Method for anchoring biomechanical implants to muscle tendon and chest wall

Current State and Future Perspectives of Energy Sources for Totally Implantable Cardiac Devices

There is a large population of patients with end-stage congestive heart failure who cannot be treated by means of conventional cardiac surgery, cardiac transplantation, or chronic catecholamine infusions. Implantable cardiac devices, many designated as destination therapy, have revolutionized patient care and outcomes, although infection and complications related to external power sources or routine battery exchange remain a substantial risk. Complications from repeat battery replacement, power failure, and infections ultimately endanger the original objectives of implantable biomedical device therapy - eliminating the intended patient autonomy, affecting patient quality of life and survival. We sought to review the limitations of current cardiac biomedical device energy sources and discuss the current state and trends of future potential energy sources in pursuit of a lifelong fully implantable biomedical device.

FiberSecure suture compared to braided polyester suture

Reliability of wound closure is limited primarily by the capacity of tissues to support conventional sutures (or staples), not by strength of either material per se. We developed FiberSecure™ for closures to surpass tissue strength. We assessed and compared the mechanical and histological performance of FiberSecure™ suture versus commercially available braided polyester suture (Mersilene) in the closure of abdominal muscle incisions in miniature swine at approximately 3 months postsurgery. Four incisions were closed in the external oblique muscle of eight Sinclair minipigs. Two wounds were closed with FiberSecure™ suture size 0 and the remaining two with Mersilene suture size 0. At 90 days, specimens were removed for biomechanics and histology. In destructive tensile testing, in the 16 abdominal muscle specimens for the FiberSecure™ suture, muscle tear was not near the suture implantation region, which remained intact. Wound strength met or exceeded strength of neighboring tissue in FiberSecure™ groups, which had peak force of 55.7 ± 22.1 N (mean ± SD) and peak stress of 579.0 ± 159.2 KPa (mean ± SD). For Mersilene, 3 of the 16 samples tore at the suture site and the remaining samples tore through the abdominal muscle not near the implantation region. The wound strength was similar to surrounding tissue, and these specimens had peak force of 51.8 ± 21.7 N and peak stress of 550.3 ± 239.4 KPa (mean ± SD). No significant difference was observed in peak force or stress between groups (p > 0.05), most repairs having met or exceeded native tissue strength by this time point. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1126-1130, 2017.

Extended healing validation of an artificial tendon to connect the quadriceps muscle to the Tibia: 180-day study

Whenever a tendon or its bone insertion is disrupted or removed, existing surgical techniques provide a temporary connection or scaffolding to promote healing, but the interface of living to non-living materials soon breaks down under the stress of these applications, if it must bear the load more than acutely. Patients are thus disabled whose prostheses, defect size, or mere anatomy limit the availability or outcomes of such treatments. Our group developed the OrthoCoupler™ device to join skeletal muscle to prosthetic or natural structures without this interface breakdown. In this study, the goat knee extensor mechanism (quadriceps tendon, patella, and patellar tendon) was removed from the right hind limb in 16 goats. The device connected the quadriceps muscle to a stainless steel bone plate on the tibia. Mechanical testing and histology specimens were collected from each operated leg and contralateral unoperated control legs at 180 days. Maximum forces in the operated leg (vs. unoperated) were 1,400 ± 93 N (vs. 1,179 ± 61 N), linear stiffnesses were 33 ± 3 N/mm (vs. 37 ± 4 N/mm), and elongations at failure were 92.1 ± 5.3 mm (vs. 68.4 ± 3.8 mm; mean ± SEM). Higher maximum forces (p = 0.02) and elongations at failure (p=0.008) of legs with the device versus unoperated controls were significant; linear stiffnesses were not (p=0.3). We believe this technology will yield improved procedures for clinical challenges in orthopedic oncology, revision arthroplasty, tendon transfer, and tendon injury reconstruction.

An artificial tendon to connect the quadriceps muscle to the tibia

No permanent, reliable artificial tendon exists clinically. Our group developed the OrthoCoupler™ device as a versatile connector, fixed at one end to a muscle, and adaptable at the other end to inert implants such as prosthetic bones or to bone anchors. The objective of this study was to evaluate four configurations of the device to replace the extensor mechanism of the knee in goats. Within muscle, the four groups had: (A) needle-drawn uncoated bundles, (B) needle-drawn coated bundles, (C) barbed uncoated bundles, and (D) barbed coated bundles. The quadriceps tendon, patella, and patellar tendon were removed from the right hind limb in 24 goats. The four groups (n = 6 for each) were randomly assigned to connect the quadriceps muscle to the tibia (with a bone plate). Specimens were collected from each operated leg and contralateral unoperated controls both for mechanical testing and histology at 90 days post-surgery. In strength testing, maximum forces in the operated leg (vs. unoperated control) were 1,288 ± 123 N (vs. 1,387 ± 118 N) for group A, 1,323 ± 144 N (vs. 1,396 ± 779 N) for group B, 930 ± 125 N (vs. 1,337 ± 126 N) for group C, and 968 ± 109 N (vs. 1,528 ± 146 N) for group D (mean ± SEM). The strengths of the OrthoCoupler™ legs in the needled device groups were equivalent to unoperated controls (p = 0.6), while both barbed device groups had maximum forces significantly lower than their controls (p = 0.001). We believe this technology will yield improved procedures for clinical challenges in orthopaedic oncology, revision arthroplasty, tendon transfer, and tendon injury reconstruction.

A soft-tissue coupling for wound closure

Wounds often cannot be successfully closed by conventional means of closure such as sutures or staples. Our group developed the FiberSecure™ device to close soft tissue wounds reliably, surpassing native tissue strength. We closed cross-fiber muscle incisions, to evaluate (1) four different configurations of FiberSecure™ for 30 days, then (2) the resulting preferred configuration for 180 days. The four treatment groups each placed 21,504 polyester (PET) 12-μm fibers (cross-sectional area 1% of muscle) traversing the incision, in the form of (A) Four large (No.7 suture) non-textured bundles, (B) Eight small (No.2 suture) non-textured, (C) Four large textured, or (D) Eight small textured. Four incisions were closed in the external oblique muscle of 16 Sinclair minipigs. At 30 days, specimens were removed for biomechanics, histology, and total collagen content. Group (B) was selected for 180-day evaluations in the same wound model in eight animals, four closures each (n = 32), again with biomechanics and histology. In strength testing, every specimen tore through muscle remotely, while the repair region remained intact. Maximum forces were (A) 37.8 ± 3.9 N, (B) 37.1 ± 4.7 N, (C) 39.0 ± 5.3 N, and (D) 32.4 ± 3.4 N at 30 days, and 37.2 ± 11.3 N at 180 days (mean ± SEM). No significant difference was observed among the groups or time points (p > 0.05).

Design Improvements and In Vitro Testing of an Implantable Muscle Energy Converter for Powering Pulsatile Cardiac Assist Devices

Harnessing skeletal muscle for circulatory support would improve on current blood pump technologies by eliminating infection-prone drivelines and cumbersome transcutaneous energy transmission systems. Toward that end, we have built and tested an implantable muscle energy converter (MEC) designed to transmit the contractile energy of the latissimus dorsi muscle in hydraulic form. The MEC weighs less than 300 g and comprises a metallic bellows formed from AM350 stainless steel actuated by a rotary cam (440C) attached to a titanium rocker arm (Ti–6Al–4V). The rocker arm is fixed to the humeral insertion of the muscle via a looped artificial tendon developed specifically for this purpose. The device housing (Ti–6Al–4V) is anchored to the ribcage using a perforated mounting ring and a wire suture. Lessons learned through seven previous design iterations have produced an eighth-generation pump with excellent durability, energy transfer efficiency, anatomic fit, and tissue interface characteristics. This report describes recent improvements in MEC design and summarizes results from in silico and in vitro testing. Long-term implant studies will be needed to confirm these findings prior to clinical testing.

An artificial tendon with durable muscle interface

A coupling mechanism that can permanently fix a forcefully contracting muscle to a bone anchor or any totally inert prosthesis would meet a serious need in orthopaedics. Our group developed the OrthoCoupler device to satisfy these demands. The objective of this study was to test OrthoCoupler's performance in vitro and in vivo in the goat semitendinosus tendon model. For in vitro evaluation, 40 samples were fatigue-tested, cycling at 10 load levels, n = 4 each. For in vivo evaluation, the semitendinosus tendon was removed bilaterally in eight goats. Left sides were reattached with an OrthoCoupler, and right sides were reattached using the Krackow stitch with #5 braided polyester sutures. Specimens were harvested 60 days postsurgery and assigned for biomechanics and histology. Fatigue strength of the devices in vitro was several times the contractile force of the semitendinosus muscle. The in vivo devices were built equivalent to two of the in vitro devices, providing an additional safety factor. In strength testing at necropsy, suture controls pulled out at 120.5 +/- 68.3 N, whereas each OrthoCoupler was still holding after the muscle tore, remotely, at 298 +/- 111.3 N (mean +/- SD) (p < 0.0003). Muscle tear strength was reached with the fiber-muscle composite produced in healing still soundly intact. This technology may be of value for orthopaedic challenges in oncology, revision arthroplasty, tendon transfer, and sports-injury reconstruction.

In vivo demonstration of a self-sustaining, implantable, stimulated-muscle-powered piezoelectric generator prototype

An implantable, stimulated-muscle-powered piezoelectric active energy harvesting generator was previously designed to exploit the fact that the mechanical output power of muscle is substantially greater than the electrical power necessary to stimulate the muscle's motor nerve. We reduced to practice the concept by building a prototype generator and stimulator. We demonstrated its feasibility in vivo, using rabbit quadriceps to drive the generator. The generated power was sufficient for self-sustaining operation of the stimulator and additional harnessed power was dissipated through a load resistor. The prototype generator was developed and the power generating capabilities were tested with a mechanical muscle analog. In vivo generated power matched the mechanical muscle analog, verifying its usefulness as a test-bed for generator development. Generator output power was dependent on the muscle stimulation parameters. Simulations and in vivo testing demonstrated that for a fixed number of stimuli/minute, two stimuli applied at a high frequency generated greater power than single stimuli or tetanic contractions. Larger muscles and circuitry improvements are expected to increase available power. An implanted, self-replenishing power source has the potential to augment implanted battery or transcutaneously powered electronic medical devices.

Design Considerations for an Implantable, Muscle Powered Piezoelectric System for Generating Electrical Power

A totally implantable piezoelectric generator system able to harness power from electrically activated muscle would augment the power systems of implanted functional electrical stimulation devices by reducing the number of battery replacement surgeries or by allowing periods of untethered functionality. The generator design contains no moving parts and uses a portion of the generated power for system operation. A software model of the system was developed and simulations performed to predict the output power as the system parameters were varied within their constraints. Mechanical forces that mimic muscle forces were experimentally applied to a piezoelectric generator to verify the accuracy of the simulations and to explore losses due to mechanical coupling. Depending on the selection of system parameters, software simulations predict that this generator concept can generate up to 690 microW of power, which is greater than the power necessary to drive the generator, conservatively estimated to be 46 microW. These results suggest that this concept has the potential to be an implantable, self-replenishing power source and warrants further investigation.

Improved mechanism for capturing muscle power for circulatory support

Although it is now understood that trained skeletal muscle can generate enough steady-state power to provide significant circulatory support, there are currently no means by which to tap this endogenous energy source to aid the failing heart. To that end, an implantable muscle energy converter (MEC) has been constructed and its function has been improved to optimize durability, anatomic fit, and mechanical efficiency. Bench tests show that MEC transmission losses average less than 10% of total work input and that about 85% of this muscle power is successfully transferred to the working fluid of the pump. Results from canine implant trials confirm excellent biocompatibility and demonstrate that contractile work of the latissimus dorsi muscle-measured to 290 mJ/stroke in one dog-can be transmitted within the body at levels consistent with cardiac assist requirements. These findings suggest that muscle-powered cardiac assist devices are feasible and that efforts to further develop this technology are warranted.

A durable load bearing muscle to prosthetic coupling

Skeletal muscles have been successfully linked to power mechanical support devices acutely. However, the required load bearing muscle to prosthetic interfaces have not been consistently durable. Tissue simply may not tolerate the repetitive pressure generated, ranging to 40,000 mm Hg, when necessary forces meet the crosssectional areas accessible by suture or clamp fixation. Dramatically increasing the force transfer surface by dispersing ultrafine polymer fibers in the distal muscle substance is the principle of a coupling device termed the MyoCoupler. Earlier, effective force transfer was computationally projected and confirmed in a pilot 30 day rabbit trial, with pull-out strength several times need. This investigation tested bonding strength after longer periods and examined the postulated fiber tissue integration. Devices and controls (buttressed suture fixation alone) were implanted contralaterally in the posterior tibial muscles of 28 rabbits for up to 90 days. Of the 28 rabbits, 21 were used for bond strength testing, and 3 were used for histology. Infection or procedural error disqualified 4 of the rabbits. Pull-out strength levels at 10-30 days (n = 7), 31-60 days (n = 10), 61-90 days (n=4), and all (n=21) were, respectively, 107.1 +/- 58.1, 111.4 +/- 42.7, 97.0 +/- 21.3, and 107.2 +/- 43.9 for MyoCouplers and 58.4 +/- 19.4, 52.3 +/- 34.7, 40.5 +/- 13.0, and 52.1 +/- 26.9 for the control animals. Differences were statistically significant (one-tailed t-test for paired data) and at progressively higher standards of probability for each successive period (p < 0.05 at 10-30 days, p < 0.01 at 31-60 days, p < 0.001 at 90 days, and p < 0.00001 for all). Histology showed fibrous tissue insinuation. Of 360 random fiber surface sites, 88% were closer to fibrous tissue structures than to other fibers. These findings support the aggressive pursuit of muscle powered mechanisms for artificial hearts, assist devices, and heart wall actuators.