21-09-2020 | | By Robin Mitchell
Medical devices implanted or swallowed need to be able to withstand the environment of the human body while causing no impact to bodily function. What are radio pills, how do they work, and what environments do they need to protect against?
Digital Pills, sometimes known as Radio Pills, are medical devices that can be swallowed and provides sensory readings as it travels through the human body. Developed in 1957, Radio Pills include all hardware needed to properly function, including sensors, transmitter, and battery. Since the firsts Radio Pills, technology now allows for incredibly advanced features including processors, cameras, lights, and gas sensors which allows them to perform a wide range of medical monitoring procedures.
As previously stated, digital pills include all the hardware needed to wirelessly transmit information from inside the body to an external receiver. Older digital pills utilised analogue circuitry whereby a sensor’s output directly affects the oscillator module for the radio transmitter. However, modern systems can take advantage of small silicon dies which allow for the use of microcontrollers, peripherals, and sensors. The first radio pill utilised pressure sensors, and this was demonstrated by a doctor who showed how the received data from the pill changed as he applied pressure to a patients stomach.
Imaging capsules integrate a small digital camera that allows for the monitoring of internal organs and streaming live images from inside the body requires a high bitrate (up to 2.7Mbit/s). Gas sensing pills require the use of semi-permeable membranes that allow gas to diffuse through, and these house the sensor inside the pill. From there, readings from the sensor are sent to a microcontroller which can preprocess the data before wirelessly transmitting it. What makes digital pills possible is the shrinking of electronics, and the ability to create biologically inert casings that do not upset the internal bodily functions.
Designing a digital pill has two main factors to consider; the internal electronics and the housing. Unlike implanted medical devices, ingested digital pills are subject to all harsh environments produced by the entire digestive system. If the pill is damaged during transit, it is essential that the body sees the damaged pill as benign and passes it with no side effects.
The first harsh environment encountered by the pill is the stomach where the pH can reach as low as 1 (for perspective, car batteries have a pH of 0.7, but remember that the pH scale is logarithmic). As a result, the casing of the pill needs to be able to handle acidic solutions. However, the acidity of the stomach can be a bonus to designers; the acid can be used to activate the pill and thus only consume power once swallowed. However, the acidity of the stomach is not the only obstacle faced by the pill; the stomach also has a grinding action whereby it tries to break food down with compression mechanically. Therefore, it is essential that the capsule not only resist acid but also resit mild crushing forces. After passing through the stomach, the pill needs to survive the long intestinal tract where it will be subjected to alkali solutions produced by the liver (i.e. bile), constant compressive actions from the walls of the intestine, and potentially volatile gasses such as methane.
However, the fun does not stop there; the internal components of the pill need to be carefully chosen, just like any implanted medical equipment. While the digestive tract is far more forgiving than the immune system, the components of the pill must not be toxic. If the pill opens up, the contents must be able to be safely passed through without harming the body. This is why silver-based batteries are used as these are non-toxic. While lithium-ion batteries provide superior energy densities, they are toxic to consume.
Anything that enters the body must meet a strict set of safety criteria to minimise risk, and traversing the human body is no easy feat. While the environmental temperature may be consistent, the various organic compounds, fluids, and mechanical stresses can give any electronic system a hard time. On top of that, the end device must be both easy to swallow and pass while containing its power source and continue transmitting information along the entire journey.