Current Scenario
Long-term and reliable power harvesting (or generation) is a key challenge for active IMDs. Most implantable devices are powered solely using batteries, which need eventual surgical replacement that can result in trauma.
Some of the other limitations of these devices include their size, lifespan, and the risk of batteries leaking toxic substances. Therefore, current miniaturization efforts are focused on ensuring a reduction in the weight and size of these devices [4], besides ensuring all-round safety.
Due to the limited battery capacity of the active IMDs in current use, there is an urgent need for transducers that can harvest power from the human body or the ambient environment to extend battery lifetime. There are a variety of approaches for harvesting energy from the subcutaneous environment using photovoltaic (PV) cells, radio frequency (RF) harvesters, piezoelectric generators (PEGs), thermal electric generators (TEGs), biofuel cells (BC), as well as other hybrid energy harvesting techniques [5]. Unfortunately, many of these techniques have limitations due to their large size, low output power density, or unstable energy output.
Figure 3 - Existing WPT Approaches for Implantable Power Applications
Recent advances in wireless power transfer (WPT) (Figure 3) provide an alternative method to power implantable electronic devices [6]. WPT not only helps eliminate the need for periodic surgical replacements of a depleted battery but also reduces the size of the implant by enabling micro-level size reductions. It also leads to a simplification of the implantation procedure and allows for the device to be placed in restricted anatomic locations that are infeasible for larger implants.
Minimizing power consumption and losses can improve the lifespan of implantable biomedical devices. In one instance, it was found that decreasing the power consumption of an implantable device from 10 mW to 8 µW increased the lifespan of the implantable
medical device from 3 days to 10 years [7].
The choice of power harvesting technology depends on the application. Therefore, before selecting a particular power harvester, it is necessary to investigate the specifications and power requirements of different implantable applications.
Advanced capabilities of the midfield WPTs enables them to operate at frequencies over orders of magnitude higher than inductive power transfer, allowing the implants to become smaller and deliver power delivery deeper into the body. The external component for midfield transfer is constructed by trying to approximate the ideal current pattern. Midfield wireless powering enables electronics to be designed at the millimeter-scale and operated at nearly any location in the body.
Because semiconductor components can be readily miniaturized, these devices can have processing capabilities equal to or exceeding their macroscopic, battery-powered counterparts. New structures, and new materials (meta-materials, flexible materials, biocompatible materials, and dissolvable materials) could generate the optimal field pattern and increase efficiency [8-10].
These days, wireless modules have become an intrinsic part of many modern IMDs. So, doctors can use device programmers wirelessly to configure parameters in the IMD. However, such a wireless technology exposes the IMDs to security attacks. The serious security and privacy risks in IMDs that could compromise the implant and even the health of the patient who carries it.
Some of the newest IMDs have started to incorporate numerous communication and networking functions — usually known as “telemetry”, as well as increasingly more sophisticated computing capabilities. This has provided implants with more intelligence and patients with more autonomy, as medical personnel can access data and reconfigure the implant remotely (i.e., without the patient being physically present in medical facilities). Apart from a significant cost reduction, telemetry and computing capabilities also allow healthcare providers to constantly monitor the patient’s condition and to develop new diagnostic techniques based on an Intra Body Network (IBN) of medical devices.
Figure 4 - Typical usage scenarios with security model - Source [11]
The exploitation of vulnerability in the IMD by an attacker can cause negative medical effects to the patient. Such effects are commonly known as “adverse events”. In order to prevent attacks, it is therefore necessary for the new generation of IMDs to be equipped with strong mechanisms guaranteeing basic security properties such as confidentiality, integrity, and availability. For instance, mutual authentication between the IMD and medical personnel is essential, as both parties must be confident that the other end is who they claim to be.
In the case of the IMD, only commands coming from authenticated parties should be considered, while medical personnel should not trust any message claiming to come from the IMD unless sufficient guarantees are given. Figure 4 presents the main entities involved in the system and shows the possible communication interactions (linked to the usage scenarios) between these devices.
