The development of biosensors for Lyme disease diagnosis is an area of emerging research and is set to see numerous advances over the next few years. While efforts to develop a Lyme biosensor date back to 2008 with the initial SPR-based biosensor [44
], the last few years have seen the most intense progress in the field; this claim is supported by the observation that over half of the research articles reviewed here were published during or after 2019. This focus on improved Lyme diagnostics is fueled by both the increasing prevalence of the disease and the rapidly developing field of biosensor technology. Through this research, scientists aim to harness chemical and biochemical techniques to develop cheap, rapid, and effective tests for Lyme that can help stem the spread of the disease.
3.1. Achievements & Challenges
While the STT serology approach remains the most used method to diagnose Lyme disease, this standard dates back to 1994 [48
] and has seen minimal improvement since then. A certain proverb may suggest that this methodology is fine as is—so long as it is not broken; however, while the test provides satisfactory results for those with disseminated Lyme, it struggles to diagnose those with early Lyme. The reported 46.3% sensitivity is considerably low, with a potentially large number of false-negative results within the first month of infection [15
]. Likewise, the MTT approach demonstrates only slightly improved sensitivity, at approximately 50%, for patients with early Lyme disease [18
]. Some of the biosensors presented here (Table 1
) report sensitivities for early Lyme that exceed the STT/MTT approaches and have the potential to develop into a much-needed alternative [32
]. However, some biosensors also demonstrate reduced specificity in comparison to the two-tiered approaches. Given the ambiguity of many of the Lyme disease symptoms, it is essential that any new sensor has high specificity to reduce false positives for other diseases.
In addition to their initial low sensitivity, the STT and MTT approaches are also quite expensive, both in analysis time (>24 h) and cost (>$
400 per test) [42
]. A rapid, inexpensive, and portable biosensor would prove to be an extremely beneficial tool, especially for rural areas where access to healthcare is limited. Several of the biosensors reviewed here have the potential to develop [37
], or have already developed [39
], into commercial Lyme sensors that approach or exceed the sensitivity and specificity of the STT and MTT methods. As well, some of the biosensors presented here utilize methods that are relatively inexpensive [39
] in comparison to the STT/MTT approaches. These biosensors represent a substantial advance in the improvement of Lyme diagnostics and open the door to new inventions and innovations for both the diagnosis of Lyme disease and other infectious agents.
Lyme biosensors have the potential to lead to improved diagnostics; however, there are still challenges that must be overcome before they can provide reliable alternatives to STT/MTT testing. Deficient analyte concentration is one issue that often plagues the biosensor development process, especially those that target bacterial/viral antigens. While sensors may provide extremely accurate results for spiked samples, unless that success can be transitioned into real-world samples, the biosensor has limited applicability. Ideally, selected biomarkers should have high concentrations in the sample type (e.g., plasma), which can be approximated by bacterial count. B. burgdorferi
cell estimates in various fluids/tissues span a wide range (Table 2
); this variation can lead to difficulties in diagnosing certain individuals and thus, lower sensitivity. Besides appropriate sample selection, biomarkers should also be abundant (many per cell), highly specific (unique to the bacteria), and easily accessed (secreted/surface protein)—especially if the samples are not treated (e.g., lysed) prior to analysis.
Sensors that target antibodies, rather than bacterial antigens, risk experiencing similar issues to STT/MTT testing, with low sensitivity to early Lyme infection. Host-developed antibodies are excellent biomarkers as they are typically abundant, highly specific, and very stable. However, the time required for these markers to reach adequate detection levels may be invaluable to the patient’s health. The selection of alternative antibody targets and improvement of antibody visualization are just some of the potential steps that can be taken to counteract this delay.
While biosensors and other POC devices possess many advantages, including portability, inexpensiveness, and the decentralization of testing centers, there exist several issues that must be accounted for prior to wide-spread biosensor implementation. Decentralization of testing is great for expediting sample analysis time; however, it also leads to an increased risk of improper sample handling and analysis, including inadequate device calibration and insufficient quality control [53
]. To address this, healthcare workers would need to undergo mandatory training or devices that would need to be simple enough, with detailed documentation, that they could be used by the consumer. While biosensors are less expensive in theory, some methods of biosensing require expensive analytic devices that would be impractical in an overly decentralized setting without a miniaturized version—which may be expensive to develop and produce. Biosensors may also utilize biological components that are highly susceptible to changes in temperature, pH, or other factors; this would make POC implementation difficult as results may vary depending on the environment where the analysis is taking place [55
The development of new biosensors, both for Lyme disease and other infectious diseases, must include answers to the previously mentioned issues to have any hope of eventual commercialization. Since the ultimate goal of biosensors is typically deployment at the POC—either by health workers in a clinical setting or by oneself at home—the device must be thoroughly optimized, tested, and stable. This requires a thorough examination of potential factors that may influence the device’s outcomes, testing of the device with well-pedigreed, high-quality samples, and testing to ensure that the device is stable over a potentially long shelf-life. Scientists looking for examples of biosensor implementation need look no further than common home pregnancy tests and glucose sensing devices [56
]. These biosensors began as in-lab assays and developed, through thorough testing and optimization, into wide-spread, inexpensive devices that are used by millions of people annually.
3.2. Looking Ahead
The current review represents the first consolidation of Lyme biosensor research to date, and functions to both summarize the field thus far and provide direction for future research. While several biosensing techniques have been explored so far, there exist many additional combinations of biorecognition elements and transducers that have shown high potential with the diagnosis of other infectious diseases. Optical methods (e.g., fluorescence, SPR) are the most common approach and are particularly good at high-throughput analysis [57
]. Electrochemical methods (e.g., impedimetric, potentiometric) have shown great promise in developing sensors that are portable and easy-to-use—for example, glucose sensors for diabetes [58
]. Piezoelectric methods (e.g., quartz crystal microbalance, acoustic wave), while not heavily commercialized as of yet, are excellent at effectively monitoring affinity interactions [59
]. In addition to the transducing method, there also exists a wide array of potential recognition elements, including enzymes, aptamers, whole cells, and antibodies. Biosensors represent a subfield of chemistry that is both practical and continually evolving with the potential to solve the Lyme disease diagnostic dilemma.