Active Implantable Medical Devices Research Articles

Ultra-Compact Pulse Charger for Lithium Polymer Battery with Simple Built-in Resistance Compensation in Biomedical Applications.

Active implantable medical devices (AIMDs) rely on batteries for uninterrupted operation and patient safety. Therefore, it is critical to ensure battery safety and longevity. To achieve this, constant current/constant voltage (CC/CV) methods have been commonly used and research has been conducted to compensate for the effects of built-in resistance (BIR) of batteries. However, conventional CC/CV methods may pose the risk of lithium plating. Furthermore, conventional compensation methods for BIR require external components, complex algorithms, or large chip sizes, which inhibit the miniaturization and integration of AIMDs. To address this issue, we have developed a pulse charger that utilizes pulse current to ensure battery safety and facilitate easy compensation for BIR. A comparison with previous research on BIR compensation shows that our approach achieves the smallest chip size of 0.0062 mm2 and the lowest system complexity using 1-bit ADC. In addition, we have demonstrated a reduction in charging time by at least 44.4% compared to conventional CC/CV methods, validating the effectiveness of our system's BIR compensation. The compact size and safety features of the proposed charging system make it promising for AIMDs, which have space-constrained environments.

Exploration of Application of Reliability Standard YY/T 1837-2022 in Development of Active Implantable Medical Devices

In the field of medical devices, there has been a long-term lack of a general technical requirements framework for reliability that can be applied in the development of high-risk active implantable medical devices. This study combines the requirements of YY/T 1837-2022 to comprehensively explain and explore the requirements for reliability work that can be performed at each stage of development of active implantable medical device products, and provides a reference for product reliability work in the industry.

Investigating SAR reference levels to better inform MRI protocols for SAR restricted Active Implanted Medical Devices (AIMDs)

Investigating SAR reference levels to better inform MRI protocols for SAR restricted Active Implanted Medical Devices (AIMDs)

Computational Assessment of the Deposited Power and the Temperature Increase around Two Coupled Implanted Leads Inside a 1.5 T MRI Scanner

A specific method for the estimation of the induced power at the tip by an active implantable medical device (AIMD) during an MRI scan is not defined in ISO/TS 10974:2018 for the cases of devices with multiple leads or the presence of abandoned and retained leads, where coupling between two leads could take place. Therefore, the aim of this work is to evaluate a numerical method, similar to Tier 3 of ISO/TS 10974:2018, for the estimation of the deposited power at the tips of two coupled implanted leads. Specifically, the proposed methodology applies the same parameters as the standard Tier 3 process. However, these parameters are calculated for each implant in the presence of its neighbors in order to include the coupling between them. Numerical electromagnetic simulations were performed, in which a pair of generic implant leads was excited and placed at various positions relative to each other. The deposited power at the tips of the pair was analyzed based on the standard Tier 3 approach, and the results show that this could overestimate the induced power by more than 6 dB. Therefore, a modified Tier 3 numerical method for the estimation of the deposited power at their tips is proposed, which can produce a reliable calculation. This method was confirmed by evaluating various configurations of the two leads excited by orthogonal electrical fields, and also its uncertainty budget was developed. Finally, the method was repeated for different properties of the generic implants and the surrounding tissue, as well as for a pair of realistic implants and in vivo implantation trajectories.

On the Relationship Between Impedances of Active Implantable Medical Devices and Device Safety Under MRI RF Emission

Impedances of different portions of active implantable medical devices (AIMDs) can influence its magnetic resonance imaging (MRI) radio-frequency (RF) safety dramatically. To better understand this behavior, in this article, we analytically derive relationships between the impedances of AIMDs and the MRI RF safety. Numerical simulations and measurements are performed to validate the proposed relationships. In addition, effective measurement setups for the AIMD impedances are proposed. The analysis in this article gives a clear design strategy for MR-conditional safe AIMDs.

Retrospective study of sacral neuromodulator implantations in a French hospital center: Lifespan and hospital costs assessment.

Sacral nerve neuromodulation (SNM) is a safe and effective therapy for the management of fecal and/or urinary incontinence. The generators InterStim™ and InterStim™ II (Medtronic™) are non-rechargeable active implantable medical devices with a limited lifespan. The aims of this study were to assess the generators' median lifespan for all indications and the long-term hospital costs of the therapy. This was a retrospective monocentric study that included 215 patients aged over 18years who were treated by SNM for fecal incontinence and/or urinary incontinence. Lifespan was considered as the amount of time between definitive implantation and observed battery depletion by the surgeon and was assessed by the Kaplan-Meier method. Costs were assessed according to the activity-based pricing of the French public health care system. The median observed lifetime of stimulators implanted in our center was 7.29years and 5.9years for InterStim™ and InterStim™ II, respectively. The difference observed between the two generations was statistically significant. The modelling of primary implantation and renewal costs allowed us to observe that the decrease in the lifetime of Interstim™ II is associated with an increase in hospital costs over time. The retrospective study design is one limitation and we did not take into consideration stimulation's settings. The InterStim™ II lifespan is shorter than the first-generation device. This is associated with an increase of the long-term hospital costs. Additional information about the new neuromodulator will be required to choose the most appropriate IPG for the patient while optimizing the costs.

Rechargeable Active Implantable Medical Devices (AIMDs)

Active Implantable Medical Devices (AIMDs) act as lifesaving devices. They provide electrical signals to tissues as well as perform data-logging operations. To perform these operations, they need power. The battery is the only source for such devices, as they are placed invasively inside the human body. Once the battery drains out, the patient wearing the device has to undergo medical surgery for the second time, where there are many chances of infections, and it could be life-threatening too. If the AIMDs, e.g., pacemakers are designed using rechargeable batteries, then the devices can be recharged regularly, which can increase the life of the device as well as reduce its size. Wireless charging of AIMDs such as ICDs or pacemakers is proposed in this paper using magnetic resonant coupling. The selection of frequency for power transfer is the most crucial part, as the basic restriction (BR) criteria proposed by ICNIRP guidelines and the IEEEC95.1 standard need to be followed, which ensures the safety of the patient. This is suggested by considering some basic restriction parameters, such as specific absorption rate (SAR) and current density, as suggested by guidelines. In this paper, experimentation using two frequencies is shown, i.e., 1.47 MHz (the high frequency) and 62 KHz (the low frequency). For experimentation, goat flesh and saline solution are used. Secondary coil and flesh are dipped in the saline solution. Battery recharging performed at a lower frequency took less time than with a frequency in the MHz range. All BR criteria are fulfilled for both frequencies, so the proposed methodology is safe to use.

Wirelessly interfacing sensor-equipped implants and MRscanners for improved safety and imaging.

To investigate a novel reduced RF heating method for imaging in the presence of active implanted medical devices (AIMDs) which employs a sensor-equipped implant that provides wireless feedback. The implant, consisting of a generator case and a lead, measures RF-induced -fields at the implant tip using a simple sensor in the generator case and transmits these values wirelessly to the MR scanner. Based on the sensor signal alone, parallel transmission (pTx) excitation vectors were calculated to suppress tip heating and maintain image quality. A sensor-based imaging metric was introduced to assess the image quality. The methodology was studied at 7T in testbed experiments, and at a 3T scanner in an ASTM phantom containing AIMDs instrumented with six realistic deep brain stimulation (DBS) lead configurations adapted from patients. The implant successfully measured RF-induced -fields (Pearson correlation coefficient squared [R2 ] = 0.93) and temperature rises (R2 = 0.95) at the implant tip. The implant acquired the relevant data needed to calculate the pTx excitation vectors and transmitted them wirelessly to the MR scanner within a single shot RF sequence (<60 ms). Temperature rises for six realistic DBS lead configurations were reduced to 0.03-0.14 K for heating suppression modes compared to 0.52-3.33 K for the worst-case heating, while imaging quality remained comparable (five of six lead imaging scores were ≥0.80/1.00) to conventional circular polarization (CP) images. Implants with sensors that can communicate with an MR scanner can substantially improve safety for patients in a fast and automated manner, easing the current burden for MR personnel.

Thermal safety considerations for implantable micro-coil design

Micro magnetic stimulation of the brain via implantable micro-coils is a promising novel technology for neuromodulation. Careful consideration of the thermodynamic profile of such devices is necessary for effective and safe designs. Objective. We seek to quantify the thermal profile of bent wire micro-coils in order to understand and mitigate thermal impacts of micro-coil stimulation. Approach. In this study, we use fine wire thermocouples and COMSOL finite element modeling to examine the profile of the thermal gradients generated near bent wire micro-coils submerged in a water bath during stimulation. We tested a range of stimulation parameters previously reported in the literature such as voltage amplitude, stimulus frequency, stimulus repetition rate and coil wire materials. Main results. We found temperature increases ranging from <1 °C to 8.4 °C depending upon the stimulation parameters tested and coil wire materials used. Numerical modeling of the thermodynamics identified hot spots of the highest temperatures along the micro-coil contributing to the thermal gradients and demonstrated that these thermal gradients can be mitigated by the choice of wire conductor material and construction geometry. Significance. ISO standard 14708-1 designates a thermal safety limit of 2 °C temperature increase for active implantable medical devices. By switching the coil wire material from platinum/iridium to gold, our study achieved a 5–6-fold decrease in the thermal impact of coil stimulation. The thermal gradients generated from the gold wire coil were measured below the 2 °C safety limit for all stimulation parameters tested.

Rapid prediction of MRI-induced RF heating of active implantable medical devices using machine learning.

The interaction between an active implantable medical device and magnetic resonance imaging (MRI) radiofrequency (RF) fields can cause excessive tissue heating. Existing methods for predicting RF heating in the presence of an implant rely on either extensive phantom experiments or electromagnetic (EM) simulations with varying degrees of approximation of the MR environment, the patient, or the implant. On the contrary, fast MR thermometry techniques can provide a reliable real-time map of temperature rise in the tissue in the vicinity of conductive implants. In this proof-of-concept study, we examined whether a machine learning (ML) based model could predict the temperature increase in the tissue near the tip of an implanted lead after several minutes of RF exposure based on only a few seconds of experimentally measured temperature values. We performed phantom experiments with a commercial deep brain stimulation (DBS) system to train a fully connected feedforward neural network (NN) to predict temperature rise after ~3 minutes of scanning at a 3 T scanner using only data from the first 5 seconds. The NN effectively predicted ΔTmax-R2 = 0.99 for predictions in the test dataset. Our model also showed potential in predicting RF heating for other various scenarios, including a DBS system at a different field strength (1.5 T MRI, R2 = 0.87), different field polarization (1.2 T vertical MRI, R2 = 0.79), and an unseen implant (cardiac leads at 1.5 T MRI, R2 = 0.91). Our results indicate great potential for the application of ML in combination with fast MR thermometry techniques for rapid prediction of RF heating for implants in various MR environments.Clinical Relevance- Machine learning-based algorithms can potentially enable rapid prediction of MRI-induced RF heating in the presence of unknown AIMDs in various MR environments.

Computational Modeling of Diffusion-Based Delamination for Active Implantable Medical Devices.

Delamination at heterogeneous material interfaces is one of the most prominent failure modes in active implantable medical devices (AIMDs). A well-known example of an AIMD is the cochlear implant (CI). In mechanical engineering, a multitude of testing procedures are known whose data can be used for detailed modeling with respect to digital twins. Detailed, complex models for digital twins are still lacking in bioengineering since body fluid infiltration occurs both into the polymer substrate and along the metal-polymer interfaces. For a newly developed test for an AIMD or CI composed of silicone rubber and metal wiring or electrodes, a mathematical model of these mechanisms is presented. It provides a better understanding of the failure mechanisms in such devices and their validation against real-life data. The implementation utilizes COMSOL Multiphysics®, consisting of a volume diffusion part and models for interface diffusion (and delamination). For a set of experimental data, the necessary diffusion coefficient could be derived. A subsequent comparison of experimental and modeling results showed a good qualitative and functional match. The delamination model follows a mechanical approach. The results of the interface diffusion model, which follows a substance transport-based approach, show a very good approximation to the results of previous experiments.

Pacemakers, Implantable Defibrillators, and 5G Technology: What We Need to Know.

Pacemakers (PM) and implantable cardioverter defibrillators (ICD) are active implantable medical devices (AIMD) needed in case of cardiac arrhythmias. Given their potentially life-sustaining nature, the interaction between any source of electromagnetic field and these AIMDs is an ongoing concern of patients, industry, and regulators. According to the current regulatory framework, the required immunity of PM and ICD provides a reasonable unperturbed behavior in the presence of cell phones and base stations that use pre-5G technologies. PM/ICD international standards do not consider some peculiar characteristics of 5G technology and some of the 5G technology frequency bands (those above 3 GHz), since these frequencies are considered not to add risks to the AIMD functioning. Here we analyze the theoretical issues about the interference between 5G technology and PM/ICD and propose an experimental approach to perform a measurement campaign.

Seamless Feedthroughs for Neurotechnologies Using Diffusion Processes (Adv. Mater. Technol. 9/2023)

Electrical Feedthroughs In article number 2201887, Linda Rudmann, Sebastian D. Huber, and Thomas Stieglitz demonstrate that active implantable medical devices and, in particular, their hermetic packages, which house the electronics and other sensitive parts, rely on reliable hermetic electrical feedthroughs. As the size of the hermetic housing decreases, as required for the optical probe design, the requirements for the package increase. A proof-of-concept study presents a novel diffusion-based process that is non-destructive to the bulk hermetic material.

Pre-Clinical Requirements for Drug-Eluting Stents for CE Certification

Abstract: Drug Eluting Stents (DES) are small, expandable tubes that are implanted into a diseased, blocked peripheral or coronary artery through angioplasty to widen and increase the blood flow by slowly releasing the drug from device. They are covered in a medication that stops scar tissue from expanding upon that artery. In medical practice DES are frequently used for vascular disorders, exhibiting adequate potency and suitability within the acceptable range. Medical device regulations in European Union (EU) are covered by Directives 93/42/EEC, 98/79/EEC and 90/385/EEC which were recently replaced by new regulations (EU) 2017/745 on medical devices and (EU) 2017/746 on In vitro diagnostic medical devices. Across all the EU legislation, “CE Mark” or “Certificate of Compliance” has become a recognised term. The Competent authority’s (CA) certifies that a product conforms to required standards of EU, then the manufacturer display the “CE mark” on the label. The DrugDevice combination products like DES are included in Class-III high risk active implantable medical devices. The pre-clinical investigations of drug eluting stents often follow International Organization for Standardization (ISO) requirements, these vascular stents are covered under ISO 25539-2: 2020 - Cardiovascular Implants - Endovascular Devices, ISO 10993:2018- Biological Evaluation of Medical Devices and ISO 10555-1: 2013 – Intravascular catheters, sterile and single use medical devices.

RF-induced heating for active implantable medical devices in dual parallel leads configurations at 1.5 T MRI.

The Radiofrequency (RF)-induced heating for an active implantable medical device (AIMD) with dual parallel leads is evaluated in this paper. The coupling effects between dual parallel leads are studied via simulations and experiments methods. The global transfer function technique is used to assess the RF-induced heating for dual-lead AIMDs inside four human body models. RF-induced heating for spinal cord stimulator systems with 60 and 90 cm length leads are studied at three parallel dual-lead configurations (closely spaced, 8 mm spaced, and 40 mm spaced) and a single-lead configuration. The global transfer function method is used to develop the AIMD models of different configurations and is used for lead-tip heating assessments inside human body models. In simulation studies, the peak 1g specific absorption rate/temperatrue rises of dual parallel leads systems is lower than those from the single-lead system. In experimental American Society for Testing and Materials phantom studies, the temperature rises for the single-lead AIMD system can be 2.4 times higher than that from dual-lead AIMD systems. For the spinal cord stimulator systems used in the study, the statistical analysis shows the RF-induced heating of dual-lead configurations are also lower than those from the single-lead configuration inside all four human body models. For the AIMD system in this study, it shows that the coupling effects between the dual parallel leads of AIMD systems can reduce RF-induced heating. The global transfer function for different spatial distance dual-lead configurations can potentially provide a method for the RF-induced heating evaluation for dual-lead AIMD systems.

Seamless Feedthroughs for Neurotechnologies Using Diffusion Processes

AbstractElectrical feedthroughs are a major part of active implantable medical devices. They are responsible for connecting the implants’ active sites with the processing electronics that are usually hermetically packaged. They deliver conductive pathways through the package wall and have to avoid premature device failure due to ingressing humidity. With decreasing device size, the requirements on the single parts increase, including the electrical feedthroughs. As conventional electrical feedthroughs rely on opening the hermetic bulk material of the package that introduce leak paths, they always raise the potential that water enters the package inside alongside these interfaces. Therefore, a proof‐of‐concept study of a new hermetic electrical feedthrough design that leaves the hermetic bulk material, that is, silicon, intact is presented. The concept is based on diffusing noble metals, Pt and Au, into the semiconductor bulk, resulting in a quasi‐localized resistance increase of the silicon. It is shown that by variation of diffusion temperature, time, the substrate cooling, and others, a parameter combination is found with which optoelectronic components can be driven through an intact silicon substrate.

High-Fidelity 3D Stray Magnetic Field Mapping of Smartphones to Address Safety Considerations with Active Implantable Electronic Medical Devices.

Case reports indicate that magnets in smartphones could be a source of electromagnetic interference (EMI) for active implantable medical devices (AIMD), which could lead to device malfunction, compromising patient safety. Recognizing this challenge, we implemented a high-fidelity 3D magnetic field mapping (spatial resolution 1 mm) setup using a three-axis Hall probe and teslameter, controlled by a robot (COSI Measure). With this setup, we examined the stray magnetic field of an iPhone 13 Pro, iPhone 12, and MagSafe charger to identify sources of magnetic fields for the accurate risk assessment of potential interferences with AIMDs. Our measurements revealed that the stray fields of the annular array of magnets, the wide-angle camera, and the speaker of the smartphones exceeded the 1 mT limit defined by ISO 14117:2019. Our data-driven safety recommendation is that an iPhone 13 Pro should be kept at least 25 mm away from an AIMD to protect it from unwanted EMI interactions. Our study addresses safety concerns due to potential device-device interactions between smartphones and AIMDs and will help to define data-driven safety guidelines. We encourage vendors of electronic consumer products (ECP) to provide information on the magnetic fields of their products and advocate for the inclusion of smartphones in the risk assessment of EMI with AIMDs.

Occupational Exposure Assessment of the Static Magnetic Field Generated by Nuclear Magnetic Resonance Spectroscopy: A Case Study.

Magnetic resonance (MR) systems are used in academic research laboratories and industrial research fields, besides representing one of the most important imaging modalities in clinical radiology. This technology does not use ionizing radiation, but it cannot be considered without risks. These risks are associated with the working principle of the technique, which mainly involves static magnetic fields that continuously increase—namely, the radiofrequency (RF) field and spatial magnetic field gradient. To prevent electromagnetic hazards, the EU and ICNIRP have defined workers’ exposure limits. Several studies that assess health risks for workers and patients of diagnostic MR are reported in the literature, but data on workers’ risk evaluation using nuclear MR (NMR) spectroscopy are very poor. Therefore, the aim of this research is the risk assessment of an NMR environment, paying particular attention to workers with active implantable medical devices (AIMDs). Our perspective study consisted of the measurement of the static magnetic field around a 300 MHz (7 T) NMR research spectrometer and the computation of the electric field induced by the movements of an operator. None of the calculated exposure parameters exceeded the threshold limits imposed by legislation for protection against short-term effects of acute occupational exposure, but our results revealed that the level of exposure exceeded the action level threshold limit for workers with AIMD during the execution of tasks requiring the closest proximity to the spectrometer. Moreover, the strong dependence of the induced electric field results from the walking speed models is shown. This case study represents a snapshot of the NMR risk assessment with the specific goal to increase the interest in the safety of NMR environments.

Interference thresholds for active implantable cardiovascular devices in occupational low-frequency electric and magnetic fields: a numerical and in vitro study

In light of concerns regarding the occupational safety and health of workers wearing active implantable medical devices (AIMDs), this study aims to investigate the potential risks of electromagnetic interference (EMI) between AIMDs and low-frequency 50/60 Hz electromagnetic fields (EMFs) in the workplace. A total of 58 AIMDs, consisting of pacemakers (PMs) and implantable cardiac defibrillators (ICDs) of different brands, models, and configurations were tested to determine the immunity thresholds for high-voltage electric fields (EFs) and magnetic fields (MFs) at 50/60 Hz. The EFs and MFs at the levels in workplaces are reproduced by setups using Helmholtz coils and aluminum plates, respectively, to ensure that the EM/MF exposures are controllable and reproducible. The EMI thresholds were recorded by observing the occurrences of PM or ICD dysfunctions. In addition, numerical studies on anatomical models were carried out using CST® software. The results indicate that the recorded thresholds all exceed the EF and MF public exposure limits given in the ICNIRP 2010 guidelines. No dysfunction was observed among four ICDs tested under MF exposure up to 2750 µT at 50 Hz and 2480 µT at 60 Hz. However, among the 43 PMs and 11 ICDs tested under EF exposures, potential hazards may occur below the occupational exposure level proposed in the ICNIRP guidelines.

A Cascaded Heterogeneous Equivalent Network for Evaluating RF-Induced Hazards on Active Implantable Medical Devices

This article presents a cascaded heterogeneous equivalent network for the evaluation of radiofrequency (RF) induced safety hazards on electrically long active implantable medical devices (AIMDs) exposed to magnetic resonance imaging systems. AIMDs in a highly heterogeneous human body are represented by cascaded transmission line models consisting of segments with different values of effective wavenumbers and characteristic impedances. Two terminations, implantable pulse generator (IPG), and lead-tip are modeled as lumped elements. Methods to extract values for lead-tip impedance, IPG impedance, wavenumber, and characteristic impedance of the AIMDs are proposed and validated through numerical simulations and experimental investigations. Based on these values, one can build up a unique AIMD model along each implanted trajectory inside human body models. The developed models can be used to accurately evaluate the RF-induced voltage and heating for each implantation scenario.