An Implantable Electrical Stimulation Probe with Bendable Split Anchors via Residual Stress

• Main Authors: Yu-Yen Chen, 陳宥延
• Other Authors: 施文彬
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/55347043609989574583

碩士 === 國立臺灣大學 === 機械工程學研究所 === 102 === Literature statistics showed that the chance for a person to suffer from low back pain could be up to 54%~80%, and the annual cost on low back pain treatment has 7% annual growth since 1990. These statistics indicate that the needs for low back pain treatment have been increasing. Low back pain can be effectively relieved by pharmacological treatment. However, this type of treatment may cause lots of side effects. Thus, researchers have been seeking other ways for low back pain relief. Neural probes have been widely used for recording and stimulating the specific sites of brains through electrical signal. Studies showed that electrical stimulation on specific neural tissues can evoke different reactions. For clinical low back pain treatment, the target nerve is treated with one-time stimulation, whose therapeutic effect is not permanent. Patients will need the stimulation surgery every 3 to 6 months to suppress the pain. Thus, implantable stimulation systems have been developed for different long-term neural electrical stimulation treatments, such as for Parkinson’s disease and sciatica. However, since implanting surgeries are often invasive, many minimally invasive surgical procedures are conducted nowadays, in order to reduce the size of incisions. This paper presents a bipolar electrode probe used for implantable nerve stimulation treatments in minimally invasive surgeries. The probe is composed of a flexible printed circuit substrate and a fabricated SU-8 layer. This probe features a tweezer-like mechanism caused by residual stress in SU-8 film, which will fix the probe to the target nerve. There are stripes on the SU-8 film so that the residual stress nets in a single direction and forms a curve. In addition, a film of gelatin nanofibers, produced via electrospinning, covers the fixed ends of the probe anchors for cell growth to ensure long-term fixation in the body. In the experiment of the cell culture, the gelatin fibrous membrane is proven to have good biocompatibility and mechanical properties for cell culture. Adhesion and growth of 3T3 fibroblast cells on the membrane is effective. Thus, this cell scaffold attached on the probe should reach expected efficacy. A modified Stoney’s formula is established for estimating residual stress in the SU-8 film. The SU-8 thickness was measured with a scanning electron microscopy (SEM); the radius of curve was computed by trigonometry, and the Young’s moduli of SU-8 and flexible printed circuit (FPC) were determined by a nanoindenter. The thicknesses and radii are variable, while the Young’s moduli for FPC and SU-8 were mostly constant at 1.37 GPa and 4.7-5.2 GPa, respectively. With the revised Stoney’s formula, the residual stress could then be calculated. The results of experiment show that there are no obvious relationship between the thickness of the film and the radius of curvature. The residual stress of the SU-8 film is affected by the film thickness. Thus, the deformation of bendable split anchor in the probe cannot be controlled by SU-8 film thickness, but the residual of the SU-8 film increase with increasing film thickness. In the experiment of holding strength test, the breaking force is dominated by the deformation of the bendable split anchors in the probe. The deformation of the bendable split anchor has an optimum holding strength. The experiment results show that breaking forces of the curved probe are higher than plane probe. Breaking force of the plane probe is 0.299 N, and normalized breaking force is 0.198. The probes with the curvature heights of 10.29 mm and 14.24 mm have the breaking forces of 0.368 N and 0.398 N, respectively; the normalized breaking force is 0.24 and 0.26, which are 22.7% and 31.3% higher than that of the plane probe, respectively. The probes with curvature heights between 13 mm and 14 mm have maximum average breaking force about 0.258. In the implanting process, the impedance of the probe should be measured using low-frequency signal in order to determine whether the probe touches the target nerve.