Transferrable tattoos—or decal transfers—are a familiar part of childhood and industrial design. The technology is straightforward. Transfers consist of a thin film of ethyl cellulose polymer stuck to a sheet of paper by a sacrificial layer of water-soluble starch or dextrin.
Placing the transfer in water dissolves the sacrificial layer, allowing the ethyl cellulose sheet to be “transferred” to human skin or numerous other objects. A key property of ethyl cellulose polymer film is that it can carry an image or text created using conventional inkjet printing.
That sparked the imagination of a team headed by Giorgio Bonacchini at the Instituto Italiano di Tecnologia (IIT) in Genoa, Italy. These guys have printed organic electronic components onto transfer paper and then tested the properties of the resulting circuits. They’ve even transferred the circuits onto edible objects such as pharmaceutical pills and pieces of fruit.
Electronic devices that operate inside the digestive tract are by no means new. For many years, medical professionals have had access to pills containing electronic devices such as cameras and batteries.
But these devices are made exclusively from silicon-based components that are expensive and inflexible. By contrast, materials scientists have made much recent progress in developing conducting polymers that can be inkjet-printed into powerful electronic devices such as plastic displays.
Bonacchini and company use the same inkjet-printing technique to create electronic circuits on transfer paper.
Of course, an important question is the biocompatibility of the resulting devices. Bonacchini and his colleagues point out that ethyl cellulose film has long been used as an edible coating on things like pharmaceutical pills.
But the circuits have other components; for example, the transistors contain metallic materials. Bulk silver is thought to be bioinert and has a recommended dietary allowance of 350 micrograms per day for someone weighing about 155 pounds. A single transistor requires just four micrograms of silver, so simple circuits should contain well below the daily limit.
However, the silver is printed in nanoparticle form and then sintered to create a continuous layer. Bonacchini and company assume this will be biocompatible, based on other research with silver nanoparticles, but that will presumably need confirmation at some point in the future.
The team also uses four different semiconducting polymers, including poly(3-hexylthiophene), or P3HT, and polystyrene, which are known to be biocompatible. The other two polymers, 29-DPP-TVT and P(NDI2OD-T2), have only recently been developed and have not yet been tested for biocompatibility.
Although these are used in picogram quantities, they still raise obvious questions regarding biocompatibility. Bonacchini and his colleagues are well aware of this and are taking on the task of assessing how the polymers interact with the human body. The results have been positive so far, but more research is needed.
The team uses these materials to print a variety of organic field-effect transistors and logic inverters on transfer paper; then it tests their properties.
The results throw up some challenges. For example, the transfer process exposes the circuits to air, light, and water, which appears to dope the active P3HT layer in unwanted ways.
But the team was able to mitigate this effect to some extent by blending the active polymers with more-stable semiconductors. This reduces the effect of the transfer process, but the stability of the final device is crucially sensitive to the stability of the active material during the transfer process.
Nevertheless, the team is confident that these problems can be overcome and that the work is a proof-of-principle demonstration for a new generation of edible electronics. “This result paves the way for the realization of robust complementary circuits,” the researchers say. “This system constitutes a simple and versatile platform for the integration of fully printed organic circuitry on food and pharmaceutical drugs.”
The technology may even be digestible, which means the components would be less likely to build up in the body over time.
All in all, this is exciting work. These circuits could monitor the ripeness of fruit or the edibility of other perishable products across their entire lifetime. They could also deliver drugs in specific circumstances or carry out assays of various kinds inside the digestive tract.
Of course, much work remains to be done, particularly on edible batteries that could provide the power for these kinds of circuits. Clinical trials are also needed to ensure biocompatibility.
But the future for edible electronics looks—well—tasty.