In cases of severe lung or heart failure, extracorporeal membrane oxygenation (ECMO) is a life-saving therapy in which a patient’s blood is passed into a circuit outside of their body to provide respiratory support. The circuit’s main component is the membrane oxygenator that drives oxygen into the blood from a sweep gas source and removes excess carbon dioxide from the blood. At present, clinical use of ECMO is limited by its high risk profile, owing to two intertwined risks: thrombosis from the large circuit, and bleeding from the anticoagulation needed to prevent thrombosis. Improvements to the gas exchange efficiency and hemocompatibility of the oxygenator could enable the development of a longer-term supportive ECMO therapy, intended as a bridge-to-transplant or destination therapy for chronic lung failure. Here we describe a novel blood oxygenator concept based on parallel plate silicon membranes developed for high precision geometry, mechanical rigidity, and high efficiency membrane transport. Using these membranes, we create blood oxygenator prototypes consisting of arrays of silicon membranes, and endeavor to improve the efficiency and hemocompatibility of this concept.

First, multiple types of silicon membranes were evaluated systematically for mechanical rigidity and oxygen exchange efficiency, indicators of suitability for a future oxygenator. The combination of a silicon micropore membrane (SµM) and a 5 µm-thick polydimethylsiloxane (PDMS) layer maximized both qualities, withstanding over 260 cmHg of applied pressure and producing 0.03 mL/min of O2 flux. These membranes were then assembled into prototype flow cells, and tested for in vitro and in vivo oxygenation, successfully yielding an oxygen permeability of 1.92 ± 1.04 ml O2 STP/min/m2/cmHg. From this benchmark, we then attempted to optimize the surface hemocompatibility of the Si-PDMS composite through application of multiple polyethylene glycol (PEG)-based coatings. Although successful application of PEG to the surfaces was demonstrated, none of the coatings appeared to reduce protein adhesion to the SµM -PDMS membranes. Finally, we inserted turbulence-inducing spacer meshes into the channels of the SµM-PDMS prototypes to disrupt the transport boundary layer adjacent to the membranes, with the goal of substantially improving oxygenation. Though a threefold increase in oxygen flux was observed in vitro with the spacer meshes, the disruptive turbulence resulted in thrombosis and channel occlusion within the channels despite heavy anticoagulation of the blood. In summary, the work in this dissertation demonstrates the successful construction and testing of SµM-PDMS oxygenator prototypes, laying the foundation for future work to optimize this concept and create a large-scale blood oxygenator that can expand the clinical use of this life-saving therapy.

Macroencapsulation technology has been an attractive topic in the field of treatment for Type 1 diabetes due to mechanical stability, versatility, and retrievability of the macro-capsule design. Macro-capsules can be categorized into extravascular and intravascular devices, in which solute transport relies either on diffusion or convection. Failure of macroencapsulation strategies can be due to limited regenerative capacity of the encased insulin-producing cells, sub-optimal performance of encapsulation biomaterials, insufficient immunoisolation, excessive blood thrombosis for intravascular devices, and inadequate modes of mass transfer to support cell viability and function. However, significant technical advancements have been achieved in macroencapsulation technology, namely reducing diffusion distance for oxygen and nutrients, using pro-angiogenic factors to increase vascularization for islet engraftment, and optimizing membrane permeability and selectivity to prevent immune attacks from host's body. This review presents an overview of existing macroencapsulation devices and discusses the advances based on tissue-engineering approaches that will stimulate future research and development of macroencapsulation technology. This article is protected by copyright. All rights reserved.

Congenital Diaphragmatic Hernia (CDH) is a life-threatening developmental defect of the diaphragm. In CDH, a hole is formed in the diaphragm during fetal life and the abdominal organs pass through it, limiting the space the lungs have to grow. Roughly one in 3000 children born in the United States suffer from this disease, and one third of these will die as an infant. Those that do survive are often met with lifelong disability.

Over the last decades, fetal surgery---surgery performed on an unborn fetus---has been developed as an attempt to treat CDH. Specifically, the airway of a developing fetus can be obstructed with a balloon or other removable device, trapping the natural fluid of lung development inside the fetal lungs. While this has been shown to reverse some deleterious effects of CDH, there has not yet, to date, been a randomized controlled study demonstrating the efficacy of this treatment. This dissertation presents the first steps toward an evolution of this fetal therapy.

First is a discussion of the epidemiology and history of CDH and its fetal therapies. This is followed by a primer to those concepts of pulmonary physiology relevant to CDH and fetal surgery. Subsequently, a novel fetal therapeutic device is discussed, representing both a new therapy and a window of scientific inquiry into the origins of CDH. The creation of specific scientific instruments to measure pulmonary development is then expounded, followed by methods of analyzing data from these instruments. The dissertation is concluded with a physiologic analysis of these data, and the proposal of a novel design methodology for sensors for clinical studies.

Respiratory diseases are a leading cause of death worldwide. Despite modern antibiotics,

treatment of pneumonia and other lung diseases is often limited by the

tools available to diagnose these disorders in low resource settings. While tools such

as chest x-ray and CT scans are highly accurate, their high cost provides a high barrier

for many patient populations. On the other hand, the physical exam provides

a time-honed method for diagnosis of many common lung diseases. Unfortunately,

due to limited sensitivity, the pulmonary physical exam is often insufficient for diagnosis.

The goal of the research presented in this dissertation is to take advantage of

the simplicity of the clinical physical exam, but to quantify its findings using modern

sensors. We present a standardized approach to analysis which characterizes

healthy from diseased lung with 91.7% accuracy. For pneumonia specifically, we

demonstrated 92.3% accuracy in distinguishing between healthy subjects and pneumonia

subjects in a pilot study. In addition to these findings, we also review this

work in the context of the current work in the field and provide suggestions for next

steps to continue quantified acoustic analysis of the lungs.

Silicon-based bio-microelectromechanical systems (bioMEMS) have become increasingly attractive for medical implants due to their relative low cost and capacity for fabrication of intricate micro- and nanofeatures. However, when silicon surfaces encounter blood, device failure can occur as plasma proteins adhere to the surface and the coagulation cascade is activated leading to thrombosis. Modifying silicon surfaces with hemocompatible coatings may reduce protein adsorption and platelet activation, increasing the lifetime of the implant.

Using bioMEMS technology, our group has developed silicon nanopore membranes (SNMs) for application in bioartificial organs. Highly uniform pores of controllable size make SNMs attractive for immunoisolation and blood filtration. To improve hemocompatibility of SNMs, this dissertation investigates sub-5 nm biomimetic polymer surface coatings. Specifically, two ultrathin zwitterionic coatings, poly(sulfobetaine methacrylate) (pSBMA) and poly(2-methacryloyloxyethyl phosphorylcholine) (pMPC), were developed and characterized via x-ray photoelectron spectroscopy, contact angle, atomic force microscopy and ellipsometry. The coatings were tested under biological shear conditions and after exposure to five standard sterilization methods. Anti-fouling and hemocompatible characteristics of the coatings were evaluated by measuring protein adsorption from single protein solutions of human serum albumin and fibrinogen, and examining platelet activation after exposure to fresh human blood and implantation in porcine model for up to 26 days.

Characterization results showed successful sub-5 nm pSBMA and pMPC surface modifications. In this thickness range, SNMs remained patent and modified silicon substrates resisted bio-fouling from single protein solutions of human albumin and fibrinogen, with pSBMA reducing protein adsorption by >80% compared to uncoated silicon. Shear data suggests the coatings are robust and functional following shear rates up to 2000/s over 24 hours. Additionally, they withstood sterilization procedures, with best performance of pSBMA and pMPC after electron-beam sterilization and ethylene oxide gas treatment, respectively. Scanning electron microscopy and immunohistochemistry following exposure to fresh human blood flow demonstrated pSBMA-silicon reduced platelet adhesion and activation by >97%. Porcine implants with pSBMA-silicon substrates remained patent over 26 days, with no gross clots in the flow path and minimal presence of blood activation. These promising in vitro and in vivo results will lead to further studies coupling SNM filtration and surface coatings, as well as clinical testing to determine the human response for use in bioartificial kidney and pancreas device applications.

The goal of this dissertation is to develop a bioartificial pancreas device consisting of silicon nanopore membrane (SNM)-encapsulated insulin-producing pancreatic islets under convective transport to treat Type 1 Diabetes (T1D). Problems associated with islet transplantation for T1D such as shortage of donor cells, and use of immunosuppressive drugs remain as major challenges. Immune isolation using encapsulation may circumvent the use of immunosuppressants and prolong the longevity of transplanted islets. The encapsulating membrane must block the passage of host’s immune components while providing sufficient exchange of glucose, insulin and other small molecules.

This research effort investigated the feasibility of encapsulating islets with SNM to provide: (1) middle molecule selectivity against pro-inflammatory cytokines; and (2) sufficient nutrients and oxygen with convective transport to overcome the mass transfer limitations associated with diffusion through nanometer-scale pores. The selectivity analysis revealed 80% reduction in cytokines passage through SNM under convective transport. Moreover, the SNM protected encapsulated islets from infiltrating cytokines and retained islet viability and remained responsive to changes in glucose levels unlike non-encapsulated controls. The glucose-stimulated insulin response showed that membrane-encapsulation of islets with convection outperformed the diffusive conditions in terms of the magnitude of insulin secreted (1.49-fold increase in stimulation index & 3.86-fold decrease in shut-down index) and the rate of insulin production during high (1.19-fold increase) and low glucose (6.45-fold decrease) challenges. As a result of these data, the small-scale SNM-based intravascular bioartificial pancreas (iBAP) device was presented to support high cell viability and function at clinically relevant islet densities utilizing convective mass transfer both in vitro and in vivo. The hemocompatibility of the iBAP blood flow path after intravascular implantation was also demonstrated in the porcine model. These studies address the critical challenges faced by macroencapsulation and pave the way for the development of a full-scale SNM-encapsulated iBAP to treat T1D in the future.

Extracorporeal membrane oxygenation (ECMO) is a life support technology capable of providing full replacement of respiratory and, if necessary, cardiac function. ECMO allows a patient to remain hemodynamically and metabolically stable while recovering from acute respiratory distress or surgical intervention, or while awaiting a lung transplant. In essence, ECMO provides time to heal.

In its simplest form, an ECMO circuit withdraws blood with low oxygen content and high carbon dioxide content from the body and passes it over a gas exchange membrane. A continuously flowing gas supply, or sweep gas, flows on the opposite side of the membrane from the blood. The gas concentration gradients existing between the blood and sweep gas enables diffusive transport of oxygen into the blood (oxygenation), while carbon dioxide diffuses out of the blood (ventilation). The oxygenated and ventilated blood is returned to the patient’s systemic circulation, where it can satisfy the metabolic needs of tissues and organs.

While a number of vital components comprise an ECMO circuit, including blood pumps, heat exchangers, and blood-contacting tubing, the most critical part is the oxygenator, which contains the gas exchange membranes and guides blood flow in contact with the membrane surface. Current clinically used oxygenators typically employ hollow fiber membranes; blood flows around bundles of thousands of individual membrane fibers while sweep gas flows through the fiber lumens. This design provides a large cross-sectional area for the blood flow, with millions of potential paths for red blood cells to pass through the interwoven plastic fibers, resulting in a low pressure drop and correspondingly low fluid shear stress through the oxygenator. However, this design consequently allows blood to follow a path of least resistance through the oxygenator, leading to incomplete utilization of the gas exchange capability of all hollow fibers. This inefficiency requires the use of excessive membrane area, on the order of 1 m2, to meet the gas exchange requirements of adult patients.

The purpose of the work outlined in this dissertation is to evaluate an alternative gas exchange membrane, in which a micron-thin polydimethylsiloxane (PDMS) film is mechanically supported by a silicon micropore membrane (SμM) with highly uniform pore geometry and distribution. The PDMS-SμMs revisit some of the earliest work on membrane oxygenators using new technologies derived from the semiconductor industry and the field of soft lithography to create a robust, planar gas exchange membrane. These membranes allow rapid gas exchange per unit area and also open the possibility for new oxygenator geometries in which blood flow is tightly controlled, as in the alveolar capillaries of the native lung, enabling highly efficient gas exchange in parallel plate oxygenators with surface areas less than one tenth of existing designs.

Using microfabrication techniques, I fabricated an oxygenation membrane consisting of a 4.63 μm thick PDMS film supported by a rigid SμM with 500 nm wide rectangular pores, which provided a planar gas exchange surface without the need for structural supports in the blood channel, capable of tolerating transmembrane pressure loads of 76 cmHg. Membrane mass transfer coefficients for oxygen and carbon dioxide were measured to be 5.49 ± 0.89 and 31.76 ± 0.80 mL STP min-1 m-2 cmHg-1, respectively. As a proof of concept, full oxygen saturation of porcine whole blood was achieved in a miniaturized ex vivo circuit for 3 hours without formation of gross clots, as well as demonstration of respiratory assist in a live ovine model, with an observed maximum oxygen exchange with blood of 1.94 ± 0.02 mL min-1 m-2 cmHg-1.

Introduction

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Expert opinion

It is important to accurately model drug disposition in the preclinical setting before novel drug compounds are introduced into humans. However, it is evident from current statistics on the drug development process that there is much room for improvement in the preclinical assessment of drug candidates in order to avoid clearance or toxicity issues when the drug is used in the clinic. One organ that plays a significant role in drug disposition and is therefore important to model is the kidney, which filters blood and is responsible for elimination of over one third of all drugs. The focus of this dissertation was to improve in vitro modeling of the kidney. First, multiple proximal tubule cell lines were systematically compared in order to identify the most physiologically relevant line. It was found that while the human cell lines hold some promise, non-human transfected cell lines remain more robust models of proximal tubule drug transport. Compared to the human cell lines, non-human transfected cell lines have superior monolayer formation, higher levels of transporter expression and function and are easier to grow in culture. Second, the role of one particular aspect of the in vivo environment, shear stress, was studied to better understand the effect on drug transporters and the possible biological pathways involved. Results indicated that shear stress affects active transport of organic cations and expression of relevant transporters in renal cells in a cilia dependent manner. Lastly, the design of a 1:1000 scaled model of the kidney that can be incorporated into a 1:1000 scaled human-on-a-chip system is presented along with the results of initial prototyping and future directions. In summary, the studies in this dissertation provide insight into the limitations of existing renal cell models, expand our current knowledge of the interaction between the cellular environment and drug transport functionality and provide a foundation for the future development of a scaled kidney on a chip module.