Abstract
Introduction
An increasing trend in the incidence of primary and revision bone replacements has been observed throughout the last decades, mainly among patients under 65 years old.10-year revision rates are estimated in the 5–20% range, mainly due to peri-implant bone loss. Recent advances allow the design of implants with custom-made geometries, nanometer-scale textured surfaces and multi-material structures. Technology also includes (bio)chemical modifications of the implants' surfaces. However, these approaches present significant drawbacks, as their therapeutic actuations are unable to: (1) perform long-term release of bioactive substances, namely after surgery; (2) deliver personalized stimuli to target bone regions and according to bone-implant integration states.
The Innovative Concept
Here we propose the design of instrumented active implants with ability to deliver personalized biophysical stimuli, controlled by clinicians, to target regions in the bone-implant interface throughout the patients' lifetime. The idea is to design bone implants embedding actuators, osseointegration sensors, wireless communication and self-powering systems. This work proposes an advanced therapeutic actuator for personalized bone stimulation, and a self-powering system to electrically supply these advanced implants.
Novel Capacitive Stimulators and Self-Powering Systems
A novel circular capacitive stimulator was designed for personalized stimulatory therapies based on the delivery of electric fields to bone cells. Its architecture is composed by 3 coplanar electrodes, 2 mm wide, 1 mm thick, and 0.5 mm apart from each other. It enables the delivery of controllable stimuli, as different stimuli (varying waveform, strength, frequency, etc.) can be delivered to target regions of bone. Numerical biophysical models were developed using COMSOL Multiphysics (v. 5.2) to analyze the osteogenic effects of stimulation delivered in vitro to MC3T3-E1 bone cells. 8 domains (electrodes, petri dish, substrate, air, cellular medium and physiological medium) were considered to simulate an apparatus to stimulate cell cultures. Simulations were carried out by applying low and high frequency (14 Hz and 60 kHz) sinusoidal excitations with 10 V of amplitude.
A motion-driven and maintenance-free self-powering system was designed using magnetic levitation-based electromagnetic energy harvesting. A semi-analytical non-linear mathematical model of its complex energy transduction was developed (it includes modelling of the magnetic field produced by levitating hard magnetic elements, repulsive force between two magnets, electrical and mechanical damping, induced voltage, mechanical and electric dynamics) to estimate the energy harvested during gait patterns.
Results
This cosurface stimulator is able to deliver similar magnitude stimuli to bone cells as those already recognized as osteogenic by previous studies. Heterogeneous stimuli is delivered both for low and high excitations, although quite different stimuli distributions are found along the cellular layer. Maximum stimuli occur over the electrode-anode region and its magnitude is approximately 0.3 V/mm. The electrode thickness influence must also be highlighted: the use of electrodes with 0.1 mm thick result in 2.5-fold magnitude increases in high-frequency stimulation.
Excellent agreement was obtained between simulations and experiments with mean energy errors around 6% and cross-correlations higher than 85%. These results indicate that the design of this self-powering system can be optimized prior to fabrication and according to gait patterns of patients.
