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Printing technology promises a viable solution for the low-cost, rapid, flexible, and mass fabrication of biosensors. Among the vast number of printing techniques, screen printing and inkjet printing have been widely adopted for the fabrication of biosensors. Screen printing provides ease of operation and rapid processing; however, it is bound by the effects of viscous inks, high material waste, and the requirement for masks, to name a few. Inkjet printing, on the other hand, is well suited for mass fabrication that takes advantage of computer-aided design software for pattern modifications. Furthermore, being drop-on-demand, it prevents precious material waste and offers high-resolution patterning. To exploit the features of inkjet printing technology, scientists have been keen to use it for the development of biosensors since 1988. A vast number of fully and partially inkjet-printed biosensors have been developed ever since. This study presents a short introduction on the printing technology used for biosensor fabrication in general, and a brief review of the recent reports related to virus, enzymatic, and non-enzymatic biosensor fabrication, via inkjet printing technology in particular.
Biosensors: The first report of an enzyme electrode (oxygen biosensor) dates back to 1962 [1], and the term biosensor was introduced by Karl Cammann in 1977 [2]. By definition, a biosensor is “a chemical sensing device in which a biologically derived recognition entity is coupled to a transducer, to allow the quantitative development of some complex biochemical parameter” [3, 4]. In other words, it is a device that detects an analyte (enzyme, DNA/RNA, tissue, antibodies, antigen, proteins, etc.) by transducing a biological response into an electrical signal. As such, a biosensor contains a sensor (molecular detection system) that interacts with the analyte, and a physiochemical detector (receptor) that transduces the analyte interaction into a quantifiable electrical signal. In general, there are three categories of biosensors: (i) Mass-based biosensors, also known as gravimetric biosensors, work on the principle of change in mass. These are highly sensitive sensors that respond to minimal mass fluctuations and are especially suited for biomolecules that are neither fluorescent nor electroactive [5, 6]. (ii) Optical biosensors work on the principle of optical transduction [7], in which bio-recognition elements, such as enzymes, antibodies, proteins, etc., are fed to an optical transducer, which subsequently produces an electrical signal proportionate to the concentration of the measured substance [8, 9]. The measurements can be based on fluorescence, luminescence, or color change. (iii) Electrochemical biosensors, as suggested by the name, convert the electroactive analytes into a quantifiable electrical signal through three comprising bio-recognition elements: analyte, transducer, and instrumentation. After recognition of the specific analyte, the electrode, acting as a transducer, detects the generation of ions during the chemical reactions and converts them into electrical current or voltage [10, 11, 12]. In simple words, in the case of optical biosensors, photon measurements of the analytes are collected, while in the case of electrochemical biosensors, electron measurements are collected instead. An electrochemical biosensor is further classified as a potentiometric, amperometric, conductometric, immunosensor magnetic, or optical biosensor based on the transduction mechanism [13]. Compared to the optical and mass-based biosensors, electrochemical sensors offer miniaturization, simple fabrication, and customized readout circuits [14]. A detailed description of the transduction systems and types of biosensors can be found in a recent review article [15].
Biosensors applications: The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has undoubtedly been devastating, causing approximately 5.4 million deaths globally as of 30 December 2021 according to the World Health Organization (WHO). This extremely contagious virus has symptoms similar to the common flu virus [16, 17], subsequently making flu patients COVID-19 suspects and requiring highly expensive molecular-precision virus testing for each individual. In light of such an urgent issue, an economic, rapid, and high-precision assay method for remote point-of-care (POC) testing and detection of SARS-CoV-2 was recently developed (Figure 1) [18]. Within 5–15 min of incubation, using DNA concentrations of 3.0 or 30 aM, respectively, a 0.7 aM limit of detection (LOD) was determined, which is equivalent to the performance of a conventional PCR test. Being a POC device, the need for electricity is also removed.
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In other words, the device is capable of being used anywhere and anytime for a rapid, 15 min test for SARS-CoV-2. As such, biosensors can be considered as promising tools for the rapid and easy detection of not just COVID-19, but other viruses as well, for the early diagnosis of infectious diseases [19, 20]. Moreover, the ad hoc detection and monitoring of bacterial infections [21, 22] can provide convenience for the patients and improved healthcare in hospitals. Testing food and water contamination is another area where biosensors excel in terms of cost effectiveness and as rapid assay tools [23, 24, 25]. In short, biosensors provide portability and the rapid, low-cost, and early diagnosis of pathologies such as cardiac or neurodegenerative diseases, infectious diseases, cancer, fatal viruses, bacterial infections, pregnancy tests, and more, anywhere and anytime. All these conveniences come without the need for specific technical expertise. With the rapid advancement in patterning, microfabrication, and nanofabrication techniques, diverse biosensors production will see a market value of USD 36.7 billion by 2026 [26].
Biosensors Fabrication: Since the fabrication of the first transistor [27], photolithography has been the state-of-the-art choice for microelectronics fabrication due to its high resolution, accuracy, and repeatability [28, 29, 30]. The photolithographic technique has been vastly used in silicon integrated circuit (IC) manufacturing, as it can produce extremely small patterns down to the nanometer scale. However, much is left to be desired for such a widely used fabrication technique; for instance, the blanket material deposition and the subsequent photopatterning and etching process in each step cause it to be impractical, expensive, and wasteful for large-area fabrications [31, 32, 33, 34, 35] The use of a high amount of hazardous chemicals renders the process dangerous for indoor industrial employees and the outside environment [36, 37, 38, 39]. Moreover, with the increasing trend of flexible electronics [40, 41], traditional subtractive fabrication techniques are limited to the use of soft and flexible substrates. The demand for low-cost, rapid, and flexible fabrication methods has led to intensive research activities towards advanced 2D [42] and 3D [43] printing technologies. The drop-on-demand and additive nature of printing technology are its most attractive features. With the rapid development of a variety of printable materials, the area of applications for printing technology is continuously diversifying [44, 45, 46, 47]. In addition to general electronic device manufacturing, printing technology has been adopted for the fabrication of biosensors, with. the earliest report dating back to as early as 1988 [48], in which an inkjet nozzle was used to deposit an enzyme onto an ion sensitive field effect transistor (ISFET).
Printed Biosensors: The current advanced printing and deposition methods for biosensor fabrication can be classified into two categories; (i) the direct deposition and patterning of material onto the desired substrate, and (ii) the transfer deposition of pre-patterned material onto the substrate. The former technique is achieved via contact (all mask-based techniques) [49, 50] and non-contact printing (maskless printing) [15, 50, 51] technologies, while the latter is achieved by a transfer print after the patterns are brought about via the conventional lithography [52, 53, 54, 55, 56, 57] or plasma-modification techniques [58, 59, 60]. An immense amount of research has been carried out in the pursuit of fabrication techniques that (i) require a minimum number of fabrication steps, (ii) are flexible in the usage of printable inks, and (iii) are compatible with the appropriate application-based substrates [61]. The reason for such diversity in the printing and deposition techniques is because each technique possesses its own merits and demerits, such that one standalone technique does not suffice for the mass production of all the nano-sized, complicated patterns with high throughput, low cost, and simplicity [62]. For instance, inkjet printing promises mass production with great simplicity and flexibility in terms of ink and substrate selection; however, it is limited in the nozzle sizes required for producing nano-sized features. On
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