Transmitting coils for neurostimulation

1. Apparatus comprising:

an implantable medical device, comprising a receiving coil; and

a transmitting device, comprising:

a first transmitting coil;

a second transmitting coil disposed with respect to the first transmitting coil such that a shortest distance from an edge of the second transmitting coil to an edge of the first transmitting coil is within 20% of a length of the receiving coil;

a current source; and

a control unit configured to transmit power to the implantable medical device by driving the current source to:

drive a current in a clockwise direction through one of the first and second transmitting coils, and

drive a current in a counterclockwise direction through the other one of the first and second transmitting coils.

2. The apparatus according to claim 1 , wherein the transmitting device further comprises an at least partially rigid element connecting the first and second transmitting coils.

3. The apparatus according to claim 1 , wherein a diameter of the first transmitting coil is within 5% of a diameter of the second transmitting coil.

4. The apparatus according to claim 3 , wherein the diameters of the first and second transmitting coils are equal to one another.

5. The apparatus according to claim 1 , wherein respective external diameters of the first and second transmitting coils are 20-50 mm.

6. The apparatus according to claim 1 , wherein the length of the receiving coil is 3-30 mm.

7. The apparatus according to claim 1 , wherein each of the first and second transmitting coils comprises a plurality of turns surrounding an opening, a diameter of the opening being at least 70% of an external diameter of the transmitting coil.

8. The apparatus according to claim 7 , wherein the diameter of the opening is at least 80% of the external diameter of the transmitting coil.

9. The apparatus according to claim 8 , wherein the diameter of the opening is at least 90% of the external diameter of the transmitting coil.

10. The apparatus according to claim 1 ,

wherein the implantable medical device is configured to transmit a feedback signal indicative of an amount of power received by the implantable medical device; and

wherein the transmitting device comprises a plurality of transmitting coils comprising:

the first and second transmitting coils; and

at least one additional transmitting coil,

the control unit being configured to receive the feedback signal, and select the first and second transmitting coils from the plurality of transmitting coils in response to the feedback signal.

11. A method for transmitting power to a medical implant that includes a receiving coil, the medical implant being implanted at an implantation depth beneath a skin surface, the method comprising:

placing a first transmitting coil of a transmitting device and a second transmitting coil of the transmitting device adjacent to the skin surface, such that (a) a line connecting a center of the first transmitting coil to a center of the second transmitting coil is generally parallel to a longitudinal axis of the receiving coil, and (b) a shortest distance from an edge of the first transmitting coil to an edge of the second transmitting coil is within 20% of a length of the receiving coil; and

using a control unit of the transmitting device, driving a current source of the transmitting device to transmit power to the medical implant by:

driving a current in a clockwise direction through one of the first and second transmitting coils, and

driving a current in a counterclockwise direction through the other one of the first and second transmitting coils.

12. The method according to claim 11 , further comprising implanting the medical implant at the implantation depth.

13. The method according to claim 12 , wherein implanting the medical implant at the implantation depth comprises implanting the medical implant such that the receiving coil is parallel to the skin surface.

14. The method according to claim 11 , further comprising selecting the first and second transmitting coils from a plurality of transmitting coils that includes (a) the first and second transmitting coils, and (b) transmitting coils that have diameters that are different from diameters of the first and second transmitting coils, in response to the implantation depth.

15. The method according to claim 14 , wherein selecting the first and second transmitting coils in response to the implantation depth comprises selecting the first and second transmitting coils such that a mean radius of each of the first and second transmitting coils is within 15% of the implantation depth,

the mean radius being a mean of an internal radius of the coil and an external radius of the coil.

16. The method according to claim 11 , further comprising transmitting data to the medical implant by driving the currents through the first and second transmitting coils.

17. The method according to claim 11 , wherein:

the medical implant is configured to transmit a feedback signal indicative of an amount of power received by the medical implant,

the first and second transmitting coils belong to a plurality of transmitting coils that includes at least one additional transmitting coil, and

the method further comprises:

placing the additional transmitting coils adjacent to the skin surface; and

using the control unit, receiving the feedback signal, and selecting the first and second transmitting coils from the plurality of transmitting coils in response to the feedback signal.