Reactive Inkjet Printing — An Introduction

PATRICK J. SMITH AND AOIFE MORRIN

1.1 Introduction

'Reactive inkjet (RIJ) printing', or 'reactive inkjet' is a type of inkjet technique whose history in the research literature has been documented for less than ten years. Despite its recent beginnings, it has attracted much attention as a cost-effective and highly controllable method to fabricate patterns, where it elegantly combines the processes of material deposition and chemical reaction. The combination of these two processes creates boundless possibilities for the fabrication of 2-dimensional and indeed 3-dimensional structures whereby it allows functional materials to be synthesized in situ at the same time as their final device geometries are patterned.

The concept of micro-dispensation of reactants actually dates back to the 1990s when the first report of a solenoid-based inkjet chemical dispenser, ChemJet, was made for combinatorial library synthesis. Since then, the field has grown from this application to become a combinatorial library synthesis tool for rational coating design, and a tool used for patterning materials directly within devices. Materials that have been reactively printed for direct device integration include conjugated polymers, fluorescent quantum dots, metallics and silk. It is the precursors to these materials that are dispensed (e.g., monomer and oxidant) and subsequently react in solution droplets (micro- or pico-litre sized) on a solid support. While performing chemistry in patterned droplets using this printing approach promises high controllability, technical challenges do still remain. Fundamental droplet analysis in terms of size, shape and kinetics must be understood in each application in order to design optimal precursor ink formulations and selection of appropriate print parameters.

A wide spectrum of reactive inks that have been reported allows one to fabricate various patterns that perform different functionalities. In terms of print resolution, RIJ will theoretically have the same resolution as inkjet printing. However, in practice, given the multi-head combinatorial approach often used, maintaining a resolution of single-layer, single-head inkjet printing, is more challenging. Nonetheless, patterning 2D structures at resolutions at the low micron-scale are easily achievable using RIJ.

As this book is concerned with RIJ, it seems prudent that the first chapter introduce to the reader what is meant by this term. By its nature, RIJ requires the use of an inkjet printer, the first chapter will, therefore, survey the various types of inkjet printing (such as piezo and thermal), as well as describing droplet ejection mechanisms and ink considerations. The following chapters will discuss a number of fundamental inkjet areas that will help inform the reader.

The second chapter discusses droplet behaviour from the moment a droplet impacts a surface to where it coalesces with a second or more to form a feature. The various drying phenomena, such as 'coffee staining' are discussed, as well as possible phase changes. The third chapter is fascinating; typically, RIJ involves two dissimilar droplets being joined, with their contained reactants meeting to form a product. Chapter 3 discusses how two droplets interact; what is intriguing is the observation that two droplets don't appear to mix! What this chapter illustrates is the youth of the field of RIJ and that the early focus on applications has now opened up some fascinating physics to explore. The fourth chapter concludes the Fundamentals section with a discussion of the behaviour of high molecular weight polymers in ink. These polymers tend to break apart during jetting which offers the prospect that radical chemistry can be performed.

The third and largest section of the book is given over to a high level overview of many of the application areas that RIJ can be employed in. Alison Lennon illustrates the attraction of using RIJ in metallisation and the production of etchants for use in solar panel manufacture. The use of RIJ to generate HF in situ is a particularly compelling example. Ghassan Jabbour and co-authors continue the exploration of RIJ's use in inorganic chemistry with the synthesis of gold nanoparticles, zinc oxide nanostructures and lead sulphide quantum dots. They begin their chapter by illustrating that RIJ offers organic chemistry applications in their modification of the sheet resistivity of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT : PSS) by selectively dispensing droplets of aqueous sodium hypochlorite onto the polymer, thereby tailoring the degree of oxidation.

The next two chapters move the focus on to biology with the preparation of silk by RIJ forming the core. The first of this duo looks at the preparation of dental barrier membranes. Here, the degradation rate of silk II can be tuned by RIJ. The second chapter looks at the preparation of Janus-particle like structures, silk micro-rockets, which could be employed in a range of applications such as environmental monitoring, Lab-on-a-Chip diagnostics, and in vivo drug delivery.

The final three chapters continue to portray the breadth of application areas where RIJ can make a contribution. Christopher Tuck and colleagues describe the use of RIJ in additive manufacture, whilst Paul Calvert looks at the use of RIJ to form conductive features of either copper or nickel. The use of copper is of particular interest since it is a cheaper alternative to silver. Finally, the focus returns to biology with a discussion of the use of RIJ in the production of alginate- and fibrin-based systems for tissue engineering.

1.2 Reactive Inkjet Printing — The Concept

In simple terms, the concept of reactive inkjet printing (RIJ)...