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.