A Subcutaneously Injectable Implant for Multimodal Physiological Monitoring in Animals

Unobtrusive acquisition of physiological and behavioral data from freely moving animals is important to many applications including animal research, veterinary science, animal husbandry, and pet monitoring. This article reports a miniaturized, injectable, and multimodal implant for real-time measurements of heart rate (HR), breathing rate (BR), movement, and subcutaneous temperature with future extensions to blood pressure and oxygen saturation. To estimate these vital signs, the presented device incorporates sensors of various modalities: photoplethysmography, electrocardiography, accelerometry, magnetometry, and thermometry. A rechargeable battery drives the system containing a system-on-chip with Bluetooth low energy capability and multiple sensor front-end circuits. The implant electronics are isolated from the animal’s extracellular fluid by a dual-layer encapsulation of biomedical epoxy and poly(methyl methacrylate) that fits into a six-gauge surgical needle to allow for subcutaneous injection. Electrically conductive epoxy is used to create electrodes on the surface of the encapsulation for biopotential measurements. With a 3-m wireless range from a custom receiver, this implant can continuously transmit data from all the sensors for 20 h, which can support two–three months of duty-cycled and intermittent recording between battery recharges. The system was tested in vivo where the acquired HR and BR estimations showed an error of less than two beats per minute (BPM) compared to the gold standard. Longer-term evaluation of tissue reaction showed an acceptable level of immune responses with minimal effect on the sensing performance. This novel system has the potential to provide new insights with greater depth in veterinary research and practice, and animal welfare management.