By Deborah Borfitz
April 14, 2020 | Researchers at Monash University in Melbourne, Australia have developed the world's first ultrasound biosensor that can be inserted deep into the tissues to measure diagnostic markers in real time. The technology has been shown to work in vivo to detect changes in pH levels, but the long-term goal is to give clinicians a way to monitor drug levels or organ response in patients and adjust dosing accordingly, according to Simon Corrie, Ph.D., a senior lecturer in chemical engineering at the university as well as chief investigator at the Monash-led ARC Centre of Excellence in Convergent Bio-Nano Science and Technology.
The new imaging approach is currently using a solid nanoparticle developed by Julia Walker, one of Corrie’s Ph.D. students, which alters its stiffness in response to pH changes in the body and does so consistently over time, he says. Those signals can be detected by common ultrasound scanners.
Nanoparticles are utilized in a range of FDA-approved medicines, but only a handful for in vivo biosensing, Corrie notes. The poster-child example is continuous glucose monitoring involving the analysis of electrochemical signals. In a 2017 review published in ACS Sensors, he and his colleagues at Northeastern University identified roughly a half dozen groups worldwide doing biosensing of a critical biomarker in a live animal model.
The detection modality for biosensors has traditionally been an optical or electrochemical system, continues Corrie. But ultrasound offers the advantages of being widely available and relatively inexpensive, thereby broadening the accessibility potential of the technology.
In the clinic, gas-filled microbubbles have been injected into people to help clinicians better visualize blood flowing through various organs via ultrasound, he adds. Gas-producing nanomaterials have also been developed that can be used to enhance ultrasound images and for ultrasound-induced drug delivery. Additionally, one or two ultrasound biosensors have been designed where a nanoparticle generates oxygen to amplify signals.
The problem with those ultrasound approaches is that the particles only work for 10 to 20 minutes because patients breathe the gas back out of their body, he says. That makes measuring biomarker fluctuations over time an impossibility, except using implantable monitors in conjunction with expensive CT scans or MRI.
The longest Monash University researchers have used their ultrasound biosensor to monitor pH changes is about two hours, Corrie says, which is more an animal ethics issue than a limitation of the technology. They have already demonstrated that when put into a hydrogel, the particles will produce a stable pH signal over three days, he adds.
Seeking Partners
Now the biosensor has been successfully tested (doi: 10.1021/acssensors.0c00245) in a healthy animal model to detect changes in pH levels, the next step is to test it in a diseased animal model to determine whether it can accurately monitor rapidly changing pH levels, says Corrie. The three “obvious” targets that researchers might focus on initially are ischemia, stroke, and cancer.
“We’re pivoting to work on coronavirus diagnostics for a few months,” he adds, after which the team will look for a partner lab to begin the new round of animal studies. The platform will also be used to begin testing of more “specific and critical biomarkers.”
Detecting proteins such as troponin is going to be a “significant challenge” using the new technique, Corrie acknowledges. But rising troponin levels is a reliable indication of a heart attack so “the dynamics of how troponin goes up and down over time is very important for clinicians to help manage the condition, and a really good target for us.”
The technology has been granted an international provisional patent, he says, which is held by Monash University and several of the researchers. It could be in clinical trials in another three years and a viable commercial product within the decade—but not without a clinical partner to co-direct the project or commercialization partners to help bring it to market.
Beyond pH Detection
Designing the biosensor for the detection of pH was a “steppingstone to more complex sensors,” such as oxygen as an indicator of stroke injury or disease-related proteins, Corrie says. “There are some useful and probably some unexplored uses for pH sensors—for example, tumors are known to have lower pH environments, as do areas of inflammation. The gastrointestinal tract also has quite a broad range of pH levels.”
But changes in pH can occur in some areas of the body in both the presence and absence of disease, he says. “The issue with pH is that it is relatively non-specific, so it can be very difficult to really lock down what’s going on.”
It is also not obvious how the ultrasound biosensor could be used to diagnose a cancer, says Corrie. “But we might be able to use it to monitor the response of a tumor to a specific therapy—[e.g.,] Is the tumor shrinking? Is pH getting back to normal?” The ability to pair a line trace of biomarker levels with anatomical information from imaging remains an “underappreciated area” in the field of in vivo biosensing.
The pairing also has the advantage of providing a “geography-specific” picture of what’s going on in the body, he adds. Conversely, the chemistry approach of continuous glucose monitoring provides “an average of what is going on around an electrode.”
Dynamic testing versus snapshot-in-time detection of a critical biomarker is the “really exciting part of this field,” Corrie says. “It could potentially change the way patients get managed.”
Sooner And Later
Currently, nanoparticles are introduced into the body through a simple subcutaneous injection into the upper layers of the skin, Corrie says. Skin-based tattoos, already being used to ensure radiotherapy treatments get precisely delivered to cancer patients, are an attractive alternative. “We want to seamlessly work into the current medical flow as much as we can.”
A more “blue sky application” of the technology would be to use intravenous injection of sensor particles that home to a particular organ, he adds. It’s also quite possible the technology will eventually be readable by “something as simple as a mobile phone.”
Companies are already making transducers that hook up to an iPhone or tablet, enabling healthcare providers to monitor patients in remote areas without the need for big hospital labs, he says. “Even without mobile smartphone applications, ultrasound imaging systems are portable and ubiquitous across the healthcare system.”
Clinical Questions
Like all biosensors, the device would at some point be subject to materials breaking down or issues of “fouling” when the environment around the device changes, either of which could lead to faulty information output, says Corrie. While not an issue for envisioned uses over the near term, fouling would need to be addressed if the ultrasound biosensor finds applications that require continuous, weeks-long monitoring.
Once researchers know the biosensor’s lifetime, they’ll need to consult with clinicians to learn if that’s long enough for the intended use, Corrie says. If not, it’s back to the drawing board to tweak the chemistry for improved resilience.
How often a biosensor will need replacing will depend on the stability of the silica-based materials used in its manufacturing as well as the clinical need, says Corrie. He imagines the ultrasound biosensor will initially find utility in the management of clinical conditions characterized by a rapid decline in pH and related metabolites, such as sepsis, as well as in therapeutic drug monitoring—and certainly in the intensive care unit and emergency room where clinicians want as much information as possible over the course of hours rather than days.