Printable, Flexible and Tissue Equivalent Wearable X-Ray Detectors—A New Biomedical Frontier for Solution Processable Organic Semiconductors

Detecting ionising radiation is of critical interest for a wide range of endeavours in science and engineering, finding use in increasingly varied applications in modern society such as medical diagnostics and therapy, environmental monitoring, space exploration, and wearable monitors. There is a growing demand for affordable, flexible, and large area sensors that can detect radiation on complex geometric surfaces in real time with high sensitivity and photostability.^[1]<br/>Inorganic semiconductors have been the traditional gold standard for radiation detection due to excellent sensitivity; however, they are expensive to manufacture and cannot be easily fabricated into flexible large-area sensors. They are also composed of heavier elements, which exhibit much stronger X-ray absorption than the lighter elements comprising human tissue. Consequently, their use for medically relevant dosimetry or wearable health monitors requires complex, often unreliable calibrations. New hybrid materials must therefore be developed for radiation detection, combining a tissue equivalent response with other desirable properties such as mechanical flexibility, high sensitivity, and good stability under ionising radiation.<br/>Here we report here the development of printable organic semiconductors into efficient X-ray detecting devices. We have tuned the material nanostructure to demonstrate a world-first combination of high sensitivity to X-rays, operation with no external power, fast temporal responses, and tissue-equivalency in the same material system.^[2] We also show these materials can be printed into functional devices and demonstrate they possess high mechanical stability and radiation hardness, providing new information that is vital to develop flexible X-ray sensing materials.^[3] The photodetectors are composed of donor polymer P3HT and two different acceptors: fullerene derivative PCBM and non-fullerene acceptor o-IDTBR. The non-fullerene acceptor material shows both higher charge carrier mobility and a greater radiation hardness, which we establish is due to greater crystallinity and improved nanoscale morphology using synchrotron-based scanning X-ray transmission microscopy.<br/>X-ray nanosensors, fabricated by coupling the photodetectors with a plastic scintillator, show efficient operation without the need for external bias, a key feature of wearable electronics. The X-ray performance is shown to be energy independent between 50 keV and 6 MeV by testing with clinical orthovoltage and medical linear accelerator sources. We show that the X-ray detection sensitivity can be tuned to match that of state-of-the-art inorganic semiconductors or slightly reduced to provide almost completely radiolucent devices for wearable radiation dosimetry. The X-ray nanosensors were also fabricated into pixelated arrays using inkjet printing to provide micrometer scale spatial resolution that matches state-of-the-art medical detectors. The organic nanosensors also exhibited remarkably fast temporal responses for organic semiconductors, detecting a pulsed X-ray source with microsecond resolution. Furthermore, the device exhibited excellent radiation hardness, withstanding a total dose equivalent to a 10-year working lifetime and demonstrating successful operation in different clinical applications, including the emerging frontier of microbeam radiation therapy.^[4]<br/>The new materials science and flexible devices we report here are an exciting breakthrough, providing the first stable, printable, flexible, and fully tissue equivalent X-ray detectors with functionality that is tuneable for optimized X-ray attenuation or high radiolucency depending on the relevant application.<br/><b>References</b>:<br/>[1] M. J. Griffith <i>et. al</i>; <i>Front. Phys.</i> <b>2020</b>, <i>8</i>, 22.<br/>[2] M. J. Large, … , M. J. Griffith; <i>ACS Appl. Mater. Interfaces</i>, <b>2021</b>, <i>Accepted</i>.<br/>[3] J. A. Posar, … , M. J., Griffith; <i>Adv. Mater. Technol.</i> <b>2021</b>, <i>6</i>, 2001298.<br/>[4] J. A. Posar, … , M. J. Griffith; <i>Med. Phys.</i> <b>2020</b>, <i>47</i>, 3658.