Stephen A. Sarles

Hair cell inspired mechanotransduction with a gel-supported, artificial lipid membrane

A gel-supported lipid bilayer formed at the base of an artificial hair is used as the transduction element in an artificial, membrane-based hair cell sensor inspired by the structure and function of mammalian hair cells. This paper describes the initial fabrication and characterization of a bioderived, soft-material alternative to previous artificial hair cells that used the transduction properties of synthetic materials for flow and touch sensing. Under an applied air flow, the artificial hair structure vibrates, triggering a picoamp-level electrical current across the lipid bilayer. Experimental analysis of this mechanoelectrical transduction process supports the hypothesis that the current is produced by a time-varying change in the capacitance of the membrane caused by the vibration of the hair. Specifically, frequency analysis of both the motion of the hair and the measured current show that both phenomena occur at similar frequencies (0.1–1.0 kHz), which suggests that changes in capacitance occur as a result of membrane bending during excitation. In this paper, the bilayer-based hair cell sensor is experimentally characterized to understand the effects of transmembrane potential, the applied air flow, and the dimensions of the hair.

Membrane-based biomolecular smart materials

Membrane-based biomolecular materials are a new class of smart material that feature networks of artificial lipid bilayers contained within durable synthetic substrates. Bilayers contained within this modular material platform provide an environment that can be tailored to host an enormous diversity of functional biomolecules, where the functionality of the global material system depends on the type(s) and organization(s) of the biomolecules that are chosen. In this paper, we review a series of biomolecular material platforms developed recently within the Leo Group at Virginia Tech and we discuss several novel coupling mechanisms provided by these hybrid material systems. The platforms developed demonstrate that the functions of biomolecules and the properties of synthetic materials can be combined to operate in concert, and the examples provided demonstrate how the formation and properties of a lipid bilayer can respond to a variety of stimuli including mechanical forces and electric fields.

Tailored Current―Voltage Relationships of Droplet-Interface Bilayers Using Biomolecules and External Feedback Control

The development of a new class of active material based on the ion transport properties of functional biomolecules is introduced in this work. The new class of materials utilizes a recently developed technique known as the droplet-interface bilayer (DIB) to enable the reconstitution of biomolecules into a durable matrix. Methods to modify the current— voltage relationship across the bilayer, including the incorporation of proteins and the use of an external feedback loop, are explored. Electrical impedance spectroscopy and cyclic voltammetry measurements are used to characterize the bilayers and indicate that a single DIB can be modeled as a resistor in parallel with a capacitor. Alpha-hemolysin proteins from Staphyloccus aureus cause a reduction in the resistance of the bilayer and exhibit current-rectification at positive cis potentials. Alamethicin proteins from Trichoderma viride produce a voltage-dependent conductance allowing the specific resistance (MΩ cm2) of the bilayer to be varied reversibly by 2—3 orders of magnitude. Feedback integral current control is demonstrated on pure 1,2-diphytanoyl-sn-glycero-3-phosphocholine DIBs and provides accurate current tracking at a driving rate of 10 mHz and less. Proportional—integral voltage control applied to a DIB establishes a second-order frequency response where the natural frequency and damping ratio of the resonance can be selected.

Consolidation of U-Nyte® Epoxy-Coated Carbon-Fiber Composites via Temperature-Controlled Resistive Heating

Temperature-controlled internal resistive heating creates on-command rigidizable materials for structural consolidation of ultra-lightweight, inflatable space structures. A PAN-based carbon fiber tow coated with a novel, low cure-temperature thermosetting resin (Hydrosize U-Nyte® Set 201 epoxy binder) was investigated for consolidation through internal resistive heating. Precise, proportional-integral (PI) temperature tracking was achieved for controlled sample heating and used to prescribe intelligently-designed curing profiles to cause resin consolidation and curing. Rigidized samples were evaluated by measuring the increase in bending stiffness as well as verifying resin cure completion through DSC. The permanent strength gained through active rigidization via internal resistive heating was demonstrated on a small, inflatable structure.

Formation and Encapsulation of Biomolecular Arrays for Developing Arrays of Membrane-Based Artificial Hair Cell Sensors

Recent research in our group has shown that artificial cell membranes formed at the base of a hair-like structure can be used to sense air flow in a manner similar to the mechanotransduction processes found in mammalian hair cells. Our previous work demonstrated that a single artificial hair cell can be formed in an open substrate. However, that study also motivated the need to develop fully-encapsulated devices that feature arrays of hair-cells. Since the transduction element in this concept is an artificial cell membrane, or lipid bilayer, this work investigates two parallel substrate designs for providing encapsulation and a method for forming arrays of bilayers. In one effort, a flexible substrate with internal compartments for hosting the biomolecules and mating cap are constructed and experimentally characterized. The regulated attachment method (RAM) is used to form interface bilayers within the sealed device. Capacitance measurements of the sealed interface bilayer show that the sealing cap slightly compresses the bottom insert and reduces the size of the enclosed bilayer. Single channel measurements of alamethicin peptides further verify that the sealed device, which is also leak-proof under water, can be used to detect the insertion and gating activity of transmembrane proteins in the membrane. The second effort pursued herein is the fabrication and initial testing of a method to form arrays of interface bilayers by using anchored hydrogel pads that support curved aqueous lenses in oil. In this fashion, the configuration of the array does not require manipulating droplets, but instead depends on the arrangement of the built-in gels used to support the aqueous lenses. As with RAM, mechanical force is used to promote contact of adjacent aqueous lenses held in the flexible substrate. Initial tests show that gel-supported lenses can be used for forming multiple lipid bilayers within the device and that these interfaces can be interrogated individually or collectively using an electrode switching circuit.Copyright

Hair cell sensing with encapsulated interface bilayers

A gel-supported lipid bilayer formed at the base of an artificial hair is used as the transduction element in a membrane-based artificial haircell sensor inspired by the structure and function of mammalian outer hair cells. This paper describes the initial fabrication and characterization of a bioderived, soft-material alternative to previous artificial haircells that used the transduction properties of synthetic materials for flow and touch sensing. Under an applied air flow, the artificial hair structure vibrates, triggering a picoamp-level electrical current across the bilayer. Experimental analysis of this mechanoelectrical transduction process supports the hypothesis that the oscillating current is produced by a time-varying change in the capacitance of the membrane caused by the vibration of the hair. Specifically, frequency analysis of both the motion of the hair and the measured current show that both phenomena occur at similar frequencies, which suggests that changes in capacitance occur as a result of membrane bending during excitation.

Characterization of porous substrates for biochemical energy conversion devices

Bimolecules have demonstrated the potential to function as active components in energy harvesting devices, biosensors and bioinspired actuators. The bilayer lipid membrane (BLM) formed from lipid molecules and supported in the pores of porous substrates is the standard platform for fabricating the biomolecule based devices. The techniques for forming BLM in an in-vitro environment like lipid painting, Lagmuir-Blodgett, Langmuir-Schaffer and lipid folding methods were developed by researchers in the biophysical community to investigate the properties of membrane bound proteins. While all of these methods can form a BLM and has been used in laboratory research for few decades, they are not equally well-suited for fabricating an engineering device. Of the different methods, the lipid deposition technique for BLM self-assembly and protein insertion is the closest in its qualities to an engineering prototyping method. This article presents a detailed electrical model of the substrates and the BLM formed in the pores from SOPC, POPS:POPE and DPhPC lipids using lipid deposition technique. The equivalent circuits of the substrates and the BLM are used to interrogate the quality of the BLM by impedance spectroscopy. The deviations of the prepared BLMs from desirable parameters are traced to the preparation procedure that could be used as a feedback information for fabricating a single BLM in the pores of the substrate. The impedance response is also used to understand the change in electrical properties of BLMs formed in an array of pores of a multi-porous substrate.

Chemoelectrical energy conversion of adenosine triphosphate

Plant and animal cell membranes transport charged species, neutral molecules and water through ion pumps and channels. The energy required for moving species against established concentration and charge gradients is provided by the biological fuel - adenosine triphosphate (ATP) -synthesized within the cell. The adenosine triphosphatase (ATPases) in a plant cell membrane hydrolyze ATP in the cell cytoplasm to pump protons across the cell membrane. This establishes a proton gradient across the membrane from the cell exterior into the cell cytoplasm. This proton motive force stimulates ion channels that transport nutrients and other species into the cell. This article discusses a device that converts the chemical energy stored in adenosine triphosphate into electrical power using a transporter protein, ATPase. The V-type ATPase proteins used in our prototype are extracted from red beet(Beta vulgaris) tonoplast membranes and reconstituted in a bilayer lipid membrane or BLM formed from POPC and POPS lipids. A pH7 medium that can support ATP hydrolysis is provided on both sides of the membrane and ATP is dissolved in the pH7 buffer on one side of the membrane. Hydrolysis of ATP results in the formation of a phosphate ion and adenosine diphosphate. The energy from the reaction activates ATPase in the BLM and moves a proton across the membrane. The charge gradient established across the BLM due to the reaction and ion transport is converted into electrical current by half-cell reference electrodes. The prototype ATPase cell with an effective BLM area of 4.15 mm2 carrying 15 &mgr;l of ATPase proteins was observed to develop a steady state peak power output of 70 nW, which corresponds to a specific power of 1.69 &mgr;W/cm2 and a current density of 43.4 &mgr;A/cm2 of membrane area.

Formation, encapsulation, and validation of membrane-based artificial hair cell sensors

Hair cell structures are one of the most common forms of sensing elements found in nature. In nearly all vertebrates hair cells are used for auditory and vestibular sensing. In humans, approximately 16,000 auditory hair cells can be found in the cochlea of the ear. Each hair cell contains a stereocilia, which is the primary structure for sound transduction. This study looks to develop and characterize an artificial hair cell that resembles the stereocilia of the human ear. Recently our research group has shown that a single artificial hair cell can be formed in an open substrate using a single aqueous droplet and a hydrogel. In this study, air was blown across the hair and analyzed using spectral analysis. The results of this study provided the foundation for our current work toward an artificial hair cell that uses two aqueous droplets. In the current study a test fixture was created in order to consistently measure various properties of the encapsulated hair cell. The response of the hair cell was measured with an impulse input at various locations on the test fixture. A frequency response function was then created using the impulse input and the output of the sensor. It was found that the vibration of the hair was only detectable if the test fixture was struck at the correct location. By changing the physical parameters of the hair sensor, such as hair length, we were able to alter the response of the sensor. It was also found that the sensitivity of the sensor was reliant on the size of the lipid bilayer.

Design and Development of a Biomimetic Jellyfish Robot That Features Ionic Polymer Metal Composites Actuators

This paper presents the design, fabrication, and characterization of a second generation biomimetic jellyfish robot that uses ionic polymer metal composites (IPMCs) as flexible actuators for propulsion. The shape and swimming style of this underwater vehicle are based on the Aurelia aurita jellyfish, which has an average swimming speed of 13 mm/s and which is known for a high swimming efficiency. The critical components of the vehicle include the flexible bell that provides the overall shape and dimensions of the jellyfish, a central hub used to provide electrical connections and mechanical support to the actuators, and flexible IPMC actuators that extend radially from the central hub. In order to provide increased shape holding ability and reduced weight, the bell is fabricated from a commercially available heat-shrinkable polymer film. A new lightweight hub has been designed and was fabricated by 3D printing using ABS plastic material. The hub features internal electrical contacts for providing voltage to the individual IPMC actuators. Finally, a new set of IPMC actuators are manufactured using the Direct Assembly Process (DAP). The IPMC actuators constructed for this study demonstrated peak-to-peak strains of ∼ 0.7% in water across a frequency range of 0.1–1.0Hz. By tailoring the applied voltage waveform and the flexibility of the bell, the completed robotic jellyfish swam at maximum speed of 1.5 mm/s.Copyright