Programa del congreso

1. Introduction and Objective

We have previously developed and in vivo validated threadlike, injectable, neuromuscular microstimulators. These devices operate wirelessly through the rectification of bursts of high-frequency current delivered to tissues by volume conduction. These implants resemble small leads: a thin, elongated and flexible silicone body (< 1 mm diameter, 35 mm length) containing in its middle a hermetic titanium capsule housing the electronics. Two platinum-iridium helical coils form the two electrodes at opposite ends and their interconnections to the capsule [1].

As any active implantable medical device (AIMD), these microstimulators will experience repeated mechanical stresses within the corrosive environment of the body, which can affect their mechanical properties, leading to failure. Long-term and complex in vivo tests are mandatory to test the reliability of AIMDs. However, mechanical tests using accelerated testing protocols may reveal mechanical failures in a short-term and present an ethical alternative to reduce the number of animal tests.

Regulatory agencies and organizations have established standards for the mechanical testing of AIMDs. The most common tests include tensile, fatigue and fracture, compression, flexion, vibration, shock and also combinations of them [2].

For these injectable intramuscular microstimulators that mostly experience flexural stresses caused by the contraction and relaxation of muscles, we have developed a customized test bench to perform flex bending fatigue tests that combine tension and compression forces.

2. Methods

The machine was designed to perform repetitive bending of the microstimulators submerged in a heated saline bath to test their reliability in simulated in vivo conditions. The support structure is made with aluminum profiles to provide stability and robustness. The fixtures, gears and ball-bearings are manufactured in polymers with good thermal and chemical resistance to avoid corrosion and reduce wear. The step-motor that actuates the machine is controlled by a Raspberry Pi that also provides an interface to select angle, frequency and number of cycles for the test.

Following guidance documents [3], the fixture dimensions are 110% larger than the diameter of the device under test and have a curvature radius of 6 mm. The machine configuration considers a maximum bending angle of 25 degrees. These parameters consider the worst case of range of motion and stresses observed in chronic experiments with rabbits.

3. Results and Discussion

The machine is capable of performing over 7.5 million cycles for over 43 days, which is equivalent to 10 years of life-span based on a median span of a person's gait of 1.03 s [4] and an adult’s average walking count of 2,000 steps per day, using a frequency of 2 Hz, as suggested by standards [5], with a contraction time of 310 ms and a relaxation time of 205 ms. The fixtures can fix the implant at different points along its length, allowing to test the interconnection points that are considered the most susceptible to mechanical failure.

References

[1] A. García-Moreno, A. Comerma-Montells, M. Tudela-Pi, J. Minguillon, L. Becerra-Fajardo, and A. Ivorra, “Wireless networks of injectable microelectronic stimulators based on rectification of volume conducted high frequency currents”, J. Neural Eng., vol. 19, no. 5, p. 056015, 2022, doi: 10.1088/1741-2552/ac8dc4.

[2] International Standard, “Implants for surgery – Active implantable medical devices – Part 1: General requirements for safety, marking and for information to be provided by the manufacturer” ISO 14708-1:2014(E), second edition, August 2014.

[3] U.S. Department of Health and Human Services, Food and Drug Administration (FDA), Center for Devices and Radiological Health (CDRH), “Guidance for the Submission of Research and Marketing Applications for Permanent Pacemaker Leads and for Pacemaker Lead Adaptor 510(k) Submissions.” Guidance for Industry, November 2000. [Available at: https://www.fda.gov/media/71740/download]

[4] J. B. Webster and B. J. Darter, “Principles of Normal and Pathologic Gait,” Atlas of Orthoses and Assistive Devices, Elsevier, 5th edition, pp. 49–62, 2019, doi: 10.1016/B978-0-323-48323-0.00004-4.

[5] European Committee for Standardization (CEN), European Electrotechnical Committee for Standardization (CENELEC), “Active implantable medical devices - Part 2-1: Particular requirements for active implantable medical devices intended to treat bradyarrhythmia (cardiac pacemakers).” Standard EN 45502-2-1, 2003.