Wearable bio-adhesive metal detector array (BioMDA) for spinal implants

Spinal hardware is crucial for treating spinal deformities, injuries, and degenerative diseases. However, current methods for monitoring implant integrity, such as computed tomography (CT), are expensive and involve radiation exposure. Alternative invasive solutions, while proposed, carry risks of infection and inflammation. Wearable technologies offer a promising non-invasive alternative for real-time monitoring. Existing wearable imaging solutions, such as ultrasound and electromagnetic-based imaging, suffer from limitations like poor image quality or lack of decoupling models. This study presents a bio-adhesive stainless-steel metal implant detector array (BioMDA) as a non-invasive, real-time monitoring system for spinal implants. The use of magnetostatic interaction eliminates heat accumulation issues. A biocompatible adhesive hydrogel interface layer and conformal designs ensure effective and reliable sensing via inductive coupling. Customized decoupling models allow precise tracking of implant position. The BioMDA system holds promise for a myriad of applications in postoperative monitoring, promoting decentralized healthcare and optimizing post-surgical outcomes.

The literature review highlights the critical need for non-invasive, real-time monitoring of spinal implants to detect hardware failures early. Current methods like CT scans are limited by cost and radiation exposure. While invasive methods have been explored, the risks of infection and inflammation limit their feasibility. The review also discusses existing wearable technologies for medical imaging, such as ultrasound and electromagnetic-based systems, and their respective limitations in terms of image quality, interference, and lack of precise localization capabilities. The authors highlight the absence of a solution that provides continuous and accurate monitoring of spinal implant position without the drawbacks of existing approaches.

The BioMDA system consists of a 16-sensor array, a biocompatible adhesive hydrogel interface layer, and customized decoupling models. The sensor array uses a 4x4 layout with 20 mm interspacing, connected via flexible Cu/PI traces. The hydrogel layer ensures secure adhesion to the patient's skin. Advanced fabrication techniques like laser cutting and transfer printing were used to create the flexible and stretchable device. The system's design allows for non-contact sensing of cervical pedicle screws (CPS) via inductive coupling between permanent magnets and metal implants. Spine movement changes the relative position between the CPS and BioMDA, generating inductive signals. These signals are analyzed using two decoupling models: a received signal strength (RSS)-based model for horizontal positioning and a custom electromagnetic-kinematic model for vertical distance. The single sensing unit was optimized via experimentation and finite element analysis to determine the optimal structural parameters (PET film thickness and central angle) for sensitivity and stability. The biocompatible adhesive hydrogel was characterized for its adhesion strength, biocompatibility, and resilience. Extensive testing was performed to assess the system's performance in various orientations and against electromagnetic interference. In-vitro experiments on a spinal prosthesis simulated normal and failure scenarios (screw loosening, screw fracture, rod fracture, and rod loosening) to evaluate the BioMDA's ability to detect these conditions.

The BioMDA system demonstrated high accuracy in tracking implant position (<0.5 mm). The customized decoupling models effectively translated the inductive signals into precise positional information, both horizontally and vertically. The biocompatible adhesive hydrogel ensured robust and stable adhesion to the skin, maintaining signal quality even after repeated bending cycles. The system showed good resistance to electromagnetic interference from common sources. In-vitro experiments successfully demonstrated the BioMDA's ability to detect common spinal implant failure modes such as screw loosening and fracture, and rod loosening and fracture. The distance decoupling model, using only six calibration factors, accurately simulated the electromagnetic response to moving implants. The horizontal mapping model, utilizing an RSS-based approach, effectively localized the implants' horizontal positions. The study provides detailed experimental data supporting the system’s performance and accuracy.

The BioMDA system addresses the critical need for non-invasive, real-time monitoring of spinal implants. Its high accuracy, non-invasive nature, and ability to detect common failure modes represent a significant advancement over existing methods. The system's potential to reduce patient radiation exposure and improve post-surgical outcomes is substantial. The successful in-vitro testing demonstrates the feasibility of using BioMDA for practical clinical applications. The modular design and biocompatible materials used ensure long-term usability and patient comfort. The findings are relevant to the field of orthopedic surgery, particularly in spinal fusion procedures, offering a novel solution for enhanced patient care and reduced healthcare costs.

The BioMDA system offers a cost-effective, real-time solution for non-invasive monitoring of spinal implants. Its unique combination of flexible design, biocompatible adhesion, and precise decoupling models allows for accurate detection of implant position and failure modes. Future work should focus on expanding the range of detectable materials, increasing the depth of detection, and incorporating AI-based algorithms for improved performance. The BioMDA holds significant promise for broader applications in orthopedic surgery and beyond.

The current study primarily focused on cervical spine implants. Further research is needed to evaluate the BioMDA's effectiveness with other types of spinal implants and in different anatomical locations (e.g., lumbar spine). The depth of detection may also be limited in certain clinical scenarios, requiring further optimization. The study used in-vitro testing; further in-vivo studies are necessary to validate the system's performance in real-world clinical settings. The current model is optimized for stainless steel; further adaptation is needed for wider compatibility with other implant materials like titanium.