(Nanowerk Spotlight) Thanks to nanotechnology, medical research is moving quickly towards a future where intelligent medical implants can continuously monitor their condition inside the body and autonomously respond to changes such as infection by releasing anti-inflammatory agents. Examples that researchers are already working on today are a sensor that can be placed under the skin for measuring the blood glucose level; the use of polypyrrole films as electrically controlled drug release devices on implant surfaces to improve bone implants; an electronic under-the-skin sensor to monitor blood flow; or pressure sensors to improve medical implants.
The two authors, Evan Wujcik, an assistant professor at Lamar University, and Chelsea Monty, an assistant professor at the University of Akron, describe various applications with an in-depth look at the implantable sensors from the viewpoints of nanomedicine, materials science, nanobiotechnology, and sensor design, both present and future. Here is a brief summary:
Multi-walled carbon nanotube (MWCNT) applications for implantable sensors
MWCNTs have shown great promise in orthopedic implant systems, for their scaffolding abilities as well as monitoring bone growth via electrochemical sensing techniques. Greater osteoblast differentiation has been shown previously using a MWCNT-Ti composite, in comparison to titanium (Ti) alone. The authors predict that using the MWCNT-Ti electrode to sense
osteoblast extracellular components may improve the diagnosis of in vivo orthopedic implant success or failure, leading to improved clinical efficacy.
Single-walled carbon nanotube (SWCNT) applications for implantable sensors
A remarkable area in which SWCNTs have shown great promise as implantable sensors is the imaging and detection of single molecules by: (1 their intrinsic bright NIR fluorescence, (2 the fact that the bandgap structure is sensitive to their local dielectric environment, and (3 the quenching capability of the CNTs.
(a) Graph showing the concentration-dependent fluorescence responses of the DNA-encapsulated single-walled carbon nanotube to divalent chloride counterions and (b) an illustration on DNA undergoing a conformational transition from the B form (top) to the Z form (bottom) on a SWCNT. (c) The mechanism by which a SWCNT acts as a molecular beacon nanoquencher. (Copyright 2006 American Association for the Advancement of Science; Copyright 2008 American Chemical Society)
SWCNTs also have a number of useful properties that makes them suitable for other applications, such as implantable SWCNT bone strain sensors, which have recently been developed for the real-time monitoring of musculoskeletal conditions, to aid in rehabilitation and advanced orthopedic operations. Neuronal interfacing is another area in which SWCNTs have become of interest for the monitoring and stimulation of neurons, as well as potential neuroprosthetic devices.
Graphene and graphene oxide applications for implantable sensors
To date, pristine graphene has found applications to implantable sensor technologies as a glucose detection system; neural stimulator and signal recorder; antigen detection system; chemiresistor; and biological molecule sensor. Researchers have also shown the development of a graphene-based microelectrode for recording neural signals.
Graphene oxide (GO) is very similar to graphene, but differs in structure slightly. However, these structural differences greatly affect its material and electronic properties. GO has been found to have a number of biosensing applications including fluorescent d-glucosamine detection; immunosensing; endonuclease/methyltransferase activity assays; enzyme inhibition and modulation; delivery vehicles; and mycotoxin detection. Silver nanoparticle-modified GO has also been found to have enhanced antibacterial activity against Escherichia coli, over that of both GO and silver nanoparticles alone, which could have applications in implantable tissue scaffolds or wound dressings.
Other allotropes of nanostructures carbon in implantable devices
Graphene foam – a 3D microporous network of monolayered to few-layered GS templated foam – has shown some unique properties of its own applicable to implantable sensor technologies. This includes both a static and dynamic hydrophobic contact angle in a GF/water/air system, as well as a superhydrophobic advancing liquid contact angle?when coated in Teflon. A material such as this could find applications in self-sterilizing or anticorrosive flexible implants, or low-friction microfluidic implantable sensors.
Fullerenes (C60) have been found to be highly electrochemically active because of the formation of sp3-like anionic centers and the fact that they have been found to undergo as many as six one-electron reversible reduction reactions. These lipophilic and relatively inert nanostructures have been modified with a number of inorganic, organic, and organometallic moieties, for a number of applications in medicine, electronics, optoelectronics, and – recently?biosensing. Some of these suggested applications include their use as redox charge transfer mediators; monitoring of chemical and biological species in supported lipid bilayer membranes; antiviral activity; and nuclear magnetic resonance imaging contrast agents.
Concluding their review, the authors mention the debate regarding the nanotoxicity of carbon nanomaterials and the conflicting evidence found in the literature. They also point out that new methods of synthesis and purification of carbon nanomaterials are another hurdle in producing cost-effective and affordable implantable sensors that rival current technologies.