Future Prospects and Conclusions

Previously, biosensors were successfully used only in laboratory analysis, but today, the need of biosensors for in situ diagnosis is growing rapidly. Especially, because of the high selectivity, sensitivity, and generally good biocompatibility, DNA-based biosensors have a bright future for surgical applications. Along this route, multifunctional materials and composite structures are the future for DNA-based biosensors. An ideal biosensor should not only complete its intended function (recognition and detection), but it also needs to possess other benefits (such as drug release, mechanical support, gene therapy, biocompatibility, etc.) (Menard-Moyon et al. 2010; Ainslie and Desai 2008; Wadhwa et al. 2006).

Take a currently developed bone sensor for example. An implantable 9-channel implant/biosensor system was studied in vivo for load measurements for orthopedic implants (Graichen et al. 2007). Briefly in the experiment, a Ti alloy (Ti4Al6V) was used for the neck and stem, cobalt-chromium-molybdenum (CoCrMo) for the tightly fitted head, and polyether ether ketone (PEEK) for the cap at the lower end of the stem (Fig. 14). Six strain gauges and the transmitter circuit were arranged inside the neck cavity with an inner diameter of 9.5 mm. The secondary power coil (30 mm length, 6 mm diameter) was combined with the AC-DC circuit and fixed

Fig. 14 Instrumented shoulder endoprosthesis (left) and a cut model with measurement electronics (right) (adapted from Graichen et al. (2007))

in a second cavity at the lower end of the stem. The rectified power supply voltages were connected to the transmitter circuit by two cables through a drill. Another pair of cables connected the two-lead feed through at the center of the lower end plate to the transmitter. The antenna loop for signal transmission consisted of a niobium wire. The wire was electron-beam welded and protected against mechanical damage by a PEEK cap, and the upper and lower endplates of the implant were also welded. This is quite a novel design to incorporate a biosensor into a normal orthopedic implant. A significant advantage of this composite system is that a sensor can be located inside the patient's body. Thus, "real time" measuring of implant performance could be accomplished by the sensor. Compared with traditional methods, like radiographs (X-rays), such biosensor system results may detect implant failures earlier and more quickly. It would also lead to less radiation to patients and a more accurate diagnosis for surgeons.

In the future, more functions can be added into such systems. For example, RNTs can be incorporated onto the implants to improve biocompatibility, while TFO can be immobilized onto the implant surface to recognize certain biomarkers and analyze cell differentiation, functions, and apoptosis to predict the outcome of the implant before clinical symptoms surface. Furthermore, controlled drug release mechanisms could also be included. A conductive polymer, like polypyrrole, can be coated on the implants and embedded with drugs. With electrical stimulation, molecular chains of polypyrrole break and can release the embedded drugs (Ainslie and Desai 2008; Wadhwa et al. 2006). Notably, by controlling the electrical current in the implant, the drug release rate is adjustable. In this manner, the diagnosis and treatment of the implant can be achieved simultaneously through such novel DNA-based nanotechnology biosensors.

In conclusion, DNA or DNA-derived materials are very promising for biosensor applications to improve their biocompatibility, selectivity, and sensitivity of the sensors, and combining with other functions (such as drug release and gene therapy) to provide earlier, real-time diagnosis and treatment.

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