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Conformal Parylene-C Media Separating Membranes for Pressure Sensing Applications in Ventricular Assist Devices
von Silvan Marc Staufert, herausgegeben von Christofer HieroldHeart failure is one of the most prevalent heath issues in todays world. Regardless, if the condition is not recognized early and allowed to progress to its end stages, there are few treatment options available. The gold standard treatment and only permanent treatment of end-stage heart failure is heart transplantation. However, transplantation has proven to be an unsustainable model, as donor organ shortage is a prevalent issue.
Ventricular assist devices (VAD) were introduced in the early 2000s and were quick to supplement the shortage of available donor organs. Over 10 years these mechanical circulatory support systems became a popular method to bridging the patients treatment until transplantation is possible. While incremental progress on the performance of the devices have been made, obvious shortcomings are present in todays VADs. One shortcoming of current generation VADs is their failure to properly respond to changes in the perfusion demand of the patient, as they are operated in a constant speed mode. The resulting adverse effects, under- and overpumping of the ventricle, have a significant impact on patient mortality.
Unfortunately, the evident solution to this problem, implementation of control algorithms, is denied due to the lack of long-term stable and safe sensors. In this work, we propose an integration approach for commercial pressure sensors in VADs. The approach is based around a thin, polymeric membrane serving as the sensing interface. It is located in the inflow cannula of the VAD and implemented as an integral part of a conformal coating, covering the entirety of the cannula. Hence, the interface does not perturb the blood flow or increase the thrombogenicity of the device. The coating material is Parylene-C, a polymer known mainly as a protective coating for electronics has a unique deposition route and outstanding mechanical stability. Its biocompatibility has been proven in multiple clinical applications and novel developments make it a promising material regarding future functionalization.
The sensing interface is produced in a newly developed fabrication process based on a sacrificial material release. The sacrificial material is shaped to conformally complement the inside surface of the inflow cannula through a shaping body. A 10 μm thick Parylene-C film is deposited and the sacrificial material is dissolved, forming the media separating membrane sensing interface. Membranes fabricated in such a manner with a diameter of 1.5 mm deviated a maximum of 12 μm from an ideally replicated cannula surface. A demonstrator pressure sensor assembly with the Parylene-C media separating membrane was fabricated and evaluated on a water based measurement bench. The sensing interface was not found to influence the measurement performance of the sensor in a significant manner and exhibited a stable performance over the duration of several weeks.
A known shortcoming of Parylene-C is its lacking long-term adhesion to metals, especially in applications with permanent liquid contact. As a loss of adhesion could lead to potentially catastrophic failure in a VAD, this work addresses the issue by introducing a novel adhesion promotion strategy. Medical grade titanium is provided with a porous nanostructure layer through hydrothermal synthesis. The resulting layers have a thickness of approximately 1.5 μm and increase the specific surface area by a factor of 256 as compared to a ideally flat surface. As Parylene-C is deposited in a chemical vapor deposition process, the polymer completely interpenetrates the nanoporous layer and is thus effectively anchored to the metal surface. Improvement in Parylene-C adhesion was measured by accelerated aging at 80° C in physiological solution and subsequent blister like adhesion testing. While the Parylene film on polished reference surfaces was easily detached after aging for 10 days, nanostructured samples did not show a significant decrease in adhesion. Investigation of the samples show, that a cohesive fracture within the polymer layer occurs, indicating optimal adhesion. The presented results provide a long-term stable and safe blood-contacting sensing interface for pressure sensors in VADs.
Ventricular assist devices (VAD) were introduced in the early 2000s and were quick to supplement the shortage of available donor organs. Over 10 years these mechanical circulatory support systems became a popular method to bridging the patients treatment until transplantation is possible. While incremental progress on the performance of the devices have been made, obvious shortcomings are present in todays VADs. One shortcoming of current generation VADs is their failure to properly respond to changes in the perfusion demand of the patient, as they are operated in a constant speed mode. The resulting adverse effects, under- and overpumping of the ventricle, have a significant impact on patient mortality.
Unfortunately, the evident solution to this problem, implementation of control algorithms, is denied due to the lack of long-term stable and safe sensors. In this work, we propose an integration approach for commercial pressure sensors in VADs. The approach is based around a thin, polymeric membrane serving as the sensing interface. It is located in the inflow cannula of the VAD and implemented as an integral part of a conformal coating, covering the entirety of the cannula. Hence, the interface does not perturb the blood flow or increase the thrombogenicity of the device. The coating material is Parylene-C, a polymer known mainly as a protective coating for electronics has a unique deposition route and outstanding mechanical stability. Its biocompatibility has been proven in multiple clinical applications and novel developments make it a promising material regarding future functionalization.
The sensing interface is produced in a newly developed fabrication process based on a sacrificial material release. The sacrificial material is shaped to conformally complement the inside surface of the inflow cannula through a shaping body. A 10 μm thick Parylene-C film is deposited and the sacrificial material is dissolved, forming the media separating membrane sensing interface. Membranes fabricated in such a manner with a diameter of 1.5 mm deviated a maximum of 12 μm from an ideally replicated cannula surface. A demonstrator pressure sensor assembly with the Parylene-C media separating membrane was fabricated and evaluated on a water based measurement bench. The sensing interface was not found to influence the measurement performance of the sensor in a significant manner and exhibited a stable performance over the duration of several weeks.
A known shortcoming of Parylene-C is its lacking long-term adhesion to metals, especially in applications with permanent liquid contact. As a loss of adhesion could lead to potentially catastrophic failure in a VAD, this work addresses the issue by introducing a novel adhesion promotion strategy. Medical grade titanium is provided with a porous nanostructure layer through hydrothermal synthesis. The resulting layers have a thickness of approximately 1.5 μm and increase the specific surface area by a factor of 256 as compared to a ideally flat surface. As Parylene-C is deposited in a chemical vapor deposition process, the polymer completely interpenetrates the nanoporous layer and is thus effectively anchored to the metal surface. Improvement in Parylene-C adhesion was measured by accelerated aging at 80° C in physiological solution and subsequent blister like adhesion testing. While the Parylene film on polished reference surfaces was easily detached after aging for 10 days, nanostructured samples did not show a significant decrease in adhesion. Investigation of the samples show, that a cohesive fracture within the polymer layer occurs, indicating optimal adhesion. The presented results provide a long-term stable and safe blood-contacting sensing interface for pressure sensors in VADs.