Nuno M. Silva

Instrumented hip implants: Electric supply systems

Instrumented hip implants were proposed as a method to monitor and predict the biomechanical and thermal environment surrounding such implants. Nowadays, they are being developed as active implants with the ability to prevent failures by loosening. The generation of electric energy to power active mechanisms of instrumented hip implants remains a question. Instrumented implants cannot be implemented without effective electric power systems. This paper surveys the power supply systems of seventeen implant architectures already implanted in-vivo, namely from instrumented hip joint replacements and instrumented fracture stabilizers. Only inductive power links and batteries were used in-vivo to power the implants. The energy harvesting systems, which were already designed to power instrumented hip implants, were also analyzed focusing their potential to overcome the disadvantages of both inductive-based and battery-based power supply systems. From comparative and critical analyses of the methods to power instrumented implants, one can conclude that: inductive powering and batteries constrain the full operation of instrumented implants; motion-driven electromagnetic energy harvesting is a promising method to power instrumented passive and active hip implants.

Instrumented hip joint replacements, femoral replacements and femoral fracture stabilizers

This paper reviews instrumented hip joint replacements, instrumented femoral replacements and instrumented femoral fracture stabilizers. Examination of the evolution of such implants was carried out, including the detailed analysis of 16 architectures, designed by 8 research teams and implanted in 32 patients. Their power supply, measurement, communication, processing and actuation systems were reviewed, as were the tests carried out to evaluate their performance and safety. These instrumented implants were only designed to measure biomechanical and thermodynamic quantities in vivo, in order to use such data to conduct research projects and optimize rehabilitation processes. The most promising trend is to minimize aseptic loosening and/or infection following hip or femoral replacements or femoral stabilization procedures by using therapeutic actuators inside instrumented implants to apply controlled stimuli in the bone–implant interface.

DESIGN METHODOLOGY FOR THE DEVELOPMENT OF LONG-TERM HIP PROSTHESIS SURVIVAL

More than 80% of failures following Total Hip Arthroplasty (THR) procedures are due to implant loosening, which is strongly related with weak osseointegration. Current instrumented prostheses were designed to store only physiologic data [Damm, 2010; Heinlein, 2009]. The design of failure detection systems for hip implants is been performed [Marschner, 2009; Alpuim., 2008]. The current methodology to optimize such implants collects real-time data from the internal environment of the implant and uses the information for the research of new designs, materials and surgical techniques [Stansfield, 2003; Dayton, 2005]. However, proposals that ensure long-term implant survival have not been reported yet. This paper proposes a new methodology to avoid irreversible aseptic loosening, which may prevent to a certain extent revision surgical procedures. Through remotely controlled and monitored osteoblast mechanical micro-stimulation, real-time supervision of the osteointegration process may be ensured through tools for telemedicine.

Multi-source Harvesting Systems for Electric Energy Generation on Smart Hip Prostheses

The development of smart orthopaedic implants is being considered as an effective solution to ensure their everlasting life span. The availability of electric power to supply active mechanisms of smart prostheses has remained a critical problem. This paper reports the first implementation of a new concept of energy harvesting systems applied to hip prostheses: the multi-source generation of electric energy. The reliability of the power supply mechanisms is strongly increased with the application of this new concept. Three vibration-based harvesters, operating in true parallel to harvest energy during human gait, were implemented on a Metabloc TM hip prosthesis to validate the concept. They were designed to use the angular movements on the flexion-extension, abduction-adduction and inward-outward rotation axes, over the femoral component, to generate electric power. The performance of each generator was tested for different amplitudes and frequencies of operation. Electric power up to 55 μJ/s was harvested. The overall function of smart hip prostheses can remain performing even if two of the generators get damaged. Furthermore, they are safe and autonomous throughout the life span of the implant.

Mathematical modelling of cylindrical electromagnetic vibration energy harvesters

In this paper the first steps for the derivation of a mathematical model to describe the mechanical behaviour of a cylindrical electromagnetic vibration energy harvester, designed to extract energy from human gait to power biomedical implantable devices, are provided. As it is usual, in the modelling of such devices, the proposed mechanical model is also based on the solution of Newtons second law, but here a nonlinear closed-form expression is used for the resulting magnetic force of the system, unlike what has been done in previous works where, traditionally, that expression is a linear or is a nonlinear approximation of the real one. The main feature of this mechanical model is that it depends on several parameters which are related to the main characteristics of this kind of devices, which constitutes a major advantage with respect to the usual models available in the literature since these characteristics can always be changed in order to optimize the device.

Numerical Simulation of Inertial Energy Harvesters using Magnets

Vibrational energy harvesters for powering wearable electronics and other electrical energy demanding devices are among the most used approaches. Devices that use magnetic forces to maintain the central mass in magnetic levitation, aligned with several coils as the emf generating transducer mechanism, are becoming a suitable choice since they do not need the usual spring that typically degrades over time. Modeling such energy harvesters poses different challenges due to the difficulty of getting the nonlinear closed-form expression that would describe the resulting magnetic force of the entire system. In this paper, modeling of the magnetic forces resulting from the system magnets interaction is presented. Results give valuable data about how the best energy harvester should be designed taking into account resonance frequency related to system’s mass and dimensions.