Veronica Sanchez-Romaguera

Water-based and biocompatible 2D crystal inks for all-inkjet-printed heterostructures

Exploiting the properties of two-dimensional crystals requires a mass production method able to produce heterostructures of arbitrary complexity on any substrate. Solution processing of graphene allows simple and low-cost techniques such as inkjet printing to be used for device fabrication. However, the available printable formulations are still far from ideal as they are either based on toxic solvents, have low concentration, or require time-consuming and expensive processing. In addition, none is suitable for thin-film heterostructure fabrication due to the re-mixing of different two-dimensional crystals leading to uncontrolled interfaces and poor device performance. Here, we show a general approach to achieve inkjet-printable, water-based, two-dimensional crystal formulations, which also provide optimal film formation for multi-stack fabrication. We show examples of all-inkjet-printed heterostructures, such as large-area arrays of photosensors on plastic and paper and programmable logic memory devices. Finally, in vitro dose-escalation cytotoxicity assays confirm the biocompatibility of the inks, extending their possible use to biomedical applications.

Inkjet printing of polymer solutions and the role of chain entanglement

The influence of polymer concentration, going from the dilute through the overlap into the concentrated regime, during drop on demand inkjet printing is investigated for a range of cellulose ester (CE) polymers from visual examination of ligament stretching as a function of applied wave form. The physical behaviour of the polymer fluids in drop formation is indicative of the dominance of viscoelastic effects within the timescale of the process, in preventing ligament break-up at the pinch point compared with a water–glycerol–isopropanol blend Newtonian fluid of similar viscosity. This has previously been described in terms of the polymer chain undergoing a coil–stretch transition at the strain rates experienced in the inkjet process. When formulated at the coil overlap concentration all polymers showed qualitatively similar behaviour with respect to time and length of ligament at rupture irrespective of polymer molecular weight. Beyond the overlap concentration the ligament rupture time continues to increase with increasing elasticity of the solution but the ligament rupture length decreases rapidly. In this regime chain entanglement becomes important, dramatically increasing the elastic nature of the ligament. Additionally it is proposed that in the case of weakly associating polymers such as cellulose esters, the effective relaxation time is further increased due to the possibility that on chain extension intramolecular H-bonds are broken and may reform as intermolecular associations whilst the polymer chains are extended. These intermolecular associations act as physical crosslinks, thereby creating a transient network structure. This network structure is capable of having a finite large viscosity. Once the strain is removed the network will decay as the chains return to the more thermodynamically stable coil state.

Inkjet printed paper based frequency selective surfaces and skin mounted RFID tags: the interrelation between silver nanoparticle ink, paper substrate and low temperature sintering technique

Inkjet printing of functional frequency selective surfaces (FSS) and radio frequency identification (RFID) tags on commercial paper substrates using silver nanoparticle inks sintered using low temperature thermal, plasma and photonic techniques is reported. Printed and sintered FSS devices demonstrate performances which achieve wireless communication requirements having a forward transmission scattering parameter, S21, depth greater than -20 dB at 13 GHz. Printed and plasma sintered RFID tags on transfer paper, which are capable of being mounted on skin, improved read distances compared to previously reported single layer transfer RFID tags fabricated by conventional thermal sintering. This journal is cop. The Royal Society of Chemistry 2015.

Towards inkjet-printed low cost passive UHF RFID skin mounted tattoo paper tags based on silver nanoparticle inks

The present work describes the inkjet printing and low temperature sintering of silver nanoparticle inks onto transfer tattoo paper. Our approach results in silver features of excellent resolution and conductivity and, subsequently the first passive UHF RFID transfer tattoo tags functional mounted on human skin of improved performance when compared to screen printed passive UHF RFID transfer tattoo paper tags. Moreover, inkjet printed passive UHF RFID transfer tattoo tags show similar performance to copper etched passive UHF RFID tags on plastic substrates. This study compares the image quality (resolution) and electrical performance of two commercial silver nanoparticle inks inkjet printed on transfer tattoo paper. The optimal printing and sintering parameters to obtain high resolution features of resistivities 20 to 57 times the resistivity of bulk silver (1.59 × 10−6 ohm cm) are described. We demonstrate how, by selectively depositing ink in specific areas of the antenna, read distance of passive UHF RFID tags can be increased from 54 to 68 cm whilst decreasing the amount of ink used by 33%. Furthermore, this approach results in inkjet printed passive UHF RFID tattoo tags with larger read distance than silver screen printed passive UHF RFID tattoo tags, 45 cm, and similar to copper etched passive UHF RFID plastic tags, 75 cm. Moreover, inkjet printed passive UHF RFID tattoo tags in this work are considerably thinner (1–5 μm) than screen and etched passive UHF RFID tags (tens of micrometers) hence, making the former more appealing to the end user. In addition to this, inkjet printing is compatible with large area manufacturing techniques and has the potential to evolve as one of the most promising RFID mass-production techniques. Therefore, this work represents a step towards the commercialization of on-body transfer tattoo paper passive UHF RFID tags.

Improving the performance of organic thin film transistors formed on a vacuum flash-evaporated acrylate insulator

A systematic investigation has been undertaken, in which thin polymer buffer layers with different ester content have been spin-coated onto a flash-evaporated, cross-linked diacrylate gate-insulator to form bottom-gate, top-contact organic thin-film transistors. The highest device mobilities, ∼0.65 cm2/V s and ∼1.00 cm2/V s for pentacene and dinaphtho[2,3-b:2′,3′-f]-thieno[3,2-b]thiophene (DNTT), respectively, were only observed for a combination of large-grain (∼1–2 μm) semiconductor morphology coupled with a non-polar dielectric surface. No correlation was found between semiconductor grain size and dielectric surface chemistry. The threshold voltage of pentacene devices shifted from −10 V to −25 V with decreasing surface ester content, but remained close to 0 V for DNTT.

Organic field effect transistors from ambient solution processed poly(triarylamine)–insulator blends

Two-component blends of amorphous poly(triarylamines) with selected amorphous and semi-crystalline polymeric binders are investigated as the semiconducting layer in organic field effect transistors (OFET). The blends can be deposited at room temperature from a good solvent and allow the fabrication of devices based on environmentally stable, low-cost organic semiconductors whose solution and solid state properties can be fine-tuned by careful binder selection making them suitable as the active layer in OFET based electronic nose arrays. A preliminary comparison of devices fabricated using drop casting and inkjet printing shows that the thin film morphology and the electronic properties of these blends are strongly dependent on the deposition method.

Study of printing errors in digitally fabricated FSS

Low cost fabrication techniques might give rise to defects in electromagnetic structures. This paper investigates the detrimental effect of producing frequency selective screens with differing percentages of absent square loop elements. It is shown that as many as one in five elements can be incomplete before the array transmission response is considered unusable.

Significant Factors in the Inkjet Manufacture of Frequency-Selective Surfaces

Additive fabrication of electromagnetic structures by inkjet printing technology is both cost effective and compatible with a wide range of environmentally friendly substrates, enabling the fabrication of frequency-selective surface (FSS) arrays with line dimensions less than 0.1 mm, which is difficult to achieve with conventional subtractive techniques. Several approaches, such as savings in ink by depositing it at the edges of dipole elements where the surface current tends to maximize, have been investigated in order to produce low-cost frequency-selective panels with acceptable level of isolation. The FSS transmission characteristics were improved by jetting multiple ink layers on the whole elements and at the edges. The electrical resistance of various arrays have been measured and analyzed and has been used to assess the performances of the FSS.

Developing inkjet printing to enable low cost UHF RFID transfer tattoo tags

This paper demonstrates the use of inkjet printing as a digital fabrication tool for the cost effective manufacture of radio frequency identification (RFID tags on low-cost flexible and porous substrates. The design presented in this work is a thin, substrate tolerant UHF RFID tag that can be mounted directly onto the skin surface in the form of a transfer patch in the same way that a temporary tattoo is applied.