Multiscale Sensing of Bone-Implant Loosening for Multifunctional Smart Bone Implants: Using Capacitive Technologies for Precision Controllability

A millimetre-scale capacitive biosensing and biophysical stimulation system for emerging bioelectronic bone implants

Bioelectronic bone implants are being widely recognized as a promising technology for highly personalized bone/implant interface sensing and biophysical therapeutic stimulation. Such bioelectronic devices are based on an innovative concept with the ability to be applied to a wide range of implants, including in fixation and prosthetic systems. Recently, biointerface sensing using capacitive patterns was proposed to overcome the limitations of standard imaging technologies and other non-imaging technologies; moreover, electric stimulation using capacitive patterns was proposed to overcome the limitations of non-instrumented implants. We here provide an innovative low-power miniaturized electronic system with ability to provide both therapeutic stimulation and bone/implant interface monitoring using network-architectured capacitive interdigitated patterns. It comprises five modules: sensing, electric stimulation, processing, communication and power management. This technology was validated using in vitro tests: concerning the sensing system, its ability to detect biointerface changes ranging from tiny to severe bone-implant interface changes in target regions was validated; concerning the stimulation system, its ability to significantly enhance bone cells' full differentiation, including matrix maturation and mineralization, was also confirmed. This work provides an impactful contribution and paves the way for the development of the new generation of orthopaedic biodevices.

The application of impantable sensors in the musculoskeletal system: a review

As the population ages and the incidence of traumatic events rises, there is a growing trend toward the implantation of devices to replace damaged or degenerated tissues in the body. In orthopedic applications, some implants are equipped with sensors to measure internal data and monitor the status of the implant. In recent years, several multi-functional implants have been developed that the clinician can externally control using a smart device. Experts anticipate that these versatile implants could pave the way for the next-generation of technological advancements. This paper provides an introduction to implantable sensors and is structured into three parts. The first section categorizes existing implantable sensors based on their working principles and provides detailed illustrations with examples. The second section introduces the most common materials used in implantable sensors, divided into rigid and flexible materials according to their properties. The third section is the focal point of this article, with implantable orthopedic sensors being classified as joint, spine, or fracture, based on different practical scenarios. The aim of this review is to introduce various implantable orthopedic sensors, compare their different characteristics, and outline the future direction of their development and application.

Sensor Technology in Fracture Healing

Introduction

SMART sensor technology may provide the solution to bridge the gap between the current radiographic determination of fracture healing and clinical assessment. The displacement and rigidity between the fracture ends can be accurately measured using strain gauges. Progressively increasing stiffness is a sign of fracture consolidation which can be monitored using sensors. The design of standard orthopaedic implants can remain the same and needs no major modifications as the sensor can be mounted onto the implant without occupying much space. Data regarding various fracture morphologies and their strain levels throughout the fracture healing process may help develop AI algorithms that can subsequently be used to optimise implant design/materials.

Materials and Methods

The literature search was performed in PubMed, PubMed Central, Scopus, and Web of Science databases for reviewing and evaluating the published scientific data regarding sensor technology in fracture healing.

Results and Interpretation

SMART sensor technology comes with a variety of uses such as determining fracture healing progress, predicting early implant failure, and determining fractures liable for non-union to exemplify a few. The main limitations are that it is still in its inception and needs extensive refinement before it becomes widely and routinely used in clinical practice. Nevertheless, with continuous advances in microprocessor technology, research designs, and additive manufacturing, the utilisation and application of SMART implants in the field of trauma and orthopaedic surgery are constantly growing.

Conclusion

Mass production of such SMART implants will reduce overall production costs and see its use in routine clinical practice in the future and is likely to make a significant contribution in the next industrial revolution termed 'Industry 5.0' which aims at personalised patient-specific implants and devices. SMART sensor technology may, therefore, herald a new era in the field of orthopaedic trauma.

Capacitive stimulation-sensing system for instrumented bone implants: Finite element model to predict the electric stimuli delivered to the interface

BACKGROUND

Prevalence of orthopaedic replacements are increasing around the world. The main cause of revision remains associated to the interface loosening. In this work, a computational study using the Finite element method was developed to predict the electric field stimuli delivered to trabecular bone structures, as well as to predict the sensing ability to detect different bone-implant interface scenarios.

METHODS

Three finite element models were developed: two simplified models, including a Gyroid TMP structure, and a realistic model based on microCT scan of a trabecular bone from sheep vertebra. Simulations were performed using a co-surface capacitive technology for stimulating and sensing bone-implant interfaces. Different fixation scenarios were considered, namely by establishing bone-stimulator gap sizes up to 1 mm (from fixation to massive loosening scenario). Electrodes were excited with sinusoidal and square electric signals up to 10V voltage and 64kHz frequency.

RESULTS

Simplification of bone geometry resulted in significant electric stimuli differences compared to the realistic bone geometry. Realistic modelling allowed to observe that, in the fixation scenario, the electric field stimuli decreased 85% from the sensor interface to a parallel plane 2 mm apart from such interface. A significant influence of the bone-stimulator distance on the electric stimuli was found: the electric stimuli magnitudes varied in the range between 0.38 V/mm (fixation scenario) and 4.8 mV/mm (massive loosening scenario) for voltages up to 10V. Strong frequency-dependent behaviours were also observed in the electric stimuli: their magnitudes can reach 106-fold decreases when the excitation frequency is decreased from 32 kHz to 14 Hz CONCLUSION: This study points out the inability of our two simplified models to predict the electric stimulation provided to different bone-implant interface scenarios. Results highlight that co-surface stimulators can deliver osteogenic electric stimuli along trabecular bone structures, ensuring low electric power excitations. Moreover, realistic models strongly enhance the sensing predictability of the bone-implant fixation states. These new and significant evidences provide a strong support to integrate co-surface capacitive into bioelectronic implants for both therapeutic and sensing operations.

Ultrasensitive capacitive sensing system for smart medical devices with ability to monitor fracture healing stages

Bone fractures are a global public health problem. A sustained increase in the number of incident cases has been observed in the last few decades, as well as the number of prevalent cases and the number of years lived with disability. Current monitoring techniques are based on imaging techniques, which are highly subjective, radioactive, expensive and unable to provide daily monitoring of fracture healing stages. The development of reliable, non-invasive and non-subjective technologies is mandatory to minimize non-union risks. Delayed healing and non-union conditions require timely medical intervention, such that preventive procedures and shortened treatment periods can be carried out. This work proposes the development of an ultrasensitive capacitive sensing system for smart implantable fixation implants with ability to effectively monitor the evolution of bone fractures. Both in vitro experimental tests and numerical simulations highlight that networks of co-surface capacitive systems are able: (i) to detect four different bone healing phases, capacitance decrease patterns occurring as the healing process progresses and (ii) to monitor the callus evolution in multiple target regions. These are very promising results that highlight the potential of capacitive technologies to minimize the individual and social burdens related to fracture management, mainly when delayed healing or non-union conditions occur.

Bioelectronic multifunctional bone implants: recent trends

The concept of Instrumented Smart Implant emerged as a leading research topic that aims to revolutionize the field of orthopaedic implantology. These implants have been designed incorporating biophysical therapeutic actuation, bone-implant interface sensing, implant-clinician communication and self-powering ability. The ultimate goal is to implement revist interface, controlled by clinicians/surgeons without troubling the quotidian activities of patients. Developing such high-performance technologies is of utmost importance, as bone replacements are among the most performed surgeries worldwide and implant failure rates can still exceed 10%. In this review paper, an overview to the major breakthroughs carried out in the scope of multifunctional smart bone implants is provided. One can conclude that many challenges must be overcome to successfully develop them as revision-free implants, but their many strengths highlight a huge potential to effectively establish a new generation of high-sophisticated biodevices.

Multifunctional Smart Bone Implants: Fiction or Future?-A New Perspective

Implantable medical devices have been developed to provide multifunctional ability to numerous bioapplications. In the scope of orthopaedics, four methodologies were already proposed to design implant technologies: non-instrumented passive implants, non-instrumented active implants, instrumented passive implants and instrumented active implants. Even though bone replacements are among the most performed surgeries worldwide, implant failure rates can still exceed 10%. Controversial positions multiply in the scientific community about the potential of each methodology to minimize the burden related to implant failures. In this perspective paper, we argue that the next technological revolution in the field of implantable bone devices will most likely emerge with instrumented active implants as multifunctional smart devices extracorporeally controlled by clinicians/surgeons. Moreover, we provide a new perspective about implant technology: the essence of instrumented implants is to enclose a hybrid architecture in which optimal implant performances require both smart instrumentation and smart coatings, although the implant controllability must be ensured by extracorporeal systems.

Towards Self-Adaptability of Instrumented Electromagnetic Energy Harvesters

Motion-driven electromagnetic energy harvesting is a well-suited technological solution to autonomously power a broad range of autonomous devices. Although different harvester configurations and mechanisms have been already proposed to perform effective tuning and broadband harvesting, no methodology has proven to be effective to maximize the harvester performance for unknown and time-varying patterns of mechanical power sources externally exciting the harvesters. This paper provides, for the first time, a radically new concept of energy harvester to maximize the harvested energy for time-varying excitations: the self-adaptive electromagnetic energy harvester. This research work aims to analyze the electric energy harvesting gain when self-adaptive electromagnetic harvesters, using magnetic levitation architectures, are able to autonomously adapt their architecture as variations in the excitation patterns occur. This was accomplished by identifying the optimal harvester length for different excitation patterns and load resistances. Gains related to electric current and power exceeding 100 can be achieved for small-scale harvesters. The paper also describes comprehensive case studies to verify the feasibility of the self-adaptive harvester, considering the energy demand from the adaptive mechanism, namely the sensing, processing and actuation systems. These successful results highlight the potential of this innovative methodology to design highly sophisticated energy harvesters, both for a small- and large-scale power supply.