Photodynamic therapy (PDT) is a promising cancer treatment modality that can selectively target unresectable tumors through optical activation of cytotoxic agents, thus reducing many side effects associated with systemic administration of chemotherapeutic drugs. However, limited light penetration into most biological tissues have so far prevented its widespread adoption beyond dermatology and a few other oncological applications in which a fiber optic can be threaded to the desired locations via an endoscopic approach (e.g., bladder). In this paper, we introduce an ultrasonically powered implantable microlight source, μLight, which enables in-situ localized light delivery to deep-seated solid tumors. Ultrasonic powering allows for small receiver form factor (mm-scale) and power transfer deep into the tissue (several centimeters). The implants consist of piezoelectric transducers measuring 2 × 2 × 2 mm³ and 2 × 4 × 2 mm³ with surface-mounted miniature red and blue LEDs. When energized with 185 mW/cm² of transmitted acoustic power at 720 kHz, μLight can generate 0.048 to 6.5 mW/cm² of optical power (depending on size of the piezoelectric element and light wavelength spectrum). This allows powering multiple receivers to a distance of 10 cm at therapeutic light output levels (a delivery of 20–40 J/cm² light radiation dose in 1–2 hours). In vitro tests show that HeLa cells irradiated with μLights undergo a 70% decrease in average cell viability as compared to the control group. In vivo tests in mice implanted with 4T1-induced tumors (breast cancer) show light delivery capability at therapeutic dose levels. Overall, results indicate implanting multiple µLights and operating them for 1–2 hours can achieve cytotoxicity levels comparable to the clinically reported cases using external light sources.

In this paper, we review new and emerging energy sources for wireless implantable microdevices. After a brief historical background, we review the developments in power sources in the decades following the pioneering works of Zworykin and Mackay in the late 1950s. These include deployment of lithium batteries and inductive powering in the 1970s, which resulted in significant growth and commercialization of implantable medical devices, such as cardiac pacemakers and cochlear implants. Recent research in nanoscale materials for energy generation has created intriguing possibilities for next generation implantable power sources in the form of flexible and biodegradable batteries and supercapacitors. In addition, energy harvesting/remote powering from various environmental physical and chemical sources within the body utilizing nanoscale materials can also offer unique possibilities for autonomous implantable microscale and nanoscale devices.

In this paper, we report on the development of an implantable pressure sensing system that is powered by mechanical vibrations in the audible acoustic frequency range. This technique significantly enhances interrogation range, alleviates the misalignment issues commonly encountered with inductive powering, and simplifies the external receiver circuitry. The interrogation scheme consists of two phases: a mechanical vibration phase and an electrical radiation phase. During the first phase, a piezoelectric cantilever acts as an acoustic receiver and charges a capacitor by converting sound vibration harmonics occurring at its resonant frequency into electrical power. In the subsequent electrical phase, when the cantilever is not vibrating, the stored electric charge is discharged across an LC tank whose inductor is pressure sensitive; hence, when the LC tank oscillates at its natural resonant frequency, it radiates a high-frequency signal that is detectable using an external receiver and its frequency corresponds to the measured pressure. The pressure sensitive inductor consists of a planar coil (single loop of wire) with a ferrite core whose distance to the coil varies with applied pressure. A prototype of the implantable pressure sensor is fabricated and tested, both in vitro and in vivo (swine bladder). A pressure sensitivity of 1 kHz/cm $H_{2}$O is achieved with minimal misalignment sensitivity (26% drop at 90° misalignment between the implanted device and acoustic source; 60% drop at 90° misalignment between the implanted device and RF receiver coil).

Transient implantable medical devices based on biodegradable electronics can be used for diagnostic and therapeutic purposes for a desired duration and undergo biodegradation, unlike their conventional counterparts. However, powering transient implants through biodegradable power sources remains under-explored. Here, we report biodegradable piezoelectric transducer fabricated using 0-3 composite film made of barium titanate nanoparticles and poly (L-lactic-co-glycolic) acid polymer (BT-PLGA). The proposed BT-PLGA can be utilized in two different powering schemes; ultrasonic powering and energy harvesting from low frequency acoustic waves. We demonstrated that the power density of the BT-PLGA transducer can reach up to 10 mW/cm2 in ultrasonic powering. The energy harvesting from low frequency acoustic waves could also readily generate sufficient power for small electronics. The fabricated transducers underwent complete biodegradation in physiological conditions within 100 days. The development of the biodegradable piezoelectric transducer potentially provides a reliable power source for transient implants, especially for deeply seated bioelectronics. The output performance, biocompatibility, and tunable biodegradation of BT-PLGA transducer demonstrate its potential as a biodegradable power source for transient implantable devices.