Adam Heller at the University of Texas at Austin has developed an implantable electrode module, the first component of a biofuel cell in which glucose is electro-oxidized at the anode and oxygen is electroreduced at the cathode at neutral pH. The volumetric power density of the cell, including the liquid passing through it, will be around 1mW/cm at the glucose and oxygen concentraions of arterial blood.
The secret to the fuel cell's size and performance is the use of microfibers rather than flat electrodes and the enzyme-based electroactive coatings. This electrode design avoids glucose oxidation at the cathode and O2 reduction at the anode, Heller points out, eliminating the need for an electrode-separating membrane, which is difficult to produce and enclose when small.
The anode coating is glucose oxidase covalently bound to a reducing-potential copolymer that has osmium complexes tethered to its backbone. The cathode coating is similar but contains the enzyme laccase and an oxidizing-potential copolymer. In the coatings, a network of osmium redox centers electrically "wires" the reaction centers of the enzymes to the carbon fibers.
The enabling breakthrough, Heller says, was the group's earlier development of the "wired" laccase cathode that facilitates the four-electron reduction of O2 to water near neutral pH (pH 5) at body temperature (37 °C) [J. Am. Chem. Soc., 123, 5802 (2001)]. Reduction of O2 to water under these conditions has been one of the long-standing problems in electrochemistry, Heller notes. Until now, only noble metal electrodes at pH 0 or carbon electrodes at pH 14 were used for the reduction.
CMOS, biochips to share International Solid-State Circuits Conference bill
In bioelectronics, one remarkable example is a flip-chip combination of a mixed-signal ASIC and an electrode array intended to be implanted in the inner ears of guinea pigs. The ASIC communicates with the outside world and draws power via a wireless interface. It relays signals to the electrode chip, which directly stimulates the animal's auditory nerve. A denser device could potentially be used for hearing-impaired humans. Two papers each will describe DNA recognition chips, retinal-implant ICs and biosignal acquisition chips
Real-world implants are arriving
Electronics engineers, surgeons and medical researchers are tackling a variety of difficult problems in their quest to blend electronics technology with physiology: how to connect to nerve nets and talk to them, how to power implanted electronic components, how to devise low-power neuromorphic circuit design methodologies and how to build sensors, such as artificial retinas, based on them.
During the past 10 years, Cyberkinetics Neurotechnology Systems (Foxborough, Mass.) has built a business out of supplying implantable electrode systems to medical researchers. The company is now planning a series of products for use in clinics. In development is the BrainGate Neural Interface System, designed to give physically disabled people the ability to control computers, robotic arms or environmental controls in a house.
The direction this capability is taking can be seen at Miguel Nicolelis' lab at the Center for Neuroengineering at Duke University (Durham, N.C.). Nicolelis and his colleagues have developed a neurochip with 128 leads that can be implanted in the brain. They are using the system, along with supporting electronics, to investigate how cognitive events are translated into the movement of limbs.
In the near term, electrodes that can be implanted and communicate with the nervous system are being used in products marketed by Medtronic Inc. (Minneapolis). Applications include controlling Parkinson's tremors, alleviating pain and controlling heart rhythms to avoid attacks.
Helping to restore sight and hearing are also high on the list of therapeutic systems that might eventually be implanted in the body. Loss of hearing has been easier to tackle (see main story); an implantable retina is much more problematic. The eye and optic nerves are complex systems and involve sophisticated mechanical feedback. Simply getting a component that can restore even rudimentary vision has been challenging.
The first success in this direction was achieved three years ago by Mark Humayun's group at the Doheny Retina Institute at the University of Southern California. Eye surgeons were able to implant a 4 x 5-mm chip with 16 electrodes that partially restored vision in a blind patient. The procedure has been successfully repeated on five more patients. Humayun's project is part of the cooperative, multipartner Artificial Retina Project, which will extend the work to more-sophisticated artificial retinas.
Other neural-implant research is targeting another frustrating area for medical therapy: paralysis. If distant robotic limbs can be controlled with implantable electrodes, it may also be possible to restore movement to the limbs of patients who have sustained spinal cord injuries.
The problem is on the same order of complexity as the artificial retina, but a number of research groups are tackling it. Dutch neurosurgeon Hans van der Aa has founded an institute called Twin, associated with the University of Twente, to develop such "neuromodulation therapy." The institute, in collaboration with several laboratories in the Netherlands, is developing complex embedded control systems and the related electrode components needed to build effective therapies for paralysis.
Nanotubes integrated with regular array of diamond atoms enable new bio-friendly IC technology
Argonne National Laboratories has found a way to make diamond a conductor as well as an insulator and semiconductor, opening the door to a new era of all-diamond chips.
In general, diamond deposition yields high-performance, long-lasting, radiation-hard dielectric films that can be thin or thick, can be etched alongside silicon components and can be doped either as n- or p-type semiconductors. Diamond's stiffness yields faster resonators, its smoothness yields friction-free microelectromechanical systems and its chemical inertness makes it ideal for bioengineered devices such as human implants.
By adjusting the ultrananocrystalline [diamond deposition] process, the lab's researchers have managed to grow nanotubes between the diamond islands, turning what would ordinarily be a dielectric that insulates as well as silicon dioxide into a conductor that conducts as well as aluminum or copper. [The nanotubes are covalently bonded to the diamond at the nanoscale]
More info on diamond as a nanoscale building material:
Water is a hazard for machines in the micron range. In a tiny device, a molecule or two of water can play havoc with the mechanics. Silicon’s chemical bonds attract water. Diamond, on the other hand, is hydrophobic; it gives water the chemical equivalent of the cold shoulder.
Because it is composed solely of carbon, diamond is suitable for biomedical and electronic applications. The body readily will accept a carbon-based MEMS device or nanomachine because carbon is present in every organic molecule. Its composition also makes diamond a good conductor of electricity.