# Inkjet-Printed and Paper-Based Electrochemical Sensors
## 2 Fabrication Methods
### Screen Printing
The sensor is referred to as a \"pop-up\" device by the authors and was designed specifically to integrate with a commercial glucometer. Among all of the devices reviewed in this article, this particular ePAD is certainly the best demonstration of a low-cost, portable sensor for point-of-care applications. The fabricated sensor before and after folding is shown in Figure 2.  
Figure 1: The first paper-based electrochemical analytical device developed using screen printing: (**a**) The basic design of each detection zone; (**b**) The fabricated sensor with three detection zones for multi-analyte detection. Reproduced with permission from [39], Copyright American Chemical Society, 2017. [PERSON] et al. developed a multilayer paper-based device for detection of metals, in which they used both colorimetric and electrochemical sensing mechanisms [11]. Filter paper served as the colorimetric sensing substrate and a polyester film served as the electrochemical sensing substrate. The two materials were essentially taped together to form an integrated device as shown in Figure 3. Like many of the other demonstrations, fluidic channels were defined on the filter paper using a wax printing technique. To form the reference electrode, unmodified carbon ink was screen-printed on the polyester film. To form the working and counter electrodes, a mixture of multi-walled carbon nanotubes, graphite powder, and carbon ink was then screen-printed on the same transparency film. [PERSON] et al. have also combined colorimetric and electrochemical techniques to enable simultaneous detection of gold and iron [24]. They patterned the microfluidic channels and detection zone using photolithography with SU-8 as the photoresist. The working and counter electrodes were screen-printed using carbon ink, and Ag/AgCl ink was utilized to fabricate the reference electrode. The fabricated sensor is shown in Figure 4.  
Figure 3: Fabrication process and sensor layout developed by [PERSON] et al. for metal detection. Reproduced with permission from [11], Copyright American Chemical Society 2017. Figure 2: Pop-up paper-based sensor fabricated by [PERSON] et al. for determination of BHB levels. Reproduced with permission from [23], Copyright American Chemical Society, 2017. The porous structure of paper and its low cost make it a desirable candidate for immobilization of enzymes on the electrodes surface. [PERSON] et al. have used this advantage in order to develop a paper-based glucose sensor [PERSON] et al. (2017). Unlike the other groups working on ePADs, this group purchased and modified a commercially available screen-printed paper-based electrochemical sensor. The working electrode was modified with platinum (Pt) nanoparticles using an electrodeposition method in order to facilitate electron transfer at the surface of the electrode. To fabricate the sensor, they soaked paper in a glucose oxidase solution to achieve an enzyme-modified paper. Next, the modified paper was placed on top of the electrode and fixed in place using a plastic holder. The mechanical properties of the enzyme-modified paper were investigated, and the results indicated that the modified paper was tougher than the unmodified paper. As demonstrated by each of these example devices, screen printing is a very suitable fabrication technique to use in developing low-cost paper-based sensors. The design and manufacturing process is relatively simple and does not require significant knowledge to execute. Despite these advantages, screen printing is not necessarily the best choice in all cases. For instance, through the screen printing process, a large volume of material is required, which may limit the application of this method when using expensive electrode materials such as noble metals. Moreover, this method requires a stencil to pattern the electrodes, which limits the achievable maximum resolution. It is also not possible to quickly alter the stencil.