Torsional Ventricular-Assist Device: Design Considerations and Prototype Development1 (2016)

A wide array of cardiac assist devices are currently used to treat patients suffering from the later stages of congestive heart failure (HF). The majority of the devices available in the market today are ventricular-assist devices (VADs), which feature a blood pump—either pulsatile or continuous flow—that is, cannulated in the left ventricle as a means to provide cardiac support. These VADs are especially effective in the short term (days to months) but are plagued by several longstanding problems with chronic use. These include: the need for long-term systemic anticoagulation strategy; invasive surgery for device deployment; complications arising from broken or worn parts; and kinked or infected percutaneous drive lines. Thus, there is a need for improved technologies that can minimize or eliminate these adverse events.Here, we report the development of a device designed to rotate the apex of the heart during the systolic portion of the cardiac cycle with the goal of reducing wall stress and increasing the ventricles' ability to empty more completely. This novel approach to HF treatment is aimed at augmenting the normal twisting motion of the ventricles, which is an important contractile component frequently missing in diseased hearts. If this method proves effective, it could be used as a means of either short-term or chronic circulatory support without the hematologic risks typically associated with blood-contacting devices. The results from preliminary computational simulations suggest that applied mechanical torsion at the apex of a failing left ventricle may significantly improve cardiovascular hemodynamics [1]. Indeed, these studies indicate that applied rotation angles up to a quarter turn can increase left ventricular ejection fraction and stroke work by 51% and 90%, respectively. Previous studies were also done with live animal hearts with a first generation device, which demonstrated that mechanical torsion applied at the apex of the heart has a significant positive effect [2]. In these experiments, a net twist of 90 deg was applied to the apex of a heart with severe drug-induced HF, resulting in a twofold increase in stroke volumes in both left and right ventricles.These early positive results have prompted the development of a second-generation torsion-based ventricular-assist device (tVAD) as an alternative to traditional VADs currently in the market. Our immediate goal is to develop a tVAD for acute cardiac support that is biocompatible, minimally invasive, easily implantable, and has no blood-contacting surfaces.We created our prototype design (Fig. 1) using the CAD software package solidworks 2015 (Dassault Systèmes Solidworks Corp., Waltham, MA). In our development of the tVAD prototype, we sought to create a device that would avoid blood contact, allow for rapid deployment through a small incision, and be simple to remove. With these three goals in mind, we developed a tVAD prototype that consists of three main components: a flexible mesh interface, a “claw” actuator mechanism, and a transcostal deployment system. The tVAD approach lends itself to minimally invasive placement across the chest wall, thus we are designing the device for deployment via a minithoracotomy. Although this approach can, in principle, be used to provide both short- and long-term care, our initial design will focus on the development of the tVAD for use as an acute therapy. In Sec. 3, we describe each of the three main system components in order of their direct interface and/or interaction with the heart.The tVAD prototype consists of three main parts: a flexible cup interface, a mechanical claw actuator, and a handle for manual insertion and deployment.The flexible cup interface (Fig. 2) is glovelike, in that it holds the “fingers” of the claw in place while providing a smooth protective layer both at the epicardial interface and the outside (pericardial-facing) surface. On the cardiac side, the cup interface spreads the distribution of the applied torsional forces evenly across the heart surface, minimizing the risk of tissue damage. The pericardial side serves to shield surrounding tissues from abrasion as the external actuator motor rotates the claw. Because of its glovelike design, a clinician can easily explant the device by simply sliding the claw and its fingers out of the cup's inner pocket. Ideally, we would want the cup to be made of a biocompatible and flexible material, such as a polymer (like silicone) or coated nitinol, patterned as a mesh.The six actuator fingers (Fig. 3) fit within the flexible “glove” and are prethreaded with sutures that fasten them to the interior webbing of the mesh interface. Should the device need to be explanted, the sutures can be unfastened and removed, allowing the device to freely slide out of the glove's pocket. The entire assembly is collapsible to allow for minimally invasive deployment and removal. When collapsed, the claw is three centimeters in diameter and can be expanded to fit snugly around the apex of the heart.The interior segments of each trusslike finger will be made less stiff than the exterior segments, thus allowing each finger to flex and conform to the surface of the heart. Possible materials to be considered for the actuator include 316L stainless steel or titanium. The flexion of the finger's truss structures could possibly be achieved by either varying the thickness of the material, patterning it so that the material is more flexible, or using a different, more flexible material. The collapsed claw and mesh interface is implanted onto the apex of the heart via the deployment system, which consists of a spring-loaded handle that allows clinicians to carefully twist open the collapsed device and position the claw and mesh onto the beating heart through a small incision.To ensure that the tVAD is securely fixed to the apex of the heart, we will first use active suction to position and secure the device to the epicardial surface. Once deployed and correctly positioned, the device will then rely on a surgical adhesive as the final means of adhering the device to the surface of the heart. The interior surface of the cup interface will be precoated with the adhesive, and the active suction will continue to be applied until the adhesive has had sufficient time to cure and set.Device rotation will be driven by a programmable servomotor (SmartMotor model SM2337DT-PLS, Animatics Corp., Santa Clara, CA), which keys into the base of the truss assembly after detachment of the deployment handle. The motor connects to the claw mechanism via a stiff metal driveshaft that traverses a fixed transcostal bushing placed to protect surrounding tissue and provide rotational stability. Rotation dynamics will be programed to mimic those rotation parameters that are found to be most effective in computational simulations.We created a preliminary tVAD prototype model with the final design to be fine-tuned based on parameters garnered from parametric studies currently being performed in parallel to this design work. Some of the parameters yet to be determined include, but are not limited to, the coverage area of the device over the epicardial surface (i.e., how far the fingers of the claw will be extending up the epicardium from the apex of the heart), the amount of applied rotation, the timing of the rotation (i.e., how quickly is the rotation applied), and direction of applied rotation. Independent of those parameters, we have created an initial design for our second-generation tVAD that consists of the glove interface, claw actuator, and deployment handle. Once the design parameters are finalized, we will then proceed to manufacture our prototype for initial testing on porcine hearts.This work was supported by the National Institutes of Health (Grant No. NIH 1R21EB017807-01A1), the Bradford and Diane Smith Fellowship Award (Carnegie Mellon University), and the Biomechanics in Regenerative Medicine (BiRM) Training Fellowship (Grant No. NIBIB 5T32EB003392-10).