160 , which consists of a wireless communication controller  162  and an antenna  164 . Wireless communication controller  162  communicates through antenna  164  to an antenna  166  that is coupled to a wireless communication circuit  168  of external device  104 . Through this communication, sensor controller  154  is able to communicate the sensed values externally where the sense values may be used by an external processor. 
         [0028]    Stimulator controller  150  may receive instructions to stimulate particular nanostimulators through communication circuit  160 . In particular, instructions may pass through wireless communication circuit  168  and antenna  166  and be received by antenna  164  and wireless communication controller  162  before being provided to stimulator controller  150 . 
         [0029]    Although communication circuit  160  is shown to be wireless in  FIG. 1 , in other embodiments, a wired connection is provided between implanted device  102  and external device  104 . 
         [0030]    Power circuit  170  provides power to sensor controller  150 , stimulator controller  154 , communication circuit  160  and memory  158  through a power bus  171 . Some of the power provided to stimulator controller  150  is routed to the nanostimulators by sensor controller  150  to cause the nanostimulators to generate a low frequency magnetic field as discussed further below. Some of the power provided to sensor controller  154  is routed to the nanosensors to read a magnetic field value sensed by the nanosensors as discussed further below. 
         [0031]    In accordance with one embodiment, power source  170  includes an induction loop  174  that generates an electrical current when it receives an alternating magnetic field  176 . The current from induction loop  174  can be used to provide power to power bus  171  directly or can be used to recharge a battery  172  that in turn provides power to power bus  171 . In accordance with one embodiment, the alternating magnetic field  176  received by induction loop  174  is generated by a corresponding induction loop  178  in a power system  180  of external device  104 . Thus, external device  104  provides power wirelessly to implanted device  102 . This power can be provided periodically to charge battery  172  or can be provided continuously to directly power the components of implanted device  102 . After battery  172  has been charged, external device  104  can be removed and does not need to be present in order for implanted device  102  to operate since battery  172  can provide power while external device  104  is not present. In accordance with other embodiments, a wired connection between implanted device  102  and external device  104  allows power to be conveyed between external device  104  and implanted device  102  either periodically to recharge battery  172  or continuously to power implanted device  102 . 
         [0032]      FIG. 2  shows implanted device  102  in contact with a neuron  200 . In particular, two nanostimulators  128  and  202  are in contact with neuron  200  and two nanosensors  204  and  206  are in contact with neuron  200 . As a result, one or both of nanostimulators  128  and  202  may stimulate neuron  200  to cause it to fire while one or both of nanosensors  204  and  206  may sense the firing of neuron  200  by sensing the magnetic field generated by neuron  200  as current passes through neuron  200 . Although direct contact with the neuron is not required to stimulate the neuron or to sense a firing event in the neuron, contact does improve the ability to stimulate the neuron and sense a firing event in the neuron. 
         [0033]    As mentioned above, the substrate  110  of implanted device  102  is flexible as shown in  FIG. 3 . Thus, implanted device  102  can be bent to form a curved shape  300  such that implanted device  102  can be wrapped around biological material to thereby focus the stimulation to a particular area in the body and to focus the sensing of cellular activity in a particular area of the body. Although flexible substrate  110  is shown flexed along a first axis, flexible substrate  110  is bendable along multiple axes. 
         [0034]    Although  FIGS. 1-3  show a combination of nanosensors and nanostimulators on implanted device  102 , in other embodiments, the implanted device contains only nanostimulators and in other embodiments, the implanted device contains only nanosensors. In addition, although the nanosensors and nanostimulators of  FIGS. 1-3  are shown in separate columns, in other embodiments, nanosensors and nanostimulators are in a same column. 
         [0035]      FIG. 4  provides a sectional side view of a nanostimulator  400 , which can be used as one of the nanostimulators such as nanostimulator  120  of  FIG. 1 . Nanostimulator  400  uses a nanowire  402  that includes a plurality of magnetic domains  404 ,  406 ,  408  and  410  with neighboring domains being separated by domain walls  412 ,  414  and  416 . In accordance with one embodiment, nanowire  402  is constructed of CoFeB. Nanowire  402  includes two ends, which are in contact with two respective electrical contacts  420  and  422 . By passing a polarized current between electrical contacts  420  and  422  through nanowire  402 , it is possible to move domain walls  412 ,  414  and  416  and thereby change the magnetic field  421  generated by nanowire  402 . This change in magnetic field  421  creates a fluctuating magnetic field that interacts with biological material. In the case of neurons, the fluctuating magnetic field can induce depolarization in a neuron, causing the neuron to fire. In other biological materials, the fluctuating magnetic field can cause the material or the material&#39;s function to be altered in some way. 
         [0036]    To change the direction of the magnetic domain in nanowire  402 , a current  430  can be passed from contact  432  to contact  434  to generate a magnetic field in nanowire  402  sufficient to set the direction of the magnetic domain. 
         [0037]    The structures of nano stimulator  400  are formed through photolithography and milling on top of an outer surface  111  of flexible substrate  110 . As a result, nanowire  402  extends transverse to outer surface  111  such that a top  403  of nanowire  402  is above substrate  110 . To isolate the electrical contacts, layers of insulating material  436 ,  440 ,  441  and  442  are deposited and milled. 
         [0038]      FIG. 5  provides a sectional side view of a nanostimulator  500 , which is similar to the nanostimulator  400  of  FIG. 4  except that nanowire  402  has been replaced with nanowire  502 , which has a greater height  504  than nanowire  402 . Nanowire  502  contains magnetic domains pointing in different directions and separated by domain walls so as to produce a magnetic field  521  that fluctuates as the domain walls are moved through nanowire  502  by a current passing through nanowire  502  from contact  420  to contact  422 . Additional domain walls may be added to nanowire  502  by applying a current between contacts  432  and  434 . 
         [0039]    Nanowire  502  forms a bridge structure  506  having legs  508  and  510  and a cross beam  512 . Legs  508  and  510  extend transverse to outer surface  111  of substrate  110  such that the top of cross beam  512  is above substrate  110 . In  FIG. 5 , bridge  506  is filled with insulating material  542 . In other embodiments, insulating material  542  is not present and the interior of bridge  506  is empty as shown in the perspective view of  FIG. 6 . 
         [0040]    In embodiments which use bridge structure  506  as the nanostimulator, different nanostimulators can have different heights  504  within implantable device  102 . Thus, the nanostimulators in columns  112 ,  114 ,  116  and  118  can each be at a different height allowing for differing levels of physical contact between the nanostimulators and the surrounding biological material. 
         [0041]      FIG. 7  provides a sectional side view of a further embodiment of a nanostimulator in the form of a half looped nanowire  702 . Nanowire  702  contains magnetic domains pointing in different directions and separated by domain walls similar to the domains and domain walls of nanowire  402  of  FIG. 4 . The different domains form a magnetic field  721  that fluctuates as the domain walls are moved by current passing from contact  420  to contact  422 . Additional domain walls may be added to nanowire  702  by applying a current between contacts  432  and  434 . Nanowire  702  is formed into a half loop  706  with a rounded top  712  and two legs  708  and  710 . Half loop  706  extends transverse to substrate  110  to a height  704  above outer surface  111  of flexible substrate  110 . In accordance with one embodiment, implantable device  102  includes multiple half loop nanostimulators, such as shown in  FIG. 7 , each with a different height  704 , such that the implantable device  102  has an array of half loop nanostimulators each at different heights. 
         [0042]    In  FIG. 7 , the interior of half loop  706  is filled with an insulating material  742 . In other embodiments, insulting material  742  does not fill half loop  706 . Instead, an empty space is present within half loop  706  as shown in the perspective view of half loop  706  shown in  FIG. 8 . 
         [0043]    The magnetic stimulation of neurons using the various embodiments causes a neuron to “fire”. Before firing, the neuron membrane has a resting potential of −70 mv. At the beginning of the firing event, a current of K+ ions flows into the membrane causing the membrane potential to rise to 30 mv. This rise in the membrane&#39;s potential is referred to as depolarization. A current of K+ ions then flows out of the membrane causing repolarization followed by hyperpolarization. The membrane then returns to its resting potential. 
         [0044]    The amount of magnetic flux required to create depolarization can be calculated using Ampere&#39;s law: 
         [0000]    
       
      
               
       {right arrow over (B)}·{right arrow over (dl)}=μ 
       0 
       l=μ 
       0 
               
       σ{right arrow over (E)}·{right arrow over (dA)} 
      
     
         [0045]    For a typical neuron: σ=0.33 s/m and E=100 V/m. If the stimulator is placed a distance 10 μm, then the required magnetic field strength to cause depolarization of the neuron is B=20 nT. 
         [0046]    Domain walls generate magnetic fields external to the magnetic material. For example, a rectangular portion of a domain wall will produce an external magnetic field at a position x, y, z from the center of the rectangle of: 
         [0000]    
       
         
           
             
                 
             
              
             
               
                 B 
                 x 
               
               = 
               
                 
                   
                     
                       μ 
                       0 
                     
                      
                     
                       M 
                       s 
                     
                      
                     t 
                   
                   
                     2 
                      
                     π 
                      
                     
                         
                     
                      
                     s 
                   
                 
                  
                 
                   log 
                    
                   
                     [ 
                     
                       
                         
                           
                             
                               
                                 
                                   ( 
                                   
                                     s 
                                     - 
                                     
                                       2 
                                        
                                       y 
                                     
                                   
                                   ) 
                                 
                                 + 
                                 
                                   
                                     
                                       
                                         ( 
                                         
                                           
                                             2 
                                              
                                             x 
                                           
                                           - 
                                           w 
                                         
                                         ) 
                                       
                                       2 
                                     
                                     + 
                                     
                                       
                                         ( 
                                         
                                           
                                             2 
                                              
                                             y 
                                           
                                           - 
                                           s 
                                         
                                         ) 
                                       
                                       2 
                                     
                                     + 
                                     
                                       4 
                                        
                                       
                                         z 
                                         2 
                                       
                                     
                                   
                                 
                               
                               
                                 
                                   ( 
                                   
                                     s 
                                     - 
                                     
                                       2 
                                        
                                       y 
                                     
                                   
                                   ) 
                                 
                                 + 
                                 
                                   
                                     
                                       
                                         ( 
                                         
                                           
                                             2 
                                              
                                             x 
                                           
                                           + 
                                           w 
                                         
                                         ) 
                                       
                                       2 
                                     
                                     + 
                                     
                                       
                                         ( 
                                         
                                           
                                             2 
                                              
                                             y 
                                           
                                           - 
                                           s 
                                         
                                         ) 
                                       
                                       2 
                                     
                                     + 
                                     
                                       4 
                                        
                                       
                                         z 
                                         2 
                                       
                                     
                                   
                                 
                               
                             
                             · 
                           
                         
                       
                       
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     - 
                                     s 
                                   
                                   - 
                                   
                                     2 
                                      
                                     y 
                                   
                                 
                                 ) 
                               
                               + 
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           2 
                                            
                                           x 
                                         
                                         + 
                                         w 
                                       
                                       ) 
                                     
                                     2 
                                   
                                   + 
                                   
                                     
                                       ( 
                                       
                                         
                                           2 
                                            
                                           y 
                                         
                                         + 
                                         s 
                                       
                                       ) 
                                     
                                     2 
                                   
                                   + 
                                   
                                     4 
                                      
                                     
                                       z 
                                       2 
                                     
                                   
                                 
                               
                             
                             
                               
                                 ( 
                                 
                                   
                                     - 
                                     s 
                                   
                                   - 
                                   
                                     2 
                                      
                                     y 
                                   
                                 
                                 ) 
                               
                               + 
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           2 
                                            
                                           x 
                                         
                                         - 
                                         w 
                                       
                                       ) 
                                     
                                     2 
                                   
                                   + 
                                   
                                     
                                       ( 
                                       
                                         
                                           2 
                                            
                                           y 
                                         
                                         - 
                                         s 
                                       
                                       ) 
                                     
                                     2 
                                   
                                   + 
                                   
                                     4 
                                      
                                     
                                       z 
                                       2 
                                     
                                   
                                 
                               
                             
                           
                         
                       
                     
                     ] 
                   
                 
               
             
           
         
       
       
         
           
             
                 
             
              
             
               
                 B 
                 y 
               
               = 
               
                 
                   
                     
                       μ 
                       0 
                     
                      
                     
                       M 
                       s 
                     
                      
                     t 
                   
                   
                     2 
                      
                     π 
                      
                     
                         
                     
                      
                     s 
                   
                 
                  
                 
                   log 
                    
                   
                     [ 
                     
                       
                         
                           
                             
                               
                                 
                                   ( 
                                   
                                     w 
                                     - 
                                     
                                       2 
                                        
                                       x 
                                     
                                   
                                   ) 
                                 
                                 + 
                                 
                                   
                                     
                                       
                                         ( 
                                         
                                           
                                             2 
                                              
                                             x 
                                           
                                           - 
                                           w 
                                         
                                         ) 
                                       
                                       2 
                                     
                                     + 
                                     
                                       
                                         ( 
                                         
                                           
                                             2 
                                              
                                             y 
                                           
                                           - 
                                           s 
                                         
                                         ) 
                                       
                                       2 
                                     
                                     + 
                                     
                                       4 
                                        
                                       
                                         z 
                                         2 
                                       
                                     
                                   
                                 
                               
                               
                                 
                                   ( 
                                   
                                     w 
                                     - 
                                     
                                       2 
                                        
                                       x 
                                     
                                   
                                   ) 
                                 
                                 + 
                                 
                                   
                                     
                                       
                                         ( 
                                         
                                           
                                             2 
                                              
                                             x 
                                           
                                           + 
                                           w 
                                         
                                         ) 
                                       
                                       2 
                                     
                                     + 
                                     
                                       
                                         ( 
                                         
                                           
                                             2 
                                              
                                             y 
                                           
                                           + 
                                           s 
                                         
                                         ) 
                                       
                                       2 
                                     
                                     + 
                                     
                                       4 
                                        
                                       
                                         z 
                                         2 
                                       
                                     
                                   
                                 
                               
                             
                             · 
                           
                         
                       
                       
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     - 
                                     w 
                                   
                                   - 
                                   
                                     2 
                                      
                                     x 
                                   
                                 
                                 ) 
                               
                               + 
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           2 
                                            
                                           x 
                                         
                                         + 
                                         w 
                                       
                                       ) 
                                     
                                     2 
                                   
                                   + 
                                   
                                     
                                       ( 
                                       
                                         
                                           2 
                                            
                                           y 
                                         
                                         + 
                                         s 
                                       
                                       ) 
                                     
                                     2 
                                   
                                   + 
                                   
                                     4 
                                      
                                     
                                       z 
                                       2 
                                     
                                   
                                 
                               
                             
                             
                               
                                 ( 
                                 
                                   
                                     - 
                                     w 
                                   
                                   - 
                                   
                                     2 
                                      
                                     x 
                                   
                                 
                                 ) 
                               
                               + 
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           2 
                                            
                                           x 
                                         
                                         + 
                                         w 
                                       
                                       ) 
                                     
                                     2 
                                   
                                   + 
                                   
                                     
                                       ( 
                                       
                                         
                                           2 
                                            
                                           y 
                                         
                                         - 
                                         s 
                                       
                                       ) 
                                     
                                     2 
                                   
                                   + 
                                   
                                     4 
                                      
                                     
                                       z 
                                       2 
                                     
                                   
                                 
                               
                             
                           
                         
                       
                     
                     ] 
                   
                 
               
             
           
         
       
       
         
           
             
               B 
               z 
             
             = 
             
               
                 
                   
                     μ 
                     0 
                   
                    
                   
                     M 
                     s 
                   
                    
                   t 
                 
                 
                   2 
                    
                   π 
                    
                   
                       
                   
                    
                   s 
                 
               
                
               
                 { 
                 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     [ 
                     
                       
                         2 
                          
                         
                           ( 
                           
                             
                               w 
                               / 
                               2 
                             
                             - 
                             x 
                           
                           ) 
                         
                          
                         
                           ( 
                           
                             
                               s 
                               / 
                               2 
                             
                             - 
                             y 
                           
                           ) 
                         
                       
                       
                         z 
                          
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     2 
                                      
                                     x 
                                   
                                   - 
                                   w 
                                 
                                 ) 
                               
                               2 
                             
                             + 
                             
                               
                                 ( 
                                 
                                   
                                     2 
                                      
                                     y 
                                   
                                   - 
                                   s 
                                 
                                 ) 
                               
                               2 
                             
                             + 
                             
                               4 
                                
                               
                                 z 
                                 2 
                               
                             
                           
                         
                       
                     
                     ] 
                   
                   + 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     [ 
                     
                       
                         2 
                          
                         
                           ( 
                           
                             
                               w 
                               / 
                               2 
                             
                             - 
                             x 
                           
                           ) 
                         
                          
                         
                           ( 
                           
                             
                               s 
                               / 
                               2 
                             
                             + 
                             y 
                           
                           ) 
                         
                       
                       
                         z 
                          
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     2 
                                      
                                     x 
                                   
                                   - 
                                   w 
                                 
                                 ) 
                               
                               2 
                             
                             + 
                             
                               
                                 ( 
                                 
                                   
                                     2 
                                      
                                     y 
                                   
                                   + 
                                   s 
                                 
                                 ) 
                               
                               2 
                             
                             + 
                             
                               4 
                                
                               
                                 z 
                                 2 
                               
                             
                           
                         
                       
                     
                     ] 
                   
                   - 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     [ 
                     
                       
                         2 
                          
                         
                           ( 
                           
                             
                               
                                 - 
                                 w 
                               
                               / 
                               2 
                             
                             - 
                             x 
                           
                           ) 
                         
                          
                         
                           ( 
                           
                             
                               s 
                               / 
                               2 
                             
                             - 
                             y 
                           
                           ) 
                         
                       
                       
                         
                           z 
                            
                           
                             
                               
                                 ( 
                                 
                                   
                                     2 
                                      
                                     x 
                                   
                                   + 
                                   w 
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                         + 
                         
                           
                             ( 
                             
                               
                                 2 
                                  
                                 y 
                               
                               - 
                               s 
                             
                             ) 
                           
                           2 
                         
                         + 
                         
                           4 
                            
                           
                             z 
                             2 
                           
                         
                       
                     
                     ] 
                   
                   - 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     [ 
                     
                       
                         2 
                          
                         
                           ( 
                           
                             
                               
                                 - 
                                 w 
                               
                               / 
                               2 
                             
                             - 
                             x 
                           
                           ) 
                         
                          
                         
                           ( 
                           
                             
                               s 
                               / 
                               2 
                             
                             + 
                             y 
                           
                           ) 
                         
                       
                       
                         z 
                          
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     2 
                                      
                                     x 
                                   
                                   + 
                                   w 
                                 
                                 ) 
                               
                               2 
                             
                             + 
                             
                               
                                 ( 
                                 
                                   
                                     2 
                                      
                                     y 
                                   
                                   + 
                                   s 
                                 
                                 ) 
                               
                               2 
                             
                             + 
                             
                               4 
                                
                               
                                 z 
                                 2 
                               
                             
                           
                         
                       
                     
                     ] 
                   
                 
                 } 
               
             
           
         
       
     
         [0000]    where M s  is the saturation magnetization of the material, μ 0  is the permeability of the material, w is the width of the magnetic material, t is the thickness of the magnetic material, and s is the width of the domain wall transverse to w and t. 
         [0047]    In a further embodiment, a triangle model of a transverse domain wall is used to determine the magnetic field at positions/external to the transverse domain wall. The triangle is an isosceles triangle with its base equal to the width of the domain wall and its height equal to the width of the magnetic material. To determine the magnetic field, the triangle broken into rectangles and the fields from the rectangles are summed to form the field for the domain wall. 
         [0048]    In order to generate a current along the axis of the neuron, the magnetic field produced by the domain wall must fluctuate. To produce this fluctuation, several embodiments move the domain wall through the material. For a magnetic nanowire, a domain wall can be moved by passing a spin polarized current through the nanowire. 
         [0049]    The use of transverse domain walls instead of current carrying loops was selected by the present inventors for power efficiency and to avoid damaging neurons. Under electrical deep brain stimulation, 1-4 Volts of electrical energy is applied to the subthalamic nucleus (STN) to trigger the neuron. From Faraday&#39;s law of electromagnetic induction, the amount of magnetic field required to induce 1 V EMF for the stimulation of the STN is 800 Gauss (with a radius of STN 2 mm and a pulse width 1 μs). To generate 800 Gauss magnetic field from current carrying wire, a large current of 3-50 Amps would have to be applied to the wire. This would require a great deal of power and would generate a substantial amount of heat that may damage the neurons. The field from one moving transverse domain wall is sufficient to stimulate the STN. In addition, the domain wall is moved by using a very small alternating current of around 74 micro-Amps. The alternating current supplied to a magnetic layer produces spin-transfer torque which induces motion of domain wall. The spin-transfer torque combined with spin-orbit torque reduces significantly the amount of current required to move the domain wall. 
         [0050]      FIG. 9  provides a sectional view of a nanostimulator  900  in accordance with a further embodiment. Nanostimulator  900  is formed of a magnetic tunnel junction nanostructure that includes two electrical contacts  902  and  904 , an antiferromagnetic layer  906 , a fixed or pinned ferromagnetic layer  908 , a free magnetic layer  910  and insulating layers  912  and  914 . Fixed magnetic layer  908  has a magnetic domain with a dipole direction  916  that is fixed while free magnetic layer  910  has a magnetic domain with a dipole direction that can be switched by applying spin polarized current between contacts  902  and  904 . Spin polarized current in a first direction causes the magnetic domain to point in a first direction  918  and spin polarized current in a second direction causes the magnetic domain to point in a second direction  920 . Free magnetic layer  910  generates an external magnetic field  922  that can interact with biological material. By alternating between directions  918  and  920  through the application of the spin polarized current, it is possible to create a low frequency fluctuating magnetic field  922  that can interact with biological material such as cells, tissues and organs, for example. In accordance with other embodiments, a giant magneto resistive nanostructure is used in place of the magnetic tunnel junction nanostructure. 
         [0051]      FIG. 10  provides a side sectional view and  FIG. 11  provides a top view of a magnetic nanosensor  1000  that can be used as one of the nanosensors on an implanted device  102 , such as nanosensor  138 . Nanosensor  1000  includes a central sensing portion  1002  and two magnetic flux guides  1004  and  1006 . Flux guides  1004  and  1006  concentrate magnetic flux into a smaller area within central sensing portion  1002  to thereby increase the sensitivity of central sensing portion  1002 . 
         [0052]    Central sensing portion  1002  is constructed of a magnetic tunnel junction similar to the magnetic tunnel junction of stimulator  900  of  FIG. 9 . In particular, the magnetic tunnel junction of nanosensor  1000  includes electrical contacts  1022  and  1024 , non-ferromagnetic layer  1026 , pinned magnetic layer  1028 , free magnetic layer  1030  and insulator layers  1032  and  1034 . Pinned magnetic layer  1028  has a fixed domain direction and free magnetic layer  1030  has a free domain direction that may be set based on an external magnetic field  1050  generated by the firing of a neuron or other living biological material. The direction of the domain of free magnetic layer  1030  can be determined by passing a current between electrical contacts  1022  and  1024 . When the domain of free magnetic layer  1030  is parallel to pinned magnetic layer  1028 , a first resistance is detected and when the domain of free magnetic layer  1030  is antiparallel to the domain direction of pinned magnetic layer  1028  a second resistance is detected. Thus, by measuring this resistance, it is possible to determine the direction of external magnetic field  1050  and thus it is possible to determine if a neuron has fired. 
         [0053]    Nanosensor  1000  may be used alone to sense neuron firing events or may be included as part of a Wheatstone bridge to sense firing events. For example, nanosensor  1000  can be used as part of Wheatstone bridge  1200  of  FIG. 12 . In Wheatstone bridge  1200 , three magnetic tunnel junctions  1202 ,  1204  and  1206  are used in cooperation with magnetic tunnel junction nanosensor  1000 . Magnetic tunnel junction  1202 ,  1204  and  1206  are protected from external magnetic fields by a shielding layer and therefore their free layer does not change its domain direction and therefore does not change its resistance. 
         [0054]    A current is passed between current contacts  1208  and  1210  along two parallel paths. A first path is through nanosensor  1000  and magnetic tunnel junction  1202  and a second path is through magnetic tunnel junction  1204  and magnetic tunnel junction  1206 . When the external magnetic field sensed by nanosensor  1000  is in a first direction it causes the domain of the free layer of nanosensor  1000  to be in a first direction and nanosensor  1000  to have a first resistance. This first resistance is balanced by Wheatstone bridge  1200  such that a voltage between voltage contacts  1212  and  1214  is zero. When the magnetic field changes directions thereby changing the domain direction of the free layer of nanosensor  1000 , the resistance of nanosensor  1000  changes causing Wheatstone bridge  1200  to become unbalanced thereby creating a voltage between contacts  1212  and  1214 . This sensed voltage change is detected to indicate a neuron firing event. 
         [0055]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.