Compliant Membrane Research Articles

Compliant membranes improve resolution in full-wafer micro/nanostencil lithography

This work reports on a considerable resolution improvement of micro/nanostencil lithography when applied on full-wafer scale by using compliant membranes to reduce gap-induced pattern blurring. Silicon nitride (SiN) membranes are mechanically decoupled from a rigid silicon (Si) frame by means of four compliant, protruding cantilevers. When pressing the stencil into contact with a surface to be patterned, the membranes thus adapt to the surface independently and reduce the gap between the membrane and the substrate even over large, uneven surfaces. Finite element modeling (FEM) simulations show that compliant membranes can deflect vertically 40 μm which is a typical maximal non-planarity observed in standard Si wafers, due to polishing. Microapertures in the stencil membrane are defined by UV lithography and nanoapertures, down to 200 nm in diameter, using focused ion beam (FIB). A thin aluminium (Al) layer is deposited through both compliant and non-compliant membranes on a Si wafer, for comparison. The blurring in the case of compliant membranes is up to 95% reduced on full-wafer scale compared to standard (non-compliant) membranes.

NON-NEWTONIAN FLOWS IN CONTRACTING/EXPANDING BLOOD VESSELS: A THEORETICAL STUDY

The paper deals with a problem related to the flow of blood in vessels. It is known that physiological pumps produce flow by alternate contraction and expansion of the vessel. When muscles start squeezing the vessel wall, the valve at the upstream end gets closed and that at the downstream end becomes open. As a result of this, blood is pumped out in the downstream direction. During systole, when the blood in the left ventricle is forced into the aorta, the mitral valve becomes closed, while the atrioventricular valve becomes open. In a situation such as this, the left ventricle takes the form of a vessel with one end closed. In order to study such a situation, a model has been developed here that consists of a fluid-filled long cylindrical pipe with one end closed by a compliant membrane (that prevents the axial motion of the fluid, leaving radial motion completely unrestricted). An approximate solution of the Navier–Stokes equation for the flow of a second-order fluid is presented. The derived analytical expressions are computed numerically. The computed values indicate that if the Reynolds number of the flow is low, the effect of fluid viscoelasticity appears in higher-order terms.

Contractile cell forces exerted on rigid substrates

Adhesive cells including fibroblasts produce contractile forces to the underlying substrate for their locomotion. Such forces are not only high enough to deform compliant membranes such as silicone but also bend rigid micro-cantilevers. The cell-induced bending of silicon micro-cantilevers was determined using laser deflection during trypsin treatment. The observed cantilever relaxation corresponds to a contractile cell force of (16±7) µN per rat-2 fibroblast. The cantilever bending approach represents a unique method for the determination of contractile cell forces on any kind of rigid substrate in desired (physiological) environment. Hence, the fundamental technique allows building cell-based biosensors or should provide a quantitative parameter for characterising the cyto-compatibility of load bearing implant surfaces.

Underwater energy harvesting from a heavy flag hosting ionic polymer metal composites

In this paper, we analyze underwater energy harvesting from the flutter instability of a heavy flag hosting an ionic polymer metal composite (IPMC). The heavy flag comprises a highly compliant membrane with periodic metal reinforcements to maximize the weight and minimize the bending stiffness, thus promoting flutter at moderately low flow speed. The IPMC is mechanically attached to the host flag and connected to an external load. The entire structure is immersed in a mean flow whose intensity is parametrically varied to explore the onset of flutter instability along with the relation between the vibration frequency and the mean flow speed. Manageable theoretical models for fluid-structure interaction and IPMC response are presented to inform the harvester design and interpret experimental data. Further, optimal parameters for energy scavenging maximization, including resistive load and flow conditions, are identified.

Multiaxial Mechanical Characterization of Interpenetrating Polymer Network Reinforced Acrylic Elastomer

The acrylic elastomer membrane VHB 4910 is a material widely used for applications as Dielectric Elastomer Actuators DEA. For suitable actuation performance however, it is necessary to pre-strain the very compliant membrane. This reduces the lifetime of DEA due to early failure of the tensioned membrane. Interpenetrating Polymer Network Reinforced Acrylic Elastomers (IPN) are produced by introducing a curable additive into the pre-strained acrylic elastomer membrane. While curing at elevated temperature, the additive forms a second polymeric network that supports part of the pre-strain in the acrylic membrane. This leads to a free standing material that combines the actuation performance of pre-strained VHB 4910 with an excellent long-term reliability. This work presents a detailed mechanical characterization of acrylic IPN membranes. To reduce the experimental effort required to characterize the nonlinear elastic behavior, we developed a unique specimen design that enables the assessment of uni- and biaxial stress states within one experiment. Slight changes in the material composition of IPN-membranes lead to substantial variations in their mechanical properties. The extraction of material behavior in different kinematic states within a single sample thus reduces the uncertainty on the determination of constitutive models. An extensive experimental campaign was carried out involving uniaxial and equibiaxial tension and relaxation. Image based local deformation measurements and iterative finite element calculations were applied to derive constitutive model parameters that describe the mechanical response in a wide range of planar strain and strain rate.

Quantification and significance of fluid shear stress field in biaxial cell stretching device

A widely used commercially available system for the investigation of mechanosensitivity applies a biaxial strain field to cells cultured on a compliant silicone substrate membrane stretched over a central post. As well as intended substrate strain, this device also provides a fluid flow environment for the cultured cells. In order to interpret the relevance of experiments using this device to the in vivo and clinical situation, it is essential to characterise both substrate and fluid environments. While previous work has detailed the substrate strain, the fluid shear stresses, to which bone cells are known to be sensitive, are unknown. Therefore, a fluid structure interaction computational fluid dynamics model was constructed, incorporating a finite element technique capable of capturing the contact between the post and the silicone substrate membrane, to the underside of which the pump control pressure was applied. Flow verification experiments using 10-μm-diameter fluorescent microspheres were carried out. Fluid shear stress increased approximately linearly with radius along the on-post substrate membrane, with peak values located close to the post edge. Changes in stimulation frequency and culture medium viscosity effected proportional changes in the magnitude of the fluid shear stress (peak fluid shear stresses varied in the range 0.09-3.5Pa), with minor effects on temporal and spatial distribution. Good agreement was obtained between predicted and measured radial flow patterns. These results suggest a reinterpretation of previous data obtained using this device to include the potential for a strong role of fluid shear stress in mechanosensitivity.

A Preliminary Theoretical Model of Hydrodynamics in the Inner Ear

Head movement should create a transient pressure imbalance across the membranous inner ear. We used basic concepts of fluid dynamics to develop a theoretical model of the inner ear. According to this model, two contiguous fluidic systems-the perilymphatic system and the endolymphatic system-are in hydrostatic equilibrium across a compliant membrane. Our model demonstrates that changes in resistance or compliance in one system results in a transient distortion of the membranous inner ear until equilibrium between the two systems is restored. The concept of hydrodynamic pressure changes in the inner ear has received little attention, but it may represent a new approach to understanding the inner ear and treating inner ear diseases.

Oscillatory motions of a prestrained compliant membrane caused by fluid–membrane interaction

Flow over a compliant membrane is a complex problem where the interaction between fluid and membrane determines the nature of the aerodynamic characteristics of the membrane wing. This investigation is concerned with the deformation and oscillatory motion of a membrane under aerodynamic loading. The approach is computational, but the analytical solution is also presented for a constant pressure loading. The computational results are compared with the experimental data available in the literature as well as with the present analytical solution. In this study, the values of Reynolds number are 38 416 and 141 500, and the angle of attack and prestrain range from 10° to 40° and from 0 to 0.074, respectively. This range of parameters makes the outcome of the investigation more relevant to applications involving the flight of micro air vehicles and the membrane wings of flying mammals such as bats. The computations indicate a mostly asymmetric deflection with the point of maximum camber located nearly at 40% of the chord length from the leading edge. The deflection is decreased with prestrain, and it is increased with Reynolds number. Moreover, the lift coefficient generally increases with the angle of attack. However, for Re=141 500, it increases first to a peak at 20–30° angle of attack, and then decreases. The drag coefficient is much higher than that of conventional airfoils. The membrane oscillates in the streamwise and vertical directions. The largest amplitude of oscillations is observed at 40° for Re=38 416. The oscillations are caused by the oscillatory nature of the flow due to fluid–membrane interaction and the formation of the leading edge and trailing edge vortices. Compared with a rigid membrane of the same camber, the compliant membrane has a smaller recirculation region which may lead to a delayed stall.

The Cardiac MR Images and Causes of Paradoxical Septal Motion

Real-time cine MRI studies using the steady-state free precession (SSFP) technique are very useful for evaluating cardiac and septal motion. During diastole, the septum acts as a compliant membrane between the two ventricles, and its position and geometry respond to even small alterations in the trans-septal pressure gradients. Abnormal septal motion can be caused by an overload of the right ventricle, delayed ventricular filling and abnormal conduction. In this study, we illustrate, based on our experiences, the causes of abnormal septal motion such as corrective surgery for tetralogy of Fallot, an atrial septal defect, pulmonary thromboembolism, mitral stenosis, constrictive pericarditis and left bundle branch block. In addition, we discuss the significance of paradoxical septal motion in the context of cardiac MR imaging.

A Latchable Phase-Change Microvalve With Integrated Heaters

This paper presents a latchable phase-change microvalve with integrated microheaters, which is suitable for lab-on-a-chip systems where minimal energy consumption is desired. The microvalve exploits low-melting-point paraffin wax, whose solid-liquid phase changes allow switching of fluid flow through deformable microchannel ceiling. Switching is initiated by melting of paraffin through an integrated microheater, with an additional pneumatic pressure used for the open-to-close switching. The valve consumes energy only during initiation of valve switching. When paraffin solidifies, the switched state is maintained passively. The microvalve was fabricated from polydimethylsiloxane through multilayer soft lithography techniques. Experiments show that the valve can switch flow within 4-8 s due to the small thermal mass and localized melting of paraffin wax; when closed, the valve can passively withstand an inlet pressure over 50 kPa without leakage. Time response of the valve can be further improved with improved heater and wax chamber designs, while the latching ability can be improved by optimizing the wax chamber/membrane design. Compared to existing latchable phase-change valves, the microvalve has no risk of cross-contamination. In addition, the improved sealing offered by the compliant membrane makes the valve robust and flexible in operation, allowing large ranges of initiation pressure from various actuation schemes.

Characteristics of highly flexible PDMS membranes for long-term mechanostimulation of biological tissue.

Measurement of mechanical properties of soft biological tissue remains a challenging task in mechanobiology. Recently, we presented a bioreactor for simultaneous mechanostimulation and analysis of the mechanical properties of soft biological tissue samples. In this bioreactor, the sample is stretched via deflection of a flexible membrane. It was found that the use of highly compliant membranes increases accuracy of measurements. Here, we describe the production process and characteristics of thin and flexible membranes of polydimethylsiloxane (PDMS) designed to improve the signal-to-noise ratio of our bioreactor. By a spin-coating process, PDMS membranes were built by polymerization of a two component elastomer. The influence of resin components proportion, rotation duration, and speed of the spinning were related to the membrane mechanics. Membranes of 22 mm inner diameter and 33 to 36 microm thickness at homogeneous profiles were produced. Isolated rat diaphragms served as biological tissue samples. Mechanical properties of the membranes remained constant during 24 h of mechanostimulation. In contrast, time- and strain-dependent mechanical properties of the diaphragms were found.

A Compliance-Based Microflow Stabilizer

This paper presents a microfluidic device that can passively reduce fluctuations in an upstream fluid flow and generate a largely steady flow in a microfluidic system. The device features a series of compliant membranes that form deformable walls of a microchannel. For an incoming pulsatile flow, passive vibrations of the membranes allow the fluid to be accumulated for an overflow and discharged for an underflow, resulting in drastically reduced fluctuations in flow rate and pressure. A lumped-parameter model is developed to simulate the device characteristics, in which the coupling of membrane vibrations and fluid flow in individual channel sections associated with a single membrane is first represented by squeeze-film theory and inertia-free structural dynamics. The entire device is next represented as a system by connecting individual channel sections in series. The model is numerically solved, and the results agree with experiments. The theoretical and experimental results both show that a five-membrane device can stabilize a pulsating flow by a factor of about 20. These results also reveal that smaller average flow rate, smaller fluctuation, and higher fluctuation frequency lead to more effective flow stabilization. With these characteristics, the flow stabilizer can be used to obtain steady flow in microfluidic systems.

Patient-Specific Compliant Vessel Manufacturing Using Dip-Spin Coating of Rapid Prototyped Molds

A procedure for manufacturing cardiovascular system models using patient-specific data, rapid prototyping, and a multistep dip-spin coating process is presented here. Improvements to a previously developed process permitted the fabrication of flexible complex vascular replicas. The primary improvement included the development of a two-axis rotation mechanism that enabled a pseudorandom rotation of the coated mold in space, providing uniform coats. Other improvements included the use of a low viscosity (1500–2000cP) silicone solution that allowed for complete coverage of the mold, and developing a procedure for fixing defects. The dip-spin coating procedure was shown to be effective for the manufacture of compliant cardiovascular membranes, such as an arterial bypass graft with an internal flow passage and an abdominal aorta with nonuniform radial geometry, tapered diameters, bifurcations, and small branches. Results from a design of experiments comparing two dipping setups demonstrated that horizontally dipping the model produced coatings with more uniform thicknesses along the length of the model when compared to vertical dipping. For a 250-mm-long model, the difference in thickness between the top and bottom of the membrane was 0.42±0.069mm and 0.09±0.077mm for vertical and horizontal dippings, respectively. Mold diameter also affected the thickness of the membrane, with membrane thickness increasing as mold diameter decreased. Thickness data comparing locations at approximately the same height of the mold but with different diameters showed thicknesses of 2.54±0.198mm and 1.95±0.140mm for 7.85mm and 15.20mm diameters, respectively. Moreover, the differences in thickness between these locations were 0.60±0.128mm and 0.58±0.231mm for vertical and horizontal dippings, respectively; thus, membrane thickness variations occurred with mold diameter irrespective of the dipping setup. Depending on the prescribed tolerance for membrane thickness, the vertical dipping setup may be recommended for use because (1) it was easier to use since only the mold was immersed in the coating solution and no special protection of the dipping mechanism was required and (2) it produced fewer defects in the coatings since the solution always dripped from downfacing surfaces of the mold. Using this dip-spin coating procedure, patient-specific cardiovascular membranes can be manufactured and used in the development of medical devices, research requiring accurate anatomical models, and education and training.

Aeromechanics of Membrane Wings with Implications for Animal Flight

Bats and other flying mammals are distinguished by thin, compliant membrane wings. In an effort to understand the dependence of aerodynamic performance on membrane compliancy, wind-tunnel tests of low-aspect-ratio, compliant wings were conducted for Reynolds numbers in the range of 0.7-2.0 x 10 5 . The lift and drag coefficients were measured for wings of varying aspect ratio, compliancy, and prestrain values. In addition, the static and dynamic deformations of compliant membrane wings were measured using stereo photogrammetry. A theoretical model for membrane camber due to aerodynamic loading is presented, indicating that the appropriate nondimensional parameter describing the problem is a Weber number that compares the aerodynamic load to the membrane elasticity. Excellent agreement between the theory and experiments is found. Measurements of aerodynamic performance show that, in comparison with rigid wings, compliant wings have a higher lift slope, maximum lift coefficients, and a delayed stall to higher angles of attack. In addition, they exhibit a strong hysteresis both around a zero angle of attack as well as around the stall angle. Unsteady membrane motions were also measured, and it is observed that the membrane vibrates with a spatial structure that is closely related to the free eigenmodes of the membrane under tension and that the Strouhal number at which the membrane vibrates rises with the freestream velocity, coinciding with increasing multiples of the natural frequency of the membrane.

Buckling pressure of “pumpkin” balloons

This paper presents a computational study of the critical buckling pressure of pumpkin balloons, which consist of a thin, compliant membrane constrained by stiff meridional tendons. The n-fold symmetric shape of a pumpkin balloon with n identical lobes is exploited by adopting a symmetry-adapted coordinate system, which leads to the tangent stiffness matrix in an efficient block-diagonal form; the smallest eigenvalue of a particular block leads to the buckling pressure for the balloon. Two different types of balloon design are considered. Extensive results are obtained for the buckling pressures of a set of 10 m diameter experimental balloons and also for an 80 m diameter flight balloon. The key findings are as follows: the same type of buckling mode, forming four circumferential waves is critical for most of the balloons that have been analysed; balloons with flatter lobes are more stable, and the buckling pressure varies with an inverse power-law of the number of lobes; increasing the Young’s modulus, the Poisson’s ratio of the membrane, or the diameter of the end fitting has the effect of increasing the buckling pressure; but increasing the axial stiffness of the tendons has the effect of decreasing the buckling pressure.

A latchable microvalve using phase change of paraffin wax

This paper presents a novel latchable phase-change microvalve that can potentially be used for flow gating in portable lab-on-a-chip systems where minimal energy consumption is desired. The microvalve innovatively exploits paraffin wax of low melting point, whose solid–liquid phase changes allow the closing and opening of fluid flow through deformable microchannel ceiling. The latchable phase-change actuation scheme can be used in an active valve to shut off and open fluid flow. Valve switching is initiated by melting of paraffin through heating, with an additional pneumatic pressure used for valve switching from open to closed state. Energy consumption is only required during the valve switching. After paraffin solidifies the switched state is subsequently maintained passively without further consumption of energy. The microvalve can be fabricated from PDMS through the multilayer soft lithography technique. Testing results demonstrate that the valve has response times of 60 s for closing and 100 s for opening; when closed, the valve can passively withstand a pressure up to 35 kPa without significant leakage. The relatively slow time response of the proof-of-concept device can be readily improved by integrating on-chip heaters, while the latchability of the microvalve can be improved by optimizing the wax chamber and membrane design. Compared to existing latchable phase-change valves, the thin compliant channel ceiling of the valve separates the fluid channel from the wax chamber. Therefore, the microvalve is free of potential contamination of the fluid by the paraffin wax. In addition, the improved sealing offered by the compliant membrane makes the valve robust and flexible in operation, allowing large ranges of initiation pressure generated from various actuation schemes.

Technique for the calibration of hydrophones in the frequency range 10 to 600 kHz using a heterodyne interferometer and an acoustically compliant membrane

A technique for the calibration of hydrophones using an optical method is presented. In the method, a measurement is made of the acoustic particle velocity in the field of a transducer by use of a thin plastic pellicle that is used to reflect the optical beam of a laser vibrometer, the pellicle being acoustically transparent at the frequency of interest. The hydrophone under test is then substituted for the pellicle, and the hydrophone response to the known acoustic field is measured. A commercially available laser vibrometer is used to undertake the calibrations, and results are presented over a frequency range from 10 to 600 kHz. A comparison is made with the method of three-transducer spherical-wave reciprocity, with agreement of better than 0.5 dB over the majority of the frequency range. The pellicle used is in the form of a narrow strip of thin Mylar©, and a discussion is given of the effect of the properties of the pellicle on the measurement results. The initial results presented here show that the method has the potential to form the basis of a primary standard method, with the calibration traceable to standards of length measurement through the wavelength of the laser light.

Compliant and low-cost humidity nanosensors using nanoporous polymer membranes

This paper proposes non-fragile compliant humidity nanosensors that can be fabricated inexpensively on various types of nanoporous polymer membranes such as polycarbonate, cellulose acetate, and nylon membranes. The nanosensor contains a pair of interdigitated electrodes deposited on the nanoporous polymer membranes. The resistance and/or capacitance between these electrodes vary at different humidity levels with a very high sensitivity due to the water adsorption (capillary condensation) inside the nanopores. The proposed sensors are low-cost in both material and fabrication. Due to its compliance, the sensors can be suitable for certain applications such as in situ water leakage detection on roofs, where people can walk on top of them. Testing results demonstrated that the sensor changes resistance within large range of relative humidity (RH) values (40–100% RH) with very high sensitivity. Design A and B sensors exhibit high sensitivities, 2.5 and 1.3 GΩ/% RH, respectively, to relative humidity changes but are not linear in response till 55% RH is reached ( R 2 ∼ 0.7391 for design A, R 2 ∼ 0.8824 for design B). For the design C sensors, sensitivity is around 20 GΩ/% RH and the response is highly linear ( R 2 ∼ 0.96) for 40–100% relative humidity range. These results showed the feasibility of the proposed compliant and low-cost nanopore polymer membrane based humidity nanosensors which could be used for in situ water leakage and humidity level detection on three-dimensional and complex surfaces such as roofs and airplane bodies in the near future.

Spatial direct numerical simulation of transitional boundary layer over compliant surfaces

An iterative implicit fractional step method is developed and employed for the simulation of transitional boundary layer over compliant surfaces. The three-dimensional perturbation Navier–Stokes equations are discretized by curvilinear finite volumes on a collocated grid system. A multigrid procedure is used for computations associated with the pressure-Poisson equation, while simulation is carried out by MPI-based parallel computation with domain decomposition. Results in the literature for oblique linear waves and the non-linear breakdown of a wave triad over a rigid wall are repeated to check the accuracy of the codes that had been developed. Oblique linear TS (Tollmien–Schlichting) waves over finite-length compliant membranes are generally found to coexist with CIFI (compliance induced flow instability) or FISI (flow induced surface instabilities) waves. The latter waves usually possess longer wavelengths and thus propagate at a larger oblique wave angles than the TS waves. Simulation reveals that compliant surfaces may slow down the development of secondary instabilities during the early stages of laminar–turbulent transition. However, during the later stages of fundamental wave breakdown, interactions with CIFI and edge-generated waves may increase the amplitudes of the original 2D and 3D TS waves, leading to an earlier breakdown on compliant surfaces. Linear interaction between the flow and compliant membranes has been assumed.

Biological inferences from a mathematical model of comedo ductal carcinoma in situ of the breast

The growth of a tumour in a duct is examined in order to model ductal carcinoma in situ (DCIS) of the breast, the earliest known stage of breast cancer. Interactions between the expansive forces created by tumour cell proliferation and the stresses that develop in the compliant basement membrane are studied using numerical and analytical techniques. Particular attention focuses on the impact that proteolytic enzymes have on the tumour's progression. As the tumour expands and the duct wall deforms, the tumour cells are subjected to mechanical and nutritional stresses caused by high pressures and oxygen deprivation. Such stresses may stimulate the cells to produce proteolytic enzymes that degrade the duct wall, making it more compliant and prone to penetration by the tumour cells. We use our model to compare these two hypotheses for enzyme production and find that mechanical stress is likely the dominant mechanism, with the wall deforming most at the centre of the duct. We then discuss the biological implications of our theoretical results and suggest possible directions for future work.