Introduction

The invention relates to inductive devices.

One application of such devices is the drop foot condition typically found in stroke patients. This is an inability to adequately dorsiflex the ankle resulting either from a weakness of the dorsiflexor muscles, spasticity in the plantar flexors, or a combination of the two. The traditional approach to this condition was to brace the ankle joint with an ankle foot orthosis. An alternative approach is to electrically stimulate the dorsiflexors during the swing phase of walking. The stimulation is applied via electrodes placed on the surface of the leg. Surface stimulation has a number of associated problems including difficulty in locating the correct points for stimulation, inability to stimulate the deeper lying nerves, lack of selectivity, variation in skin impedance, physical discomfort and low energy efficiency.

In an effort to overcome these problems, a two-channel implantable stimulator was developed. This consists of an externally worn transmitter inductively coupled to an implanted receiver unit. The two channels of the receiver are connected by electrodes to the nerves being stimulated. By inductively coupling the external transmitter to the implant there is no need for a transcutaneous wire link or a battery in the implant. With inductive coupling there are typically two resonant circuits (one in the external transmitter and the other in the implanted receiver) that are tuned to the same frequency. Each resonant circuit consists of a parallel or series arrangement of a capacitor and inductor. By applying an alternating voltage to the transmitter resonant circuit, a voltage is induced in the receiver resonant circuit and hence power is transferred from transmitter to receiver. In a two-channel device, there are two receivers in the implant, each of which is tuned to a different frequency and used to stimulate a different nerve.

The use of coils for inductive power and data transmission for other medical applications is also known. For example, US 6083174 "Implantable measuring unit for intracorporal measurement of patient data", and US 2002/0165592A1, "Induction powered in-vivo imaging device". In particular US 2002/0165592A1 discusses a three axial coil assembly with three separate orthogonal elements. This ensures that energy will be produced from a unidirectional magnetic field independently of the directionality of the energy-receiving unit.

There are some problems with the current implantable inductors. Wirewound coils which are used for the inductors in the implant tend to be bulky and there are concerns about the repeatability and reliability of such an approach.

Also, in the two-channel stimulator, each channel has to be independently monitored and controlled. Movement of the implant relative to the external stimulator could result in an increase in the signal on one channel and a decrease in the signal on the second channel. This depends on the direction and amount of relative displacement. Thus, implementing a control loop on such a setup could prove to be very difficult.

There are two ways in which other researchers have dealt with this problem in the past. One method is to use active implants whereby active components are incorporated into the implant to monitor and regulate voltage and current levels. This leads to a larger and more power-hungry implant. Another method employed is one of direct user control whereby the user simply turns up or down the transmitter voltage manually for each channel until the correct level of stimulation is achieved. This can be time-consuming and cumbersome as many of the individuals who will be using such a system may not be very agile.

The invention is directed towards providing an improved implantable inductor.

Summary of Invention

According to the invention, there is provided a receiving device comprising a plurality of coils, the coils being mutually arranged to minimise differences in coupling of the coils with another transmitting device arising from movement of the devices relative to each other. In one embodiment, the layout of the coils is substantially symmetrical.

In one embodiment, the coils are planar.

In one embodiment, each coil comprises a pair of opposed and inter-connected sectors in a disc-shaped footprint.

In one embodiment, the sectors are quadrants.

In one embodiment, the coils are on a disc-shaped substrate.

In one embodiment, the coils are of magnet wire construction.

The invention also provides an inductive coupling system comprising a receiver device as defined above.

In one embodiment, the system is a medical implant.

In one embodiment, the system comprises both planar and magnet wire receiving coils.

In one embodiment, the plurality of coils is for both receiving and transmitting.

In one embodiment, the roles of transmitting and receiving elements may be reversed.

In one embodiment, the system comprises a monitoring circuit for monitoring inductive coupling between coils.

In one embodiment, the monitoring circuit monitors coupling on only one coil, and due to the symmetric nature of the receiver coils, the power transmitted / coupling only needs to be monitored for a single receiver coil and the same percentage correction is given for all coils. Detailed Description of the Invention

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:-

Fig. 1 is a plan view of spiral receiver coils of a dual-channel stimulator;

Figs. 2 and 3 are plan views of coils of alternative stimulators; and

Figs. 4 and 5 are plots of test results.

Referring to Fig. 1 an implantable receiver inductor 1 comprises two spiral planar coils 2 and 3 on a substrate 4. Referring to Fig. 2 an alternative implantable receiver inductor 10 comprises a substrate 11 with the same footprint as that of Fig. 1. However, in this case coils 12 and 13 are D-shaped to make optimum use of this footprint, hi both of these embodiments, each coil is for one channel and there is a need to monitor each channel.

Referring to Fig. 3, in an implantable receiver inductor here are two coils, Coil 1 and Coil2, and each coil comprises two diametrically-opposed quadrant-shaped parts. Because each coil is arranged across opposed quadrants, a change in orientation will have the same effect in both coils. Therefore, only a single channel need be monitored. By arranging the two receiver coils so that each coil is spread across different physical locations, the varying effect of relative displacement between transmitter and receiver is neutralised such that if a certain displacement results in an increased signal on one channel, then there will also be a corresponding increased signal on the second channel. Similarly, a displacement leading to a decreased signal on one channel will also result in a similar decrease in the signal on the second channel. Hence, by monitoring and controlling a single channel, a second channel can also be monitored and controlled indirectly without any further resources required. Another problem that the invention overcomes is one of angular displacement between transmitter and receiver coils. If the coils are not perfectly parallel, then the transmitter coil may be preferentially coupled to one of the receiver coils. The invention provides a solution to this problem.

Planar technology offers benefits of reduced cost, reduced profile, increased reliability, and increased repeatability. A major advantage of the invention is that the control of the system is simplified and that by monitoring one channel, the two (or more) may be controlled.

Test results are shown in Figs. 4 and 5. While displacement of both receivers in the y direction has the same effect on both channels, movement in the x direction has a considerably different effect. When two single coils are used, the results in Fig. 4 show that as coupling from the transmitter to one coil increases, coupling from the transmitter to the other coil decreases. Fig. 5 shows that by employing split coils (Fig. 3 inductor) this problem is overcome.

The invention is not limited to the embodiments described but may be varied in construction and detail. Although the invention has been described for a biomedical application (dual channel drop foot stimulator) it may also find applications in other areas such as veterinary, wearable computing, and ambient electronics. The device may also have only one or more than two channels. Also, the coils may be for transmitting rather than receiving. In another embodiment, each coil comprises a pair of diametrically opposed sectors other than 90° quadrants, for example diametrically opposed 45° sectors. In the latter embodiment, there may therefore be a total of four coils (one coil per channel) in the same footprint.