We report the memory device on paper by means of an all-printing approach. Using a sequence of inkjet and screen-printing techniques, a simple metal–insulator–metal device structure is fabricated on paper as a resistive random access memory with a potential to reach gigabyte capacities on an A4 paper. The printed-paper-based memory devices (PPMDs) exhibit reproducible switching endurance, reliable retention, tunable memory window, and the capability to operate under extreme bending conditions. In addition, the PBMD can be labeled on electronics or living objects for multifunctional, wearable, on-skin, and biocompatible applications. The disposability and the high-security data storage of the paper-based memory are also demonstrated to show the ease of data handling, which are not achievable for regular silicon-based electronic devices. We envision that the PPMDs manufactured by this cost-effective and time-efficient all-printing approach would be a key electronic component to fully activate a paper-based circuit and can be directly implemented in medical biosensors, multifunctional devices, and self-powered systems.

In this study, an inkjet printing process was developed to produce thermistor arrays for temperature sensing applications. First, a formulation process was carefully performed to generate a stable nanoparticle ink for nickel oxide, a material with a large temperature coefficient of resistance. The thermistor was then fabricated by printing a square NiO thin film in between two parallel silver conductive tracks on either glass plates or polyimide films. The printed thermistor, which has an adjustable dimension with a sub-millimeter scale, can operate over a wide range from room temperature to 200 °C with great sensitivity (B values ~4300 K) without hysteretic effects. When printed on polyimide films, the thermistors can also be bent or attached to curved surfaces to provide accurate and reliable temperature measurements. Moreover, the thermistor responds quickly to small temperature changes and provides an effective tool for transient temperature measurements. Finally, a thermistor array was fabricated to show the flexibility of this inkjet printing process and to demonstrate the applicability of the printed devices for temperature sensing applications.

A simple and efficient method is developed to create conductive copper thin films on polymer surfaces. Instead of regular palladium colloid inks, micropatterns of silver nitrate inks, which serve as an activating agent for copper plating, were printed and dried on flexible plastic substrates. The printed plastic sheets were then immersed in an electroless copper plating bath at 55 °C for 2 min to create copper thin films on the printed patterns. The prepared copper films have an electrical conductivity as high as 83% of bulk copper and show good adhesion on PET or PI substrates.

In this study, a simple and effective silver ink formulation was developed to generate silver tracks with high electrical conductivity on flexible substrates at low sintering temperatures. Diethanolamine (DEA), a self-oxidizing compound at moderate temperatures, was mixed with a silver ammonia solution to form a clear and stable solution. After inkjet-printed or pen-written on plastic sheets, DEA in the silver ink decomposes at temperatures higher than 50 °C and generates formaldehyde, which reacts spontaneously with silver ammonia ions to form silver thin films. The electrical conductivity of the inkjet-printed silver films can be 26% of the bulk silver after heating at 75 °C for 20 min and show great adhesion on plastic sheets.

A new method is described for conductive silver film formation. An inkjet printing device with two ink channels was used to deposit silver thin films. Two inks, silver ammonia and formaldehyde solutions, were separately ejected, mixed, and reacted on glass slides. The so-called silver mirror reaction created smooth and continuous silver lines of 90 micron in width with an average film thickness of 200 nm. Surface profilometry was used to measure the thicknesses and cross section areas of printed silver lines. The electrical conductivity of the resulting silver lines is 6% of bulk silver at room temperature. After sintering at 150 C for an hour, electrical conductivity was enhanced to 14%. Scanning electron microscopy (SEM) showed that the microstructure of the printed thin films consists of nanoparticle grains of 50 to 200 nanometres. Results of X-ray diffraction (XRD) and Energy-dispersive X-ray spectroscopy (EDS) further confirmed our printed films purely comprised of face-centered cubic (FCC) silver crystalline. The printed conductive lines could be used for various applications, such as embedded printed circuit boards, micro-electromechanical systems (MEMS), and radio frequency identification (RFID) tags.

For printed micropatterns on plastic substrates, the decreasing volume because of solvent evaporation frequently leads to contact line receding and changes the original printed pattern. To prevent printing quality deterioration caused by contact line motions, an ink formulation method was developed. A nearly non-volatile solvent [polyethylene glycol (PEG)] with a low receding angle on polyethylene terephthalate (PET) sheets was added in water to hold the contact line. To understand the dewetting phenomena of inks, the geometrical evolution of circular liquid films under evaporation was recorded and analyzed. The results showed that the contact line receded as water evaporated for inks with low PEG concentrations but remained pinned at a moderate PEG concentration (~10 wt %). A simple model was proposed to explain the dewetting phenomena and can successfully predict the critical PEG concentration, beyond which the contact lines will be unconditionally pinned. The optimized water/PEG solvent can then be used to prepare dye- or particle-based inks, which preserved accurate features after solvent evaporation.

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A taxel device on flexible substrates based on the electrical contact resistance (ECR) variation mechanism is developed. The proposed taxel device consists of one top and one bottom substrates, which are coated with conductive polymer then face-to-face assembled. The device is tested with different substrate materials to show the viability of the ECR variation mechanism. It is found that when paper is used as the substrate material, because of its exclusive features of surface roughness and material compressibility, the ECR variation mechanism is greatly enhanced and the device exhibits higher sensitivity and larger working pressure range. Practical applications are investigated in elasticity measurement on phantom samples and biological tissues. The results are promising with our taxel device made on flexible substrates being capable of integration onto surgical tools and the potential to realize the psychical property of biological tissues.