Alan J. Snyder

Modeling of a dielectric elastomer diaphragm for a prosthetic blood pump

The electromechanical behavior of dielectric elastomers is to be exploited for medical application in artificial blood pumps. It is required that the pump diaphragm achieves a swept volume increase of 70 cc into a systolic pressure of 120 mmHg with the main design objective being volumetric efficiency. As such, a model that accommodates large deformation behavior is used. In order to design prosthetic blood pumps that closely mimic the natural pumping chambers of the heart, a dielectric elastomer diaphragm design is proposed. The elastomers change in shape in response to the applied electric field will permit it to be the active element of the pump just as the ventricular walls are in the natural heart. A comprehensive analytical model that accounts for the combined elastic and dielectric behavior of the membrane is used to compute the stresses and deformations of the inflated membrane. Dielectric elastomers are often pre-strained in order to obtain optimal electromechanical performance. The resulting model incorporates pre-strain and shows how system parameters such as pre-strain, pressure, electric field, and edge constraints affect membrane deformation. The model predicts more than adequate volume displacement for moderate pre-strain of the elastomer.

Sequence and task analysis of instrument use in common laparoscopic procedures

Background: In the area of instrument evaluation, one aspect that still requires objective assessment is the dynamics of instrument maneuver and exchange. If we could gain a better understanding of these phenomena, we could improve the design of the instruments themselves. Methods: A total of 29 laparoscopic procedures were videotaped and reviewed using time motion analysis. Instrument multifunctionality was determined using a standardized list of laparoscopic maneuvers. State transition diagrams were utilized to document the sequence of instrument exchanges. Results: The curved dissector, atraumatic grasper, and cautery scissors were identified as the most multifunctional instruments; each was able to perform five distinct maneuvers. Instrument sequences were found to consist of a three-part dissect ? clip ? cut cycle and a two-part dissect ? suction cycle of instrument exchange. Conclusion: This study demonstrated that laparoscopic instruments are often used to perform a variety of maneuvers in addition to their primary function. Furthermore, there are common patterns in instrument exchange that provide a potential source of design parameters for improved surgical efficiency.

Design of a PZT Bimorph Actuator Using a Metamodel-Based Approach

The design of a variable thickness piezoelectric bimorph actuator for application to minimally invasive surgery is proposed. The actuator is discretized into five segments along its length, where the thicknesses of the segments are used as design variables in the problem of optimizing both the force and deflection at the tip. Metamodeling techniques are used to construct computationally inexpensive approximations of finite element simulations and to rapidly explore the design space and the Pareto frontier. A prototype device and experimental verification of the analytical results are also discussed.

An electrically powered total artificial heart. Over 1 year survival in the calf

An electric motor driven orthotopic artificial heart was implanted in a 110 kg female Holstein calf as part of a series of 12 such implants intended to demonstrate the in vivo durability and compatibility of the device. The device uses pusher plates set into motion by a reversing brushless DC motor and roller screw to alternately eject two cylindrical sac type blood pumps. The pumps use Bjork-Shiley Delrindisc convexo-concave or monostrut valves. The left pump provides an 88-90 ml dynamic stroke volume. Woven Dacron grafts and polyurethane coated Dacron/Lycra cuffs are used to attach the device to the major arteries and atria, respectively. A polyurethane conduit and anchoring skin button bring motor wires percutaneously to an extracorporeal controller. The controller provides balanced cardiac output sensitive to atrial or aortic pressures, without operator intervention. The system is hermetically sealed and uses a simple compliance sac to maintain thoracic pressure between the pumps. The calf recovered uneventfully from surgery and thrived thereafter. She was killed on the 388th post-operative day because of worsening cardiac insufficiency. The previous three operative survivors in this series lived 131, 134, and 204 days. These results indicate the devices good potential for durability and body compatibility.

Pressure-volume characteristics of dielectric elastomer diaphragms

With the ultimate goal of constructing diaphragm-type pumps, we have measured pressure-volume characteristics of single-layer dielectric elastomers diaphragms. Circular dielectric elastomer diaphragms were prepared by biaxial stretching of 3M VHB 4905 polyacrylate, or spin casting and modest or no biaxial stretching of silicone rubber films, followed by mounting to a sealed chamber having a 3.8 cm diameter opening. Pressure-volume characteristics were measured at voltages that provided field strengths up to 80 MV/m in un-deformed VHB films and 50-75 MV/m in silicone films. The most highly pre-strained VHB diaphragms were found to have linear pressure-volume characteristics whose slopes (diaphragm compliance) depended sensitively upon applied field at higher field strengths. Compliance of unstretched silicone diaphragms was nearly independent of field strength at the fields tested, but pressure-volume characteristics shifted markedly. For both kinds of dielectric elastomers, pressure-volume work loops of significant size can be obtained for certain operating pressures. Each type of diaphragm may have advantages in certain applications.

Permanent circulatory support systems at the Pennsylvania State University

Two systems which provide long-term circulatory support are described: the left ventricular assist system and the total artificial heart. These systems are based on the design of a pusher plate actuated blood pump, driven by a small brushless DC electric motor and rollerscrew driver. An implantable motor controller maintains suitable physiologic flow rates for both systems and controls left-right balance in the total artificial heart. Other parts of the system include an intrathoracic compliance chamber, transcutaneous energy and data transmission system, and internal and external batteries.<<ETX>>

In vivo performance of a transcutaneous energy transmission system with the Penn State motor driven ventricular assist device.

A transcutaneous energy transmission system (TETS) has been used to power the Penn State motor driven ventricular assist device in nine calf experiments, for a total of 316 days of cumulative in vivo experience. This is seen as an important step toward a completely implantable ventricular assist system and total artificial heart. The TETS converts an external 12 volt DC source via inductive coupling to a regulated 14 volt output voltage for use by the motor controller. A maximum output power of 70 watts is available. In calf experiments, the TETS output power averaged between 8 and 12 watts. The motor controller was not implanted in these experiments, awaiting further development of the miniaturized electronics. The TETS output was returned percutaneously to the external motor controller, allowing the TETS output to be monitored directly. System efficiency, from DC source to DC output, and including losses in 12 feet of cable, ranged from 55% to 70%, depending upon supply voltage, motor load, and degree of coil coupling. The subcutaneous coil was well tolerated, demonstrating only temporary, mild, superficial induration.

Steady state hemodynamic and energetic characterization of the Penn State/3M Health Care Total Artificial Heart.

Total Artificial Heart (TAH) development at Penn State University and 3M Health Care has progressed from design improvements and manufacturing documentation to in vitro and in vivo testing to characterize the systems hemodynamic response and energetic performance. The TAH system is completely implantable and intended for use as an alternative to transplantation. It includes a dual pusher plate pump and rollerscrew actuator, welded electronics and battery assembly, transcutaneous energy transmission system, telemetry, and a compliance chamber. In vitro testing was conducted on a Penn State mock circulatory loop with glycerol/water solution at body temperature. Tests were performed to characterize the preload and afterload response, left atrial pressure control, and power consumption. A sensitive preload response was demonstrated with left atrial pressure safely maintained at less than 15 mm Hg for flow rates up to 7.5 L/min. Variations in aortic pressure and pulmonary vascular resistance were found to have minimal effects on the preload sensitivity and left atrial pressure control. In vivo testing of the completely implanted system in its final configuration was carried out in two acute studies using implanted temperature sensors mounted on the electronics, motor, and energy transmission coil in contact with adjacent tissue. The mean temperature at the device-tissue interface was less than 4 degrees C above core temperature.

In vivo testing of a completely implanted total artificial heart system.

The authors performed 14 implants of a completely implanted total artificial heart (TAH) system in calves. The system consisted of a dual pusher plate rollerscrew energy converter, two sac type blood pumps, an implanted electronic control and battery package, and a transcutaneous energy transmission system. Ten of the implants included a percutaneous lead for monitoring of the implant; the remainder made use of wireless two way telemetry between the implant and the outside. Three animals survived the perioperative period. These calves survived for 98 to 118 days, and one was still alive at 150 days. Causes for termination of the 98 and 118 day cases were abdominal pocket sepsis originating at a monitoring line, and systemic sepsis acquired perioperatively. Death or termination in the shorter cases was mainly due to respiratory complications or bleeding. The TAH system proved capable of providing adequate cardiac outputs at modest atrial pressures. Wireless monitoring and wireless intervention for weaning from cardiopulmonary bypass were readily achieved. All organ systems functioned normally in the presence of the device. Once recovery from implantation in these very young animals was achieved, the system proved its ability to reliably support these animals until body mass exceeded its cardiac output capabilities.

Multiscale analysis of surface thrombosis in vivo in a left ventricular assist system.

Thrombosis limits the success of ventricular assist devices as the demand for alternatives to heart transplants is increasing. This study mapped the occurrence of thrombosis in a left ventricular assist system (LVAS) to better understand the biologic response to these devices. Nine calves divided into two groups were implanted with LVAS for 28 to 30 days. One group was anticoagulated, whereas the second group received no long-term anticoagulation. The blood-contacting poly(urethane urea) surfaces of blood sacs in the LVAS were examined for macroscopic thrombi upon retrieval. The sac was partitioned into eight sections and imaged for thrombi by scanning electron microscopy. No difference in thrombosis was observed macroscopically between the groups. Anticoagulation appeared to result in reduction of platelet-like structures, but the presence of fibrin-like structures remained similar between groups. Regional differences correlating with high and low shear stress regions were observed. At the macroscale, fewer thrombi were recorded in the high shear stress ports. At the microscale, features resembling fibrin were observed primarily in the ports and platelet-like features were common in lower shear stress regions. These variations in thrombosis with anticoagulation and location are likely due to varied fluid dynamics within the LVAS blood sac.