Synthetic Biomaterials

Another example of tissue engineering is biomaterials or compatible materials constructed of human parts, (e.g., heart valves and arteries). Development and use of these artificial devices will require a thorough understanding of biological functions, toxicity, and other factors. An understanding of surface chemistry and its implications for biological function will be essential to ensure biocompatibility. Recent advances in tailoring and characterizing surfaces at the molecular level are providing insights into how cells and tissues organize at interfaces. Continued development will lead to new methods of integrating biological and synthetic components for the generation of hybrid devices that may facilitate in vivo communications between biological events and electronic devices. Significant research in this area is already under way, and the Army should monitor developments to determine if they can be adapted to soldier applications.

One area of special interest is the transplantation of cell lines to provide an implantable biochemical factory for the production of life-sustaining or performance-enhancing molecules. Among the former are blood-clotting factors and insulin; the latter could include proteins that influence responses to fear, sleep deprivation, and fatigue. The synthetic component of these constructs is in the immunoisolating external shield, which is designed to protect them against the rejection response. Private industry is investing heavily in immunoisolating technologies (e.g., Neocrine, Inc.) thus providing the Army with opportunities for leveraging private investments.

Bridges Between Electronics and the Nervous System

For more than three decades, researchers have been exploring the coupling of electronics and the nervous system at the cellular level, both as a means of understanding neural functions and as a means of developing prostheses to mitigate, or even “cure,” a number of debilitating neural disorders (Agnew and McCreery, 1990). Considerable work is under way to develop retinal and cortical implants to restore vision to the blind, to restore movement to paralyzed limbs via functional neuromuscular stimulation, and to restore hearing to the profoundly deaf. Of these, the hearing prosthesis is the most completely developed; more than 30,000 first-generation devices have been implanted worldwide. Success rates with these implants continue to increase, enabling many patients to function normally in a hearing world. Dramatic results are expected in neural prostheses in the next decade.

As the risks and costs associated with neural implants are reduced, they may be used to increase the visual and hearing acuity of unimpaired individuals to levels well above average. Soldiers possessing these extraordinary faculties would be well suited to gathering intelligence and performing longrange reconnaissance missions. Success for some types of implants will depend on resolving biocompatibility issues. (See section Implants and Biocompatibility in Chapter 7.)

Most current neural systems use distributed arrays of individual wire electrodes (25–50µm in diameter). Future systems with higher levels of capability and more sites are likely to use micromachined electrode arrays (see Figure 5-3) (Najafi et al., 1985). After 30 years of research, three-dimensional arrays can now be formed with as many as 1,024 stimulation or recording sites spaced on 100µm–400µm centers (Bai and Wise, 2000; Rousche and Norman, 1998). With embedded circuitry, electrode sites can be positioned electronically to couple with active neurons (Ji and Wise, 1992),