F. Umbrecht

A wireless implantable passive strain sensor system

A design study of a novel passive strain-sensor technology for the in-situ measurement of small strains on implants, bones or fixation systems is presented. The sensing principle is based on hydro-mechanical strain amplification which allows for the abandonment of any electrical circuits. Thus, the sensor can be fabricated applying solely biocompatible or bioresorbable polymeric materials. Finite element simulations are employed to validate the basic sensing principle and to optimize design parameters according to the required target specifications. Remote wireless and passive signal read-out of the sensor signal can be achieved by advanced ultrasound imaging technologies

Processing and quantitative analysis of biodegradable polymers (PLLA and PCL) thermal bonding

A quantitative analysis of the bond strength and microstructure integrity achieved when bonding the biodegradable polymers poly(L-lactide) (PLLA) and poly(e-caprolactone) (PCL) has been performed using the response surface methodology. The respective influence of the bonding parameters (temperature, pressure, duration) on the bond strength and microchannel integrity was investigated. PLLA and PCL were identified as suitable candidates for packaging materials for bioelectronic circuits of conductive biodegradable polymers. For a future packaging application, the bonding parameters were adapted to optimize the bond strength; the estimated values for the bond strength and channel integrity that were predicted by the surface plots were 2.32 ± 0.26 MPa and 33.7 ± 12.9% for PLLA, and 0.81 ± 0.11 MPa and 50.9 ± 5.7% for PCL. These values were in good agreement with the experimentally determined bond strength of 2.00 ± 1.10 MPa (PLLA) and 0.67 ± 0.22 MPa (PCL) and deformation of 31.4 ± 7.0% (PLLA) and 52.9 ± 4.1% (PCL). Microchannels with an aspect ratio of 1:12.5 were successfully fabricated. The impact of the fabrication process on the PLLA and PCL chemical properties was also investigated through differential scanning calorimetry and gel permeation chromatography measurements. It was observed that the weight average molecular weight Mw decreases after each fabrication step, as much as 68% for PLLA and 59% for PCL. The strongest reduction was observed after the compression molding (above the melting temperature) which should be kept as short as possible. An annealing step allowed increasing the crystallinity and improved the overall polymer stiffness.

Quantitative analysis of biodegradable polymers (PLLA & PCL) thermal bonding

A quantitative analysis of the bond strength and microstructure integrity achieved when bonding the biodegradable polymers poly(L-lactide) (PLLA) and poly(e-caprolactone) (PCL) is realized using the response surface methodology (RSM). For the present packaging application the bonding parameters (temperature, pressure, duration) are adapted to optimize the bond strength; The estimated values for bond strength and channel integrity that are predicted by the surface plots are 2.32±0.26MPa and 33.7±12.9% for PLLA, and 0.81±0.11MPa and 50.9±5.7% for PCL. These values are in good agreement with the experimentally determined bond strength of 2.00±1.10MPa (PLLA) & 0.67±0.22MPa (PCL) and deformation of 31.4±7.0% (PLLA) & 52.9±4.1% (PCL). Microchannels with aspect ratio of 1∶12.5 are also fabricated.

Drahtloser, implantierbarer und passiver Dehnungssensor (Wireless Implantable Passive Strain Sensor)

Wiederholtes Überbelasten orthopädischer Implantate während Rehabilitationsübungen kann zu deren Versagen führen. Um dies zu verhindern, schlagen wir einen neuartigen, implantierbaren Dehnungssensor vor, mit dessen Hilfe Deformationen solcher Implantate drahtlos erfasst werden können. Das Messprinzip basiert auf der Transformation einer Kraft in eine sich ändernde Flüssigkeitsmenge in einem Mikrokanal, welcher in den Sensor integriert ist. Mittels eines speziellen Ultraschallausleseverfahrens soll der Füllgrad des Mikrokanals durch menschliches Gewebe hindurch ermittelt werden. Monitoring the deformation of orthopedic implants during rehabilitation exercises helps preventing their possible failure due to repeated overloads. For this reason we propose a completely new sensor concept, which consists of a passive load sensor and its associated ultrasound-based read-out unit. The measurement principle is based on the transformation of a force into a varying amount of fluid in a micro channel integrated into the sensor. A novel ultrasound read-out method is applied to monitor the fill level of the micro channel.