Medical Device Link.

MICROENGINEERING PROCESSES FOR MEDICAL TECHNOLOGY

Andreas E. Guber

A significant amount of current research in medical technology is focusing on how the functionality of devices and instruments can be improved by microengineering processes and microsystems technology. Today, completely new medical products and instruments are feasible on the basis of these recently elaborated fabrication techniques.

Among other international centers, the Institute for Microstructure Technology at the University of Karlsruhe (Karlsruhe, Germany) has been at the forefront of microengineering research. The Karlsruhe Research Center has participated in the development of two novel microengineering techniques—the LIGA technique and mechanical microengineering¹—and has adapted other techniques, such as laser material processing and microelectrical discharge machining (µEDM).^2,3 The following account presents two typical examples of medical microengineering activities performed at the Institute.

MICROINSTRUMENTS FOR ENDOSCOPIC PROCEDURES

In recent years, the number of endoscope-supported clinical procedures has increased considerably. This applies especially to laparoscopy, whereby operations in the abdominal cavity are carried out in a minimally invasive manner. Current laparoscopic instruments enable several basic surgical functions to be carried out simultaneously, without an exchange of instruments being required. Product developers working on advanced endoscopic designs are equipping the instruments with even more options, while keeping the external dimensions more or less the same. This means that the individual components of an endoscope are being continually scaled down. At the same time, component and overall instrument functionality must be maintained.

Figure 1. Functioning principle of a joint-free micro-instrument made of NiTi. The distal end of the micro-forceps can be deflected continuously in the range of 0° to 40°.

Figure 2. Schematic representation of erosive cutting of the bit components of the microforceps.

The miniaturization of major endoscopic components undertaken at the Institute began with instruments such as forceps and scissors that still possessed mechanical joints.^4,5 Such components normally require considerable space, and constitute a potential contamination risk as regards the reprocessing and sterilization of the instruments after procedures. However, extremely small and joint-free instruments can be generated using novel materials based on various biocompatible alloys of nickel titanium alloys (NiTi).² Because of the superelasticity of NiTi, strains of up to 8% can be achieved in certain temperature ranges under the influence of a deformation force; following relief, these materials return to their initial state.

Figure 3. SEM of the NiTi microforceps holding a human hair. In the closed state, the thickness of the forceps is only 0.63 mm.

Figure 1 shows the setup and functioning principle of a surgical microinstrument using the example of a joint-free microforceps. Closing of the forceps is accomplished by means of a PTFE hose that is moved out over a microstructured NiTi wire. Extending the hose presses the grips together and closes the forceps; retracting the hose causes the forceps to open again. To work with the deflected forceps in a straightforward direction, a rigid deflection pipe has to be moved forwards distally, straightening the instrument. Deflection can be adjusted continuously in a range from 0° to 40°, enabling endoscopic procedures to be carried out at difficult angles or "around the corner."

Figure 4. SEM of deflectable NiTi microscissors. The entire scissors structure is made of a thin NiTi wire.

In order for the microforceps to achieve a simultaneous gripping and deflection function, a suitable bit profile with an adjacent long hole and a deflection part has to be integrated into the NiTi wire. Figure 2 depicts the fabrication of this component, with the forceps profile generated in a 0.63-mm-thick NiTi wire using the µEDM technique and a tungsten cutting wire that is a mere 30-µm thick.² The excess NiTi material is removed by a combination of electrical, thermal, and mechanical processes. The µEDM cutting of the bit configuration and the adjacent long hole can be performed in a single processing step. An SEM of the closed microforceps is illustrated in Figure 3. By precisely rotating the workpiece and varying the cutting profile, microscissors can also be generated using the same NiTi wire material. Figure 4 shows the ready-to-use microscissors with eroded blades in the open form; when closed, the scissors are only 0.63-mm thick at the distal end.

Figure 5. SEM of 250-µm-high brass micropyramids. A total of 10,000 micropyramids can be located in an area of 1 cm².

The microinstruments discussed above can survive approximately 30,000 load cycles without losing their elastic properties before they fracture at the deflection point or bits. Through a combination of different processing techniques, such as µEDM and microlaser welding, other useful microinstruments can be produced from NiTi alloys.

MICROSTRUCTURED X-RAY DETECTORS

Microengineering techniques such as LIGA or mechanical microengineering enable manufacturers to generate a variety of metal molding tool forms for low-cost mass production of plastic microstructure products. As an example, a sheet of metal micropyramids fabricated from brass is shown in Figure 5. The micropyramids are generated in a brass substrate via cutting and crosswise processing using microprofiled processing diamonds. With suitable tools, as many as 10,000 micropyramids, each 250-µm high, can be produced on a 1-cm² substrate at a modular dimension of 100 x 100 µm (smaller modular dimensions can be generated in principle). Currently, the maximum metal surface area that can be processed measures 8 x 10 cm.

Figure 6. SEM of the pyramid depressions obtained by plastic molding. In accordance with the molding tool, each depression is 250-µm deep.

Once the metal microstructures have been formed, they can be transferred to plastic using various plastic molding processes. For the tool shown in Figure 5, the metal pyramids are obtained as inverse depressions in the plastic, as can be seen in the surface of a polyoxymethylene (POM) foil shown in Figure 6. Each of these microstructures is 250-µm deep, with the dimensions of the opening being approximately 95 x 95 µm and the minimum width of the walls approximately 5 µm. Besides POM, suitable polymers for such transfer processes include polymethyl methacrylate, polycarbonate, polyamide, polyvinylidene fluoride, and polysulfone.

Figure 7. Schematic representation of the functioning principle of conventional (left) and microstructured (right) x-ray intensifying screens. By filling the microchambers with phosphor, unsharpness can be reduced.

If the microdepressions in the foil depicted in Figure 6 are filled with phosphor over an area of 2.5 x 6 cm, microstructured x-ray detectors can be produced that offer resolution much higher than that of the intensifying screens commonly used in medical diagnostics.⁶ The functional principles of microstructured x-ray intensifying screens and conventional screens are compared in Figure 7. As a result of the precise filling of the phosphor into discrete microchambers, ultimate image clarity can be considerably enhanced, especially when light transmission from one microchamber to the other is prevented by an opaque layer. Measurements of modular transmission functions demonstrate that values of up to 15 line pairs per millimeter—much larger than values for conventional systems—can be attained by microstructured x-ray intensifying screens. Such components hold great potential for clinical radiation therapy and tumor diagnostics, especially insofar as microstructured detectors and large-area microstructured scintillator surfaces could make exact dose distribution in irradiated body regions or organs easier to achieve.

REFERENCES

1. W Menz and J Mohr, Mikrosystemtechnik Für Ingenieure, (Berlin: VCH-Verlag, 1997.

2. AE Guber et al., "Mikroinstrumente aus Nickel-Titan," F&M—Feinwerktechnik, Mikrotechnik, Mikroelektronik (1997): 247–251.

3. T Haas and A Schüssler, "Laserschweißen von NiTi-Legierungen für Medizinische Anwendungen," Laser Magazin, 1, (1995): 46–50.

4. AE Guber and P Wieneke, "MINOP—Development of a Miniaturized Endoscopic Operation System for Neurosurgery," in SPIE 2676, (Bellingham, WA: SPIE, 1996), 2–13. (The research and development work reported here and in Reference 5 was carried out in part within the framework of the MINOP [Microsystem Technology for Use in Minimally Invasive Neurosurgical Operation Techniques] joint project; grant #MV 0323, funded by the BMBF.)

5. AE Guber and P Wieneke, "Innovative Instruments for Endoscopic Neurosurgery," in Minimally Invasive Techniques for Neurosurgery: Current Status and Future Perspectives, ed. D Hellwig and BL Bauer (Berlin/Heidelberg: Springer-Verlag, 1998), 12–15.

6. AE Guber: "Potential of Microsystems in Medicine," Minimally Invasive Therapy 4, (1995): 267–275.