# Inkjet-Printed and Paper-Based Electrochemical Sensors
## 1 Introduction  
In a world where new technology is introduced regularly, innovation is critical to advancement. A multitude of motivations drive this constant innovation ranging from convenience to resource limitations. In fact, one of the primary driving forces behind modern innovation is cost. In order to reduce production cost, there are two traditional paths that can be taken. In both cases, cost reduction ultimately requires developing or altering materials, methods, or processes as well as developing or obtaining new equipment. The first path involves the development of new materials or processes to further reduce cost. Of course, this preliminary development phase will likely increase cost initially, but in the long-term, more advanced materials and techniques will eventually save money. The second path involves the utilization of existing technologies including readily available materials, well-defined processes, and established equipment. By employing these existing solutions and applying them to a new field or application, both development costs and production costs can be reduced substantially. As it applies to sensor development, this continuous pursuit of cost savings has driven incredible innovation, allowing for the production and distribution of sensing devices and platforms at an affordable price. Of course, many other sensor requirements drive advancement including sensitivity and size, but it has become increasingly more important to introduce low-cost point-of-care sensing solutions to the market. In many cases, time is critical, and transporting samples to a remote laboratory may be very time-consuming and even result in sample contamination. On the other hand,the ability to transport small portable sensing devices to the field is a critical advancement allowing for a much simpler process that maintains sample integrity while improving efficiency. To meet this ever-increasing need, some research groups have focused on new materials and methods, while others have relied on tried-and-true materials, methods, and equipment. Given the notable results and advancements in this field, the latter of these approaches remains the focus of this review article. The use of existing technologies has allowed the production of many exemplary devices capable of being used in a point-of-care situation. In particular, among existing fabrication technologies, printing has become incredibly popular due to its ease of use and low-cost equipment. Although various printing methods have been investigated and demonstrated, inkjet printing has many advantages that lend itself particularly well toward the development of low-cost point-of-care analytical devices. Furthermore, with respect to materials, the use of paper to develop sensing platforms has gained significant attention in recent years due to its exceptional mechanical properties, porous structure, low cost, and natural origin [1; 2; 3; 4]. Considering the wide applications of microfluidics in point-of-care diagnostics, environmental analysis, and food processing, utilizing paper in microfluidic systems can overcome the limitations caused by conventional materials including glass, polydimethylsiloxane (PDMS), and silicon. Paper is both biocompatible and biodegradable, which opens the door for potentially disposable paper-based devices capable of chemical and biological sensing. Additionally, paper has a natural ability to wick fluids, enabling the fabrication of pump-free microfluidic paper-based analytic devices (\(\upmu\)PADs) [4; 5; 6]. The concept of guided fluid flow inside paper was first introduced by [PERSON] et al. for chromatography applications in 1949 [7]. However, the [PERSON] group at Harvard University first reported employing paper in microfluidics in 2007, where they presented the first paper-based microfluidic device with millimeter-sized channels fabricated using photolithography [8].