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BioMed X Expects to Validate Field-Effect Transistor for POC Testing


NEW YORK (360Dx) – Researchers at BioMed X have demonstrated that a transistor-based platform that they are developing can eliminate time-consuming analysis steps, opening up the possibility that testing of numerous medical conditions could move toward the point of care.

In a study published recently in ACS Sensors, the BioMed X researchers describe a proof-of-principle immunodetection system that uses a field-effect transistor within a hand-held reader connected to a disposable electrode. The device, they said, was able to measure human thyroid-stimulating hormone present in serum at particularly low levels of concentration.

Partly because the technology has been able to provide measurements over a broad dynamic range — from subpicomolar to 100 nanomolar — broadening the use of the platform to detect several different types of analytes and biomarkers, and medical conditions in physicians' clinics or beside patients in hospitals, should be possible, Alexey Tarasov, group leader at BioMed X, said in an interview.

However, while the platform has broad potential, it needs to undergo clinical validation testing using patient samples during the next year or so, to ascertain its accuracy compared with an established laboratory analyzer.

The project in progress at BioMed X, a research center in Heidelberg, Germany, is being funded by Roche Diagnostics.

It could take more than five years to commercialize the use of the platform as a diagnostic test, Tarasov said.

"The vision is to bring it closer to the market and commercialize it, but that's a few steps down the road," Tarasov said, adding that when validation studies have been completed, "it will then be time to decide whether to move the concept into product development."

At that time, if results continue to be promising, Roche could be interested in taking on product development, he said.

Importantly, the FET-based device is compatible with inexpensive large-scale manufacturing processes, such as roll-to-roll printing, and could therefore be produced at low cost using inexpensive materials, Tarasov said.

The platform could eventually provide an alternative to antibody-based laboratory tests, such as ELISAs, that use dilution and washing steps that delay results for several hours, the researchers noted.

The device uses the field-effect transistor's charge detection principles. Charged molecules that attach to the sensor's surface generate an electric field that's detected and analyzed within a reader.

Tarasov said that the ability to implement such an electronic device and eliminate washing and dilution steps was enabled by overcoming a challenge related to the phenomenon of screening — whereby ions in samples such as blood and serum make it difficult to detect the analytes of interest.

In these circumstances, molecules of interest that attach to the sensor surface cannot be detected because other ions surround them, Tarasov said.

Until now the answer to overcoming screening has been to dilute or wash a sample with a buffer solution, he said, which reduces the effect of ionic interference and boosts the sensor signal.  

To address this challenge, the researchers fabricated shorter antibody-based receptor molecules by using only the essential fragments of whole antibodies, to reduce the distance of the analyte from the sensor surface.

"Instead of being 10 nanometers, the antibody fragments were about 5 to 7 nanometers," Tarasov said, adding, "This makes your life quite a bit easier, but it still doesn't give you a clear enough signal."

He noted that the researchers, as a result, considered "other signal enhancement strategies," and they decided to add a polymer layer of polyethylene glycol. "This changes the dielectric environment close to the sensor surface," Tarasov said. The polymer was immobilized and attached to the surface with the fragment-antibody receptors, and this mixed layer helped to enhance the signal by a factor of three, he said.

"It might not sound like a lot, but it's what allows us to measure analytes in serum without dilution or washing steps," Tarasov said.

In their study, the researchers demonstrated their system by testing human thyroid-stimulating hormone, which has demanding detection requirements, and is present in blood or serum in very low concentrations. "It is relatively difficult to measure, which is what makes it interesting," Tarasov said.

However, the system is not limited to detecting human thyroid-stimulating hormone, he noted. "This platform can be interesting for many analytes especially where time is an issue and you cannot wait for an ELISA test, for example," Tarasov said.

The BioMed X group is especially interested in testing out the platform with cardiac biomarkers at the point of care. He noted that the group intends to test markers other than human thyroid-stimulating hormone going forward and they could be interested in testing for troponin, for example, as it "is one of the very important parameters for heart failure."

"The system is versatile," Tarasov said, "so if you give us a good receptor for a target, then I am sure that we can try to measure it."

FET-based biosensors have been shown to have a fast response and can be inexpensive, miniaturized, and mass-fabricated using conventional nano- and microfabrication technology, the researchers said. That makes the biosensors good candidates for point-of-care diagnostic devices.

Although these types of sensors have shown high sensitivity and selectivity in label-free detection schemes for various analytes, few have been commercialized, largely because of the issue with ionic screening that they have been working to overcome, the researchers said.

Only a few diagnostic tests that use transistors have been launched in the market, the researchers said, including Thermo Fisher's Ion AmpliSeq next-generation sequencing platform and Vista Therapeutics' nanobiosensor.

London-based DNAe's founders invented a semiconductor sequencing technology that became the basis of Ion Torrent, which was acquired by Thermo Fisher Scientific. DNAe, by itself, is validating an integrated prototype of a semiconductor-based sepsis test that could eventually identify pathogens directly from blood within three hours.

By the end of 2018, if the results of clinical validation studies go well, DNAe may be ready to seek clearance from the US Food and Drug Administration, and it will seek CE marking next summer.

Tarasov said that he also sees the use of electronics over optical systems as an advantage in the development of diagnostic tests, because many optical detection methods used in diagnostics "contribute to the expense of the systems."

As a next step, his team is planning to test out its platform using patient samples to establish sensitivity and specificity values for the platform compared with a laboratory analyzer, Tarasov said.

However, the researchers have already demonstrated high levels of performance, he said. The platform's subpicomolar limit of detection — more than 5 orders of magnitude lower than reported previously in buffer — combines with its broad dynamic range, he noted.

In addition to testing the accuracy of the platform in patient samples, the group is also looking to optimize its platform, "pushing its detection limits even further."

He noted that validation studies are likely to occur during the next year.

The system is being specially designed with point-of-care applications in mind, he said. As a result, its design involves the use of two principal components — an electronic-based system around the size of a smart phone and a disposable chip. "It's important that the disposable element is inexpensive and we have shown that it takes only a few steps to fabricate it," Tarasov said.

The field-effect transistor is placed in the system reader, rather than on the disposable chip, which simplifies the manufacturing of the disposable element and, as a result, the entire system, he added.