Micromachined nanoporous membranes for blood oxygenation systems

An oxygenator is a medical device that aids in the exchange of oxygen and carbon dioxide in the human blood during procedures and illnesses that may cause the interruption or cessation of blood flow in the body. Porous membranes currently used in blood-oxygenation systems are bulky, and the pores are relatively large in dimension. The control over the pore size is poor due to the fabrication techniques used. The standard deviation of the pore size and the placement of the pores deteriorate even further with decreasing pore diameter. It is because of inaccurate pore placement and poor control over the porosity the oxygenation of blood in current oxygenation system is low. Control of the feature size and consistent topography of the nanopores will make long-term pulmonary blood oxygenation possible, which was not within reach previously. Fabrication of nanoscale features with precise placement and control have been realized with advancements in Microelectromechanical (MEMS) fabrication techniques. This led to the development of Micromachined nanoporous membranes for blood-oxygenators, which is the focus of this work. The blood oxygenator designs were modeled, simulated, and optimized to increase the gas exchange efficiency. In order to achieve increased blood gas exchange rate, a good balance between the blood and gas volume, and the surface area of gas exchange should be increased. The blood oxygenator designs were modeled to preserve microscale blood channel dimensions, thereby permitting the red cell shape change that enhances gas exchange in the pulmonary capillary. The membrane material was experimentally identified as low-pressure chemical vapor deposited (LPCVD) silicon nitride owing to its high stability, ease of deposition and characterization. Potassium hydroxide was chosen as the etchant for silicon because of its elevated etch selectivity in the (100) plane of silicon and high etch rate to form the channels for the flow of gas. Different techniques like Focused Ion Beam (FIB) and nanoimprinting were used to fabricate pores on the membrane. The membranes were simulated for displacement followed by stress/strain analysis. The porous membranes were experimentally examined for stability under pressure and the results were verified using FEM. The experimental results and the FEM analysis showed good agreement under normal operating conditions. Due to intrinsic problems in FIB like sequential drilling of the pores and inaccurate stage control, nanoimprinting was adopted for pore patterning and etching. Upon analysis, the imprinting indicated good transfer of pattern from the mold to the resist. The pore dimensions ranged from 180 nm--220 nm. Subsequently, nanoporous membranes were fabricated and the membranes were releases by bulk micromaching of silicon. Future work involves development of blood channels and manifolds for input and output of blood and gas channels.