Device unlocks path to better implants

Researchers at The University of Queensland's Australian Institute for Bioengineering and Nanotechnology in Brisbane have developed a small sensor made from gold film that is both flexible and sensitive enough to enable a more streamlined future for electronic medical implants and real-time sensing applications.

Principle investigators, Mostafa Kamal Masud, Ph.D., and Ph.D. candidate Aditya Ashok, used a new engineering approach that represents a breakthrough in the field of flexible nanoarchitecture and, ultimately, suggests a new way to miniaturize and improve medical devices for diagnostics, biological sensing, and neurological exploration.

"Although modern implanted electronics have developed rapidly over the past 60 years, most commercially available devices are still built on relatively similar – and limiting - design concepts such as thick ceramic or titanium packaging," the researchers wrote.

They noted that implantable biomedical devices provide powerful tools for diagnosing and treating several chronic diseases, saving millions of lives and significantly improving individuals' quality of life.

Well-known examples include cardiac pacemakers that enable the regulation of the heart rate and rhythm and cochlear implants that provide sound perception to patients with sensorineural hearing loss. Introducing functional electronics with physiological recording and stimulation capabilities also has created an untraditional pathway for treating nervous system disorders such as Parkinson's disease and spinal cord injuries.

Golden biosensing discovery

Though modern implanted electronics have witnessed rapid development, most commercially available devices are still built on a relatively similar design concept consisting of a thick ceramic housing for electronics encapsulation and long metallic wires for biological tissue interfaces.

The use of thick ceramic or titanium packaging allows for relatively long-term stable operation; however, the mechanical mismatch between the implanted components and the soft nature of biological tissue poses potential risks to users and limits functional modalities, according to the scientists.

The University of Queensland team said recent advancements in material science had accelerated the research into flexible electronics platforms, creating exciting opportunities for the seamless integration of electronics into biological tissues.

For example, silicon nanomembrane transistors transferred from standard bulk wafers onto polymeric substrates can conform to the curvilinear surface of the heart and record cardio physiological signals over a large surface area. Flexible organic material-based electrode arrays were successfully demonstrated to record action potentials from superficial cortical neurons that potentially facilitate minimally invasive diagnoses and treatments for brain disorders.

"Flexible and implantable electronics hold tremendous promise for advanced healthcare applications, especially for physiological neural recording and modulations. Key requirements in neural interfaces include miniature dimensions for spatial physiological mapping and low impedance for recognizing small biopotential signals," the researchers wrote.

The scientists successfully created flexible and low-impedance mesoporous – also known as super nanoporous due to the extremely small pores – gold electrodes for biosensing and bioimplant applications.

They said mesoporous architectures developed on a thin and soft polymeric substrate provide excellent mechanical flexibility and stable electrical characteristics capable of sustaining multiple bending cycles.

Large surface areas formed within the mesoporous network allow for high current density transfer in standard electrolytes, highly suitable for biological sensing applications as demonstrated in glucose sensors with an excellent detection limit and sensitivity approximately six times higher than that of flat/non-porous films.

"We are offering a new route toward miniaturized, flexible, implanted medical devices that will diagnose and treat chronic diseases and help improve the lives of millions of people," Masud told reporters.

The sensor's design represents a novel approach to the field of mesoporous materials – highly porous substances with traits that benefit diagnostics, catalysis, and drug delivery.

Using a new hybrid fabrication process under the guidance of senior AIBN group leader Professor Yusuke Yamauchi, Masud and Ashok synthesized the mesoporous gold film, which serves as an electrode for biosensing and bioimplant applications.

The flexibility and sensitivity of the gold film make it an ideal wearable system for real-time monitoring of body glucose. However, the new material also has strong potential for implanted nerve recording applications.

"The demand for a simple and robust fabrication process with this kind of flexible electronics is enormous," Masud observed.

"Our aim here is to see this sensor embedded in wearable devices – but the potential and possibilities in this field are vast. We're going to be exploring more in our coming projects," he added.

Overcoming issues with gold films

An essential requirement of implanted devices is the low impedance at the electronic-tissue interface to enable the detection of relatively low biopotentials, which typically fall within a range of a few microvolts. In addition, to decipher the functionalities of complex neural circuits, high-density and high-resolution miniaturized electrode arrays distributed on a soft, planar substrate are desired.

To meet these requirements of implanted medical devices, several approaches have been proposed to increase the effective surface areas of electrodes and subsequently reduce their electrochemical impedance.

For example, stacking numerous layers of nanoflakes or nanopores of highly conductive 2D materials such as graphene can significantly minimize impedance. Spin-coating or drop-casting a network of metallic nanowires or carbon nanotubes also can increase the effective surface areas, thereby enhancing the impedance spectra of electrodes.

However, the unstable physical attachment of nanoflakes, nanowires or nanodots onto conductive surfaces could lead to problems in material uniformity, deposition selectivity, and scalability for mass manufacturing.

Alternative methods using top-down fabrication techniques to create high-density nanoholes on metallic materials such as gold can enhance the surface area along with well-controlled uniformity. These methods of developing small-sized nanopores require sophisticated equipment such as electron beam lithography or focused ion beam, typically increasing the processing time and device costs.

Other top-down techniques, such as laser-reduced graphene oxide, have been developed to form microelectrodes for chemical and biosensing applications. Since the fabrication process involves high-power laser irradiation, achieving high-resolution devices and integration with other functioning components that are sensitive to temperature are difficult.

In addition, drop casting is another common method to deposit nanostructured materials on different templates. For example, graphene oxide decorated with a metal-organic framework has been employed for nitrous oxide gas sensors. However, material uniformity, fabrication scalability, and the physical bonding between the functioning elements and the substrate have been challenging issues in this approach.

"We have recently successfully developed a variety of mesoporous materials, which cover a wide range of materials from metals to semiconductors," the Queensland University researchers wrote.

They said the porous network within these materials can significantly boost the overall performance of functional devices because of the significantly increased surface area. Some unique features offered by the mesoporous architecture include a high absorption spectrum for optoelectronics, excellent electrochemical properties for biosensing, and the capability of loading and releasing biomaterials for drug delivery.

Despite these significant merits, current mesoporous materials are mainly synthesized on rigid conductive substrates such as indium tin oxide and metal-coated silicon. This limits their potential for flexible electronics applications, particularly implemented biomedical applications, the scientists noted.

"To address these issues, in this work we introduce a new engineering pathway that combines top-down and bottom-up methods to selectively synthesize flexible mesoporous metallic gold films as electrodes for biosensing and bioimplant applications," they wrote.

"Taking advantage of mesoporous materials and advanced microlithography, the proposed platform exhibits a low electrochemical impedance, excellent mechanical deformability, and capability for physiological recording verified through an in vivo study on an animal model. The results confirm that our approach offers a new route toward miniaturization, high density of nano-architectured electrodes for implanted in vivo medical devices that can support both fundamental neuroscience investigation and clinical applications," the researchers observed.

Other features of the film highlight new possibilities for future novel applications, they added.