- Electromagnetic induction - Electromagnetic induction is the process of generating an electric potential across a conductor placed near to a changing magnetic field. This phenomenon has been exploited in wireless powering technologies. In this case, the rapid change in magnetic field is generated electrically by an external transmitter rather than through the physical movement of a magnet.
- Resonance (electrical) - Electrical components tuned to resonance give minimum losses when energy is transferred from one component to another. Often this resonance occurs at a specific excitation frequency. For wireless powering resonance is important to maximise efficiency. An example is the playground swing - it will attain its largest degree of motion when pushed in its direction of motion periodically.
- Back scattering - This is one of the techniques used by battery-less (or passive) implants to communicate with an external device. The external device will send out a signal (radiated power) to the implant whereas the reflected signal from the implant back to the external device allows us to understand the current state of the implant. An analogy is throwing a ball towards a target. The ball might propel the target forward (providing energy to the target) and the way the ball rebounds tells us something about the target itself – is the target soft? In what direction did the ball rebound? Back scattering telemetry involves the implant (target) dynamically changing its electrical properties while this change is detected externally (rebound pattern).
At the Hamlyn centre we develop smart implants and wearable devices for monitoring patient recovery during and after surgery. One key technological element of sensing implants, whether chronic or for deployment over a relatively short time, is power delivery and communication with external devices for data logging and/or implant calibration. Ideally, batteries in chronic implants operating continuously should be charged wirelessly, eliminating the need to surgically remove the depleted battery. On the other hand, batteries add weight and size to implants that only need to operate intermittently. To address these issues, power can be delivered to an implanted device wirelessly via a number of means including near-field inductive coupling, mid-field resonant coupling, and ultrasound. Many of these wireless powering technologies provide a means of data communication from/to the implant via back scattering. The most widely deployed wireless powering and telemetry technique to date is inductive coupling. This is used in a number of devices including cochlear implants where power is transmitted from the external device with a transmitting coil to the implant receiver coil. The transmitter coil produces a time-varying magnetic field that is detected by the receiver coil and provides energy for the implant. Inductive coupling is suitable for delivering a moderate amount of power (<100mW) over short-medium (few cm) distances. Ultrasonic powered implants such as pacemakers are also emerging. Currently, experimental work on mid-field power delivery and telemetry carried out by research groups around the world are also showing promising results. At the Hamlyn Centre we have developed external coils that are capable of delivering power to small implants a few cm away using inductive coupling.
A key application of battery-less, externally powered implants is to provide continuous monitoring after patients have returned home from hospital following surgery. An external power source as well as implant communication device can be used to take intermittent readings from the implant, revealing signs of surgical recovery. This allows doctors to be alerted immediately should the patient require urgent life-saving medical attention. These implants are particularly suited to use in surgery of the gut where it will be expelled naturally after use.
Advances in integrated circuit and sensor technology can lead to the production of implants with increased functionality, reduced size, and improved energy efficiency. The future challenge is to develop smaller devices and integrating sensing capabilities currently not available in miniaturised form - such as Raman spectroscopy for bacteria detection - as well as achieving a greater separation between the external “reader” and the implant.
The design of battery-less implants that can be powered wirelessly for multi-modal tissue ischemia detection following colorectal surgery poses several technical challenges. For wireless power delivery, we are investigating the prospect of using ultrasound as a power source as well as inductive powering. This is to extend the working distance between the implant and the external power source while keeping the transmitted power within safe limits for the patient. On the other hand, typical bench-top sensing systems, comprised of precision instruments, are capable of carrying out very accurate measurements. In the case of an implant, the instrumentation is scaled down thanks to integrated circuitry and micro-fabricated sensor technology. This permits micro-power operation of the implant suitable for operation with a wireless power source. However, the reduction of power consumption and the use of wireless powering come at the expense of measurement precision. To this end, we are developing novel systems and circuit level techniques to tackle these challenges such that precise measurements rivalling bench-top performance can be carried out using implants.