*4.2. Biosensors*

Biomolecule sensing devices, such as glucose-sensing patches, promise a transformative way to continuously assess crucial biomarkers, making their development critical in the future of healthcare development [12,14–16]. Traditional biosensing methods, as discussed in the introduction, are highly limited because of exceptional costs and the inability to record results continuously in real-time, whereas printed biosensors are well suited to long-term, continuous monitoring in an affordable and wearable package [12,13]. The clinical implication of such technology is clear, and the development of high throughput biosensor fabrication, such as the slot die process that is shown in Figure 10b [57] It is of very high importance [13]. Generally, biosensors consist of a receptor, e.g., an antibody, and transducer, e.g., a nanomaterial sheet capable of transmitting an electrical signal from the receptor to a circuit element [13,16]. To bind the receptor to the transducer, the material must be functionalized, as discussed in Section 3.4. Although many nanomaterials may be effectively functionalized, graphene is one of the most favorable materials because of the ease in which one can attach a variety of organic and inorganic functional groups [13]. For instance, graphene can be oxidized, then functionalized with 1-ethyl-3-(3 dimethylaminopropyl) carbodiimide hydrochloride (EDC)/N-hydroxysuccinimide (NHS) (EDC/NHS) to facilitate antibody binding [156]. The target molecule detection can be achieved through several methods, although electrochemistry is the most employed [12,13]. An example of a potentiometric electrochemistry analysis is provided in Figure 10c [12]. In these systems, functionalized working (where the reaction occurs) and reference (where the current is provided) electrodes are implemented, and the transducer is able to record changes in current, resistance, or potentially caused during the binding reaction [13]. Another printed biosensor method is based on FET technology, where the binding of a target molecule is used to modulate the flow of current through a channel [13].

Table 3 summarizes recent demonstrations of high throughput biosensor fabrication, with an emphasis on device performance [16,40,157–160]. Although each work incorporated high throughput fabrication methods, many did not specifically state key process parameters required to translate this technology, such as print speed, roll pressure, ink viscosity, and in some cases, curing [12]. Instead, these works focused primarily on device efficacy, likely because there are significant unanswered questions in biosensor printing relating to material choice and functionalization. However, Cagnani et al. were able

to achieve an exceptionally high 30 m/min printing speed using a slot die coating on PET for dopamine detection [151]. Bariya et al. developed a high throughput gravure printing method for wearable sweat sensor fabrication capable of 6 m/min printing [40]. The majority of additional works focused primarily on sensor stability, which itself is highly dependent on material functionalization, purity, and receptor choice. For instance, Narakathu et al. used gravure printing to fabricate AuNP electrodes for the detection of a variety of chemicals, such as mercury sulfide (HgS), lead sulfide (PbS), D-proline, and sarcosine, demonstrating high sensitivity down to pico-molar concentrations [158]. In another experiment, Favero et al. demonstrated that graphene and MWCNT functionalized electrodes can be improved with the ingrafting of AuNPs to increase conductivity, noticing a >10% increase in electroactive area. A corresponding increase in R correlations indicates linearity after the addition of AuNPs [157]. Although biosensor printing is an active area of research with significant hurdles to overcome, the study of high throughput fabrication methods for the sensors that have been well tested, such as glucose and sweat monitors, is highly needed to scale these methods into clinical practice.
