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Researchers unveil wireless power for medical implants using near-infrared light
(Nanowerk Spotlight) In a recent study published in the journal Advanced Materials ("Skin Thermal Management for Subcutaneous Photoelectric Conversion Reaching 500 mW"), a team of researchers from China introduced a new approach to harnessing the power of second near-infrared light (NIR-II) for subcutaneous photoelectric conversion. This research offers a potential alternative to the way we power implantable medical devices, possibly reducing the need for surgical replacements when batteries deplete.
Setup of subcutaneous NIR-II power supply
Setup of subcutaneous NIR-II power supply. a) Schematic of a subcutaneous NIR-II power supply device made by combining photovoltaic and thermoelectric conversions. b) The absorption of SC-Si cells in the UV–vis–NIR and middle-Infrared windows. Inset: Photograph of a SC-Si cell. Scale bar: 1.0 cm. (Reprinted with permission by Wiley-VCH Verlag)

Challenges with Current Implantable Devices
Implantable medical devices, ranging from pacemakers to cochlear implants, have become indispensable tools in modern medicine, offering life-saving and life-enhancing benefits to millions of patients worldwide. However, these devices come with their own set of challenges:
Battery Life and Replacement: Most implantable devices are powered by batteries. Over time, these batteries deplete, and replacing them often requires an invasive surgical procedure. This not only poses risks associated with any surgery, such as infections, complications, and anesthesia-related issues, but also places a financial and emotional burden on patients.
Biocompatibility: Implantable devices must be made of materials that are biocompatible, meaning they won't cause an immune response or other adverse reactions when placed inside the body. Finding and maintaining materials that remain inert over time can be challenging.
Size and Comfort: As technology advances, there's a push to make devices smaller and more comfortable for patients. However, miniaturization often comes at the cost of battery life, creating a trade-off between device size and the frequency of surgical replacements.
Wireless Communication: Some modern implantable devices can communicate wirelessly with external devices for monitoring or adjustments. However, this communication can be susceptible to interference, potentially compromising the device's functionality or the patient's safety.
Infection Risks: Any surgical procedure carries a risk of infection. Even after successful implantation, there's a small chance that bacteria can colonize the device, leading to chronic infections that are difficult to treat.
Mechanical Failures: Like all electronic devices, implantable medical devices can experience mechanical failures or malfunctions. When this happens inside the body, the consequences can be severe, necessitating emergency interventions.
Cost: High-quality implantable devices are expensive to produce, and their cost can be prohibitive for many patients, especially in regions with limited healthcare coverage or resources.
Given these challenges, the pursuit of alternative power sources, like the wireless power transmission through the skin as described in the study, becomes even more crucial. Such innovations have the potential to address several of the aforementioned issues, particularly those related to battery life and the need for surgical replacements.

Exploring the Potential of NIR-II Light
Near-infrared (NIR) light is a type of light that's not visible to the human eye but is very close to the red light we can see. Imagine a rainbow: at one end, you have violet light, and at the other end, you have red. If that rainbow extended beyond the red end, the next type of light would be near-infrared.
When we talk about NIR-II light, we're referring to the second "window" or segment of this near-infrared spectrum. Specifically, NIR-II light has wavelengths between 1000–1350 nanometers (nm). To give you a perspective, the visible light we see ranges from about 400 nm (violet) to 700 nm (red). So, NIR-II light is just beyond what our eyes can detect.
Here's why NIR-II light is particularly interesting for medical applications:
Deep Tissue Penetration: Unlike visible light, which gets absorbed or scattered easily by our skin and tissues, NIR-II light can penetrate deeper into the body. This makes it useful for reaching devices implanted beneath the skin.
Safety: While some types of light, like ultraviolet (UV) light, can damage cells and DNA, NIR-II light is much gentler on tissues. It doesn't have the harmful effects that UV light does, making it safer for medical applications.
High Permissible Exposure: Our skin and tissues can tolerate a higher intensity of NIR-II light compared to other types of light, like visible light or radio waves. This means devices can use a stronger NIR-II light source without harming the body.
Given these properties, the study's focus on using NIR-II light for wireless power transmission through the skin is a logical step. By harnessing this unique type of light, researchers aim to safely and efficiently power implantable medical devices without the need for invasive procedures.
The study highlights the capabilities of NIR-II light (1000–1350 nm) for subcutaneous photoelectric conversion. NIR-II light, compared to other electromagnetic waves, offers better tissue transmittance and a higher permissible exposure power density for skin. However, the photothermal effect that accompanies it can lead to skin hyperthermia.
To mitigate this, the researchers developed a skin thermal management system. This system optimizes the photothermal effect of a photovoltaic cell, improving photoelectric conversion efficiency while ensuring skin safety. The device demonstrated an output power of over 500 mW and a photoelectric conversion efficiency of 9.4%, suggesting potential for wirelessly recharging various implantable devices.

The Approach and Its Implications
The team employed a synergistic energy conversion (SEC) strategy, merging photovoltaic cells with a thermoelectric generator. In this structure, the photovoltaic cell on top absorbs and converts NIR-II light into electricity. The generated photothermal energy is then channeled to the thermoelectric generator below, producing additional electricity.
This combined method aims to optimize efficiency while minimizing the risk of overheating skin tissues. With thermal management, the device maintained a temperature below 44°C during extended light exposure. Tests on rabbits indicated no adverse effects on surrounding tissues after several weeks of implantation and light exposure.
The chosen near-infrared light range can penetrate tissue without causing cellular damage like UV light. The photovoltaic cell, constructed from efficient single-crystal silicon, played a crucial role in the device's performance.
In evaluations, the device converted the near-infrared light to electricity with a 9.4% efficiency, delivering over 500mW of power—a notable improvement over using only photovoltaics. The generated power was sufficient to operate a pacemaker and endoscopic camera in rabbit tests.
This wireless power output method presents an alternative to older technologies like ultrasound, radio waves, or inductive coupling. If further research confirms its safety and efficacy for human use, this technology might reduce the need for surgeries related to battery replacements in medical implants. The team is keen on refining the efficiency and adaptability of this approach for potential clinical applications.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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