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Imagine implants that detect cancer and deliver the appropriate drugs. Or read the brain's electrical signals and control motor functions affected by diseases like epilepsy and multiple sclerosis. Or restore sight to the blind. Actually, you don't have to imagine. These devices, if they're not here yet, are in advanced stages of testing. And they're taking advantage of improvements in MEMS sensors and wireless communications.

Hope For Heart Patients
Less than a year ago, the Food and Drug Administration gave Abiomed Inc. a Humanitarian Device Exemption for the AbioCor implantable replacement heart (Fig. 1). One of the most sophisticated medical implantable devices ever developed, the AbioCor can extend the lives of patients who would otherwise die of heart failure. The self-contained unit is the world's only artificial heart that operates by remote control, without the need for external wires or tubes, via an internal miniaturized electronics package and an external battery pack.

The two-pound thoracic unit (replacement heart) includes two artificial ventricles with their corresponding valves and a motor-driven hydraulic pumping system. The implantable electronics package monitors and controls the pumping speed of the heart based on the physiologic needs of the patient. The AbioCor operates on both internal and external lithium batteries.

The internally implanted battery is continually recharged from an external console or from a basic patient-carried external battery pack. This is achieved via transcutaneous energy transmission (TET). The TET system consists of internal and external coils that transmit power across the skin. External battery packs can power the AbioCor for approximately four hours.

Morbidly ill heart patients who need an immediate transplant can benefit from the CardioWest temporary total artificial heart (TAH-t) from SynCardia Systems (Fig. 2). A modern version of the Jarvik 7 artificical heart first implanted in 1982, it temporarily replaces the diseased heart until a donor is found. The TAH-t pumps more blood (up to 9.5 liters/minute) than any ventricular-assisted device, helping patients regain their strength.

The Eyes And Ears Have It
Researchers have been attempting to see if visual perception could be triggered by electrical stimulation of the retina. Germany-based Intelligent Medical Implants came up with a system for patients suffering from retinitis pigmentosa (RP). The Learning Retinal Implant consists of a retinal stimulator, a pair of spectacles with an integrated camera and transmitter for wireless transmission of signals and energy, and a photoprocessor worn at the patient's waist (Fig. 3). This device is still in clinical trials.

Second Sight Medical Products and the Doheny Eye Institute of the University of Southern California are developing the Argus II retinal prosthesis for RP patients. It consists of a camera and a microprocessor mounted in eyeglasses, a receiver implanted behind the ear, and an electrodestudded array that's attached to the retina.

The microprocessor sends images from the camera to the receiver. Using a tiny cable, the receiver transmits a signal to the array, which then emits pulses that travel along the optic nerve to the brain. The brain perceives patterns of light and darkness corresponding to the stimulated electrodes.

Last year, the National Eye Institute of the U.S. National Institutes of Health (NIH) awarded a $6.5M grant to a host of organizations to develop the National Center for the Design of Biometric Nanoconductors, located at the University of Illinois in Urbana-Champaign.

The center will design, model, synthesize, and fabricate medical devices based on natural and synthetic ion transporters—proteins that control ion motion across the membranes of living cells. The first task is the development of a nanobattery to power an implantable artificial retina (Fig. 4). Sandia National Laboratories is taking the lead in the project's theoretical and computational tasks.

The University of California at Berkeley and Lawrence Berkeley National Laboratory (LBNL) are working together to create light-sensitive switches that can be flipped on and off as easily as a television's remote control in the body's cells. The optical switches work by triggering a chemical reaction, initiating a muscle contraction, and activating a drug or stimulating a nerve cell—all at a flash of light (Fig. 5).

A major goal of the UC Berkeley-LBNL Nanomedicine Development Center is to equip cells of the retina with photoswitches. This would ideally enable blind nerve cells to see, restoring light sensitivity in people with diseases like macular degeneration.

Aural artificial implants are available for patients with moderate to severe hearing loss. In fact, the FDA has approved several devices based on pioneering work by the University of Michigan. Devices already on the market include the Nucleus Freedom from Cochlear Corp., the HR90k from Advanced Bionics Corp. (a Boston Scientific company), and the Pulsar CI100 from MED-EL Corp.

Envoy Medical's Envoy implant employs two bio-compatible piezo transducers. One is a microphone that's attached to the ear's incus and malleus bones and receives sound from the natural eardrum. The other, which is attached to the ear's stapes, receives signals from an implanted processor and stimulates the inner ear.

According to the company, it's the only implantable hearing device that leverages the natural anatomical function of the ear.

It's powered by a lithium-iodide battery with a life of three to five years, and it's currently in the final phase of testing at the St. Croix Medical Center.

The Carina hearing implant from Otologics Inc. is also undergoing final-phase testing before FDA approval. Its subdermal microphone feeds signals to an implanted processor, which drives a transducer affixed to the incus and then to the stapes. It operates from a lithium-ion (Li-ion) battery that's recharged by the patient from a belt-worn device, using transdermal inductive coupling for power delivery.

Neural Implants Are Next
Neural implants will be key as researchers learn more about the nervous system. The neurons that make up the nervous system act as pulse-rate-modulated logic elements, forming complex 3D networks that use sensory and physiological functions.

"There is great hope that neural prostheses will help in overcoming diseases and ailments like Parkinson's disease, epilepsy, paralysis, deafness, and blindness," says Kensall Wise, director of the University of Michigan's Engineering Research Center for Wireless Integrated MicroSystems.

Medtronic is working on an implantable "brain radio" system to monitor and control nervous disorders. It's part of a broad category of neural stimulators that send electrical pulses to the brain to control ailments like Parkinson's disease, Alzheimer's disease, epilepsy, and multiple sclerosis. Medtronic's engineers tackled key design issues like noise sensitivity and very low power dissipation by developing a chopper-stabilized amplifier with a noise efficiency factor of 3.6 to 4.5 to handle "popcorn" noise from silicon digital and mixed-signal circuits, as well as a 1.6-V battery that delivers 1.5 µA.

Work on a different kind of neural implant, called "BrainGate," is under way at Brown University. It will enable the mind to control movement—an incredible advantage for the paralyzed. Developed in conjunction with Cyber Kinetics Neurotechnology Systems (founded by professor John Donoghue, head of Brown University's Department of Neuroscience), so far it's allowed one patient to read e-mails, play video games, turn lights on and off, and change and adjust TV controls just by thinking about these actions (Fig. 6).

Epilepsy patients who must rely on vagus nerve stimulators (VNSs) or defibrillators because their seizures don't respond to drugs can look forward to better treatment options. The Massachusetts Institute of Technology (MIT) and the Beth Israel Deaconess Medical Center are creating new software to determine when best to activate the VNS—in this case, a model from Cyberonics Inc. They're developing an implantable detector that measures brain activity to predict when a seizure is about to occur, activating the VNS and halting the seizure before it occurs.

The University of California at Los Angeles (UCLA) David Giffen School of Medicine is creating an implantable trigeminal nerve stimulator (TNS) with Advanced Bionics Corp. According to the researchers, this unique device holds several advantages over VNSs, which only stimulate one side of the brain. A TNS can stimulate both sides.

And unlike the VNS, the TNS can be tested externally to gauge its effectiveness before implantation. Before implanting, the TNS can be worn on a patient's belt with wires attached to the stimulator, passed under clothing, and connected to electrodes attached to the face.

An Exciting Future
Many more implants loom on the horizon. It's come to the point where designers no longer ask if they can be developed, but when. For example, In-Cube Inc. is focusing on tissue engineering. In these hybrid devices, living cells from the patient are integrated with traditionally engineered systems. Researchers there believe this will lead to artificial kidneys, pancreases, lungs, and colons.

In-Cube is pursuing an implantable kidney dialysis system that incorporates implantable polymers and electronics. The patient's own cells perform the dialysis. According to Mir Imran, founder of In-Cube, the implantable kidney is a complex project that's in the bench-testing phase.

The IntelliDrug project for the European Commission's 6th Framework Programme sheds some light on the growing excitement surrounding biomedical implants. The program's aim is to develop electronically controlled intra-oral drug-delivery systems. These remote-controlled systems with replaceable reservoirs would become alternative sources of treatment for addictions and chronic diseases.

Furthermore, nanoparticle implants will monitor the growth and treatment of cancer tumors. Such is the charge of researchers at MIT. They are building implantable nano-particles that can detect specific molecules or analytes like glucose and oxygen, which are associated with tumor growth. The implants are encased in silicone, allowing them to remain in a patient's body for an extended period. They detect tumor growth, show how much of the drug has reached the tumor, and reveal the drug's effectiveness.