Clémentine M. Boutry

Towards biodegradable wireless implants

A new generation of partially or even fully biodegradable implants is emerging. The idea of using temporary devices is to avoid a second surgery to remove the implant after its period of use, thereby improving considerably the patients comfort and safety. This paper provides a state-of-the-art overview and an experimental section that describes the key technological challenges for making biodegradable devices. The general considerations for the design and synthesis of biodegradable components are illustrated with radiofrequency-driven resistor–inductor–capacitor (RLC) resonators made of biodegradable metals (Mg, Mg alloy, Fe, Fe alloys) and biodegradable conductive polymer composites (polycaprolactone–polypyrrole, polylactide–polypyrrole). Two concepts for partially/fully biodegradable wireless implants are discussed, the ultimate goal being to obtain a fully biodegradable sensor for in vivo sensing.

Junctions between metals and blends of conducting and biodegradable polymers (PLLA-PPy and PCL-PPy)

The junctions between newly developed biodegradable conducting polymers (polylactide-polypyrrole PLLA-PPy and polycaprolactone-polypyrrole PCL-PPy) and metal electrodes (Au, Au/Cu, Ag, Ag/Cu, Cu, Cr/Au/Cu, Pd/Au/Cu, Pt/Au/Cu) were studied. The objective was to determine the composite/metal combination having the lowest possible contact resistance and ohmic characteristics. In a first step, different surface treatments, adhesion and metal layers were tested in order to evaluate the contact resistance. Then the current-voltage (IV) characteristics were measured and both ohmic and rectifying behaviour were observed depending on the polymer/metal junctions investigated. The surface treatments studied included an argon sputtering step and a grinding of the polymer surface with the objective of improving the contact between the metal electrode and the polymer. It was found that the most favourable conditions resulted from a process flow without argon sputtering, without grinding for PLLA-PPy and with a slight grinding for PCL-PPy. Moreover the most favourable metal electrodes for PLLA-PPy were Pd/Au/Cu, while the best compromise for PCL-PPy was to use Au/Cu. For the rectifying polymer/metal junctions, the standard thermionic emission model modified with a series resistance was successfully applied to the measured current-voltage IV characteristics. The saturation current density J0, series resistance R, ideality diode factor n and barrier height φB were investigated. The Chot functions were computed for each rectifying junction and the corresponding threshold voltages were calculated. Finally the conductivity of both composites was evaluated as a function of temperature in the range of 30 °C to 80 °C. For PLLA-PPy a decrease of the resistivity was observed when the temperature was increasing, while no clearly recognisable pattern was identified for PCL-PPy in this temperature range. The electrical conductivity of the PLLA-PPy samples was found to follow the empirical Arrhenius model, and the difference between the Fermi energy EF and the mobility edge EC | EF - EC | as well as the conductivity at the mobility edge σC were evaluated. Moreover the electrical conductivity of the PLLA-PPy samples was found to follow the Mott variable range hopping (VRH) model, and the high temperature limit of conductivity σ1 as well as the Mott characteristic temperature T1 were calculated.

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.

RF conductivity of biodegradable conductive polymers used for a new generation of partially/fully resorbable wireless implantable sensors

RLC resonators are used for wireless power/data transmission in short-range telemetry (inductive link). The biodegradable conductive polymer composites PLLA-PPy and PCL-PPy are used to fabricate RLC resonators, with the ultimate goal of making a fully biodegradable implant for in vivo operation. Modeling the RF conductivity of PLLA-PPy and PCL-PPy as a function of frequency is required to correctly design the electrical circuits. In this paper, two models (the empirical Jonschers model and Papathanassious model), based on DC and RF measurements, are used to predict the conductivity of the polymer composites at several frequencies of interest for telemetry operation.

Development and characterization of biodegradable conductive polymers for the next generation of RF bio-resonators

The objective of this research is to develop a completely polymeric and biodegradable RF driven RLC resonator circuit. New polymer composites are fabricated and characterized: they consist on conductive polymer nanoparticles (polypyrrole PPy) embedded in a biodegradable polymer matrix (both polylactide PLLA and polycaprolactone PCL are under investigation). The influence of PPy content and polymerization conditions (temperature, atmosphere, additional doping agent) on the resistivity are evaluated. A strong decrease of the resistivity is observed for composites containing more than 12% and 6% of PPy for PLLA/PPy and PCL/PPy, respectively. Resistivities of 0.0043Ω.m (PLLA/PPy39%) and 0.0016Ω.m (PCL/PPy39%) are achieved. A Matlab modelling and HFSS simulation of the RLC resonator performances based on the measured material properties is performed. The simulation results validate the use of these composites to successfully fabricate RLC resonators.

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.