Printed electronics for food packaging sensors

Technologies for fabricating electronics require ever greater investment to produce the latest integrated circuits. Although the cost per transistor continues to decrease in line with Moore's law, it is difficult to use silicon-based electronics for certain applications. Printed electronics¹ is an alternative technology for fabricating transistors and circuits using suitable inks (for example, organic semiconductors) and fast-patterning methods borrowed from the graphic arts domain, such as screen printing and gravure. Printed electronics enables low-cost embedding of electronic functions in objects such as food packaging, making it possible to sense quantities like temperature and pH, which are important for food conservation. Using sensors printed on the package, we can then accurately determine the conservation quality of the food inside. This would make obsolete the use of a predetermined expiration date, and thus would avoid the discarding of an enormous quantity of food that is safe and good to eat. The problem is that printed electronics is still in its infancy, and the performance of printed circuits is far from optimal. In particular, the variability among different transistors, the low transistor speed, and the presence of many defects present challenges for the design and manufacture of useful printed circuits.

Our research group aims to improve printed electronics' performance and yield (the percentage of correctly working circuits) by choosing suitable circuit architectures, exploiting specifically conceived circuit techniques, and taking advantage of our understanding of the physics of printed transistors. In particular, we have concentrated on the design of a key component in a printed sensor, the so-called analog-to-digital converter (ADC). This circuit converts the sensor output in a digital word, which can be transmitted using (for example) the near-field communication (NFC) radio link found in any modern smartphone. In this way, a phone could read out the information on conservation temperature from a sensor embedded in the packaging around meat, making possible a more precise calculation of the meat's expiration date.

The ADCs in our work were manufactured using screen-printing,² which enabled printing electronics on 11×11cm² foils that we then scaled to 32×38cm² substrates. ADCs contain both digital and analog circuits. For the digital circuit, we made design choices that minimize the number of transistors needed to implement a given function, while maintaining high robustness against variability in transistors (caused by geometric defects introduced during fabrication): we thus chose a logic transmission gate style.^{3, 4}

For the analog circuits, we minimized the impact of mismatch and variability using offset cancellation techniques, and avoided the use of differential structures (a differential amplifier, for example). At the same time, we chose a simple ADC counting architecture, and used a printed resistive digital-to-analog converter to maximize linearity (printed resistors demonstrate better matching than printed transistors). The results were compelling: we were able to print the first functional 4-bit ADC ever reported^{5, 6} (see Figure 1). The main performance parameters include a signal-to-noise ratio of 25.7dB and a signal-to-noise-and-distortion ratio of 19.6dB. Later measurements revealed that this structure could provide (for example) up to 7-bit resolution, enabling 0.3°C accuracy in a 40°C temperature range. Speed is still limited, with a bandwidth of about 2Hz.

Figure 1. A 4-bit counting analog-to-digital converter printed on a transparent foil. The silver patterns are the top conductor, while the bottom conductors appear dark. The dark meanders on the top are the resistors of the digital-to-analog converter. (Photo courtesy of Bart van Overbeeke Fotografie.)

In summary, we have developed a screen-printed ADC with 4-bit resolution that is a first step toward transmitting by NFC information on food conservation from sensors incorporated in packaging materials. This information could enable monitoring of food quality using handheld devices.

In a new project, we are now teaming up with several companies and research centers to bring printed electronics a step further. We plan to reduce variability (employing polymer semiconductors instead of the small-molecule materials used until now), decrease defects (exploiting optical inspection tools during fabrication), and greatly increase speed (by using gate/source-drain self-alignment). With these improvements, we aim to realize printed sensors embedded in packaging. We will also develop and test applications such as electronic baggage tags, printed robotic skins, and large-area pressure sensors for crash tests.

The author gratefully acknowledges financial support from the European Commission for the projects COSMIC (Seventh Framework Programme, contract 247681) and ATLASS (Horizon 2020, Nanotechnologies, Advanced Material and Production theme, contract 636130).