Researchers from the Council for Scientific and Industrial Research in South Africa have presented a Letter detailing a manufacturing process developed to integrate unpacked silicon die onto a paper substrate as part of the construction of a hybrid inkjet-printed paper-based circuit. The approach offers beneficial low-cost applications with the advantages of well-established integrated circuit technology and emerging paper-based electronics platforms. Co-author Trudi Joubert discusses the background to the work: “Paper-based electronics stem from the rapidly emerging field of printed and flexible electronics, which is gaining traction for wearables and other intelligent devices towards connected, integrated systems. Printed electronics promise the integration of smart features in disposable devices on low-cost substrates for applications in diagnostic point-of-care and environmental point-of-need applications, in adherence to the WHO ASSURED principles. Printed electronics is an emerging industry, where new global markets are enabled or disrupted by the use of flexible substrates, and by low-cost integration of electronic devices into novel systems.” Left: Paper-based printed potentiostat and printed electrochemical sensor. Right: Concept of fully integrated smart paper-based diagnostic device. Left: Liquid analyte on printed interdigitated electrodes. Right: Printed electronics heater integrated in a polymer (PDMS) receptacle. Paper provides the ideal platform on which to develop functional solutions. Paper is low-cost, disposable, and environmentally friendly with well-established mass production and distribution mechanisms in place. Integrating all smart functions on a single paper substrate would enable the use of devices without requiring an external reader, an objective which may in future be fulfilled by leveraging the rapid expansion of printed electronics technology. The digital and additive nature of printing processes, the range of materials that can be formulated as inks, and the possibility to cater to various production scales, ranging from prototype to large-scale, are some of the key factors that enable the deployment of printing processes in electronics fabrication. A paper-based RFID tag was fabricated using a non-planar ramp to connect the inkjet-printed electronic and an unpackaged silicon integrated circuit (IC). The dynamic deposition methods, fine alignment and fine track features of functional inkjet-printing were utilised to fabricate the printed electronic circuits and connections. The tracks printed from the paper over the ramp and onto the chip are made from a silver nanoparticle paste. Thermal curing steps are used throughout the fabrication process to ensure desirable mechanical and electronic properties. The ramp consists of a specialised superglue with an increased temperature resistance, which is a property required during curing steps of the printed features. The fabricated paper-based RFID tag read values from a 100 mm distance and delivered results that corresponded to the results of a printed circuit development board. Although thin film metallisation offers fine pitch interconnects for direct integration of bare die onto flexible substrates, printed electronics provide digital techniques for manufacturing metal interconnects on these substrates, which are lower cost and more environmentally friendly than subtractive fabrication due to less material waste and fewer processing steps. Rapid prototyping is also enabled with digital printing because no physical mask is required. When integrating a bare IC in a printed electronics circuit, the step at the edge of the chip presents a non-conformity to the printing process. In this work the step is covered by a ramp constructed from a commercially available adhesive used for everyday bonding, where there is potential to automate the construction via printed electronics. This may be contrasted with methods where submerging the IC into a molded receptacle produced a more planar printing surface, but which involve extra process steps such as de-gassing the molding material. Furthermore, printing on a readily available adhesive helps with the advancement of conformal electronics in various sectors. The challenges to create the inkjet-printed paper-based RFID tag included determining some manufacturing parameters and selecting the ramp material with optimal surface properties, adhesions properties, and curing temperature. The surface properties and ink properties must allow for a balance between hydrophobicity and hydrophilicity to allow for a uniform line with minimal spreading while printing on a slope. The surface-ink interaction was resolved during the pre-curing step. The example application in the groups Letter is of an RFID device, which is well-suited to paper-based solutions as the tag antennas are printable and paper is well-suited for UHF and microwave applications. Although the focus is currently on paper, other low cost flexible substrates can also be used. In the short term, RFID tags printed and assembled onto paper or adhesive labels can be mounted onto various items or surfaces for investigation. This could be useful for tracking, identification, and sensing applications, to name a few. In the long term, these tags could be integrated with other paper-based systems such as rapid diagnostic tests to enable automated communication of results from the test to an external database. Doctor Joubert comments on the future of the field: “Fully printed and sophisticated electronic circuits and systems will be realised in the near future, without the need for hybrid electronic implementations, which add to manufacturing costs and complexity. Printed electronic components such as resistors, capacitors, inductors and transistors are evolving in terms of printability and performance. Data processing capabilities have been demonstrated using transistor arrays, and the speed and functionality of printed components are expected to grow substantially in the next decade. From an application perspective, the hope is that low cost diagnostics will find widespread use in low resource areas and will play a critical role in improved human and environmental health - and thereby uplift poor communities.”