4.2 Clinical biomedical applications
Because of the high sensitivity and fast response time of cell-free biosensors, they can quickly judge the diagnosis result, which occupies an important position in biomedicine. The existing cell-free biosensors mainly focus on the detection of clinically relevant carcinogens, bacteria, and pathogens (e.g., EDCs, AA, and AHL) [17, 56-58]. These sensors can be used to screen pathogens that can be detected in blood, urine, and sputum samples from clinical patients.
The well-known chemicals called endocrine-disrupting chemicals (EDCs) [59, 60], widely exist in the environment, food, and personal care products. Exposure to EDCs can lead to acute and chronic diseases, including cancer and diabetes. Salehi et al . [61] designed a hERβ-CFPS biosensor to detect endocrine xenoestrogens (XEs, one kind of the EDCs) in human blood and urine. Because these test samples (blood, urine, and sputum samples from clinical patients) are complex, they can interfere with the cell-free system to a certain degree and may alter the delicate calibration of protein synthesis reactions, making the test process less smooth. In order to solve this problem, some measurements can be taken to pretreat the sample. They added RNA enzyme inhibitors to the samples, which can reduce the impact of urine or blood samples on the CFPS system. This can simplify the detection process, which can be used for more complex analytes and expand the application range of cell-free biosensors.
The virus is rapidly pathogenic, and some are highly infectious. The severity of the virus infection and its unknown complications make timely diagnosis of the virus critical to the human health, which also can limit the rapid spread of the virus. In this context, some cell-free biosensors are designed to detect viruses. Gootenberg et al . [62] developed a rapid diagnostic cell-free biosensor (SHERLOCK) based on CRISPR-Cas13a to detect Zika virus in human serum samples. The sensitivity of the sensor is similar to that of ddPCR and qPCR, but with less variation. To improve the specificity of the sensor, they synthesized mismatched crRNA, which allowed them to distinguish between different strains (Zika virus and dengue fever). Cell-free biosensors can also be used to detect other viruses, such as Ebola virus. Now there is a kind of electrochemical DNA biosensor for detecting examples of this kind of virus [63]. In a word, cell-free biosensors can detect not only the presence or absence of a single virus but also different types of viruses.
The content of amino acids also has a great impact on human health. The lack of amino acids can lead to abnormal physiological functions, affect the normal progress of antibody metabolism, and lead to diseases. It can be used as an early detection method for several types of cancer. Janget al . [41, 64] designed a CFPS biosensor to detect amino acids in fetal bovine serum (FBS) samples. The amino acid content was successfully detected, and the detection limit was less than 100 nM. However, the types of amino acids detected by cell-free sensors based on crude extracts are limited, so they suggested using the PURE system for cell-free biosensors. It can detect more kinds of amino acids (Asp, Asn, Glu, and Gln) than the crude extract system, and the detection threshold is lower [65].
These examples show that cell-free biosensors have significant advantages in medical diagnosis and clinical sample testing. They can be used for early detection of disease and also can respond quickly to virus outbreaks or the occurrence of genetic variations and mutations to help monitor the spread of disease.