Deformational loading represents a primary component of the chondrocyte physical environment in vivo. This review summarizes our experience with physiologic deformational loading of chondrocyte-seeded agarose hydrogels to promote development of cartilage constructs having mechanical properties matching that of the parent calf tissue, which has a Young's modulus EY = 277 kPa and unconfined dynamic modulus at 1 Hz G*=7 MPa. Over an 8-week culture period, cartilage-like properties have been achieved for 60 × 106 cells/ml seeding density agarose constructs, with EY = 186 kPa, G*=1.64 MPa. For these constructs, the GAG content reached 1.74% ww and collagen content 2.64% ww compared to 2.4% ww and 21.5% ww for the parent tissue, respectively. Issues regarding the deformational loading protocol, cell-seeding density, nutrient supply, growth factor addition, and construct mechanical characterization are discussed. In anticipation of cartilage repair studies, we also describe early efforts to engineer cylindrical and anatomically shaped bilayered constructs of agarose hydrogel and bone (i.e., osteochondral constructs). The presence of a bony substrate may facilitate integration upon implantation. These efforts will provide an underlying framework from which a functional tissue-engineering approach, as described by Butler and coworkers (2000), may be applied to general cell-scaffold systems adopted for cartilage tissue engineering.

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A Paradigm for Functional Tissue Engineering of Articular Cartilage

Machines made of soft materials bridge life sciences and engineering1. Advances in soft materials have led to skin-like sensors and muscle-like actuators for soft robots and wearable devices1-3. Flexible or stretchable counterparts of most key mechatronic components have been developed4,5, principally using fluidically driven systems6-8; other reported mechanisms include electrostatic9-12, stimuli-responsive gels13,14 and thermally responsive materials such as liquid metals15-17 and shape-memory polymers18. Despite the widespread use of fluidic actuation, there have been few soft counterparts of pumps or compressors, limiting the portability and autonomy of soft machines4,8. Here we describe a class of soft-matter bidirectional pumps based on charge-injection electrohydrodynamics19. These solid-state pumps are flexible, stretchable, modular, scalable, quiet and rapid. By integrating the pump into a glove, we demonstrate wearable active thermal management. Embedding the pump in an inflatable structure produces a self-contained fluidic 'muscle'. The stretchable pumps have potential uses in wearable laboratory-on-a-chip and microfluidic sensors, thermally active clothing and autonomous soft robots.

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Stretchable pumps for soft machines

In situ mechanical characterization of nanostructures, such as carbon nanotubes and metallic nanowires, in scanning and transmission electron microscopes is essential for the understanding of material behavior at the nanoscale. This paper describes the design, fabrication, and operation of a novel microelectromechanical-systems (MEMS)-based material testing system used for in situ tensile testing of nanostructures. The device consists of an actuator and a load sensor with a specimen in between. Two types of actuators, in-plane thermal and comb drive actuators, are used to pull the specimens in displacement control and force control modes, respectively. The load sensor works based on differential capacitive sensing, from which the sensor displacement is recorded. By determining sensor stiffness from mechanical resonance measurements, the load on the specimen is obtained. Load sensors with different stiffness were fabricated. The best resolutions were achieved with load sensors that are designed for testing nanotubes, reaching 0.05 fF in capacitance, 1 nm in displacement, and 12 nN in load. For the first time, this MEMS-based material testing scheme offers the possibility of continuous observation of the specimen deformation and fracture with subnanometer resolution, while simultaneously measuring the applied load electronically with nano-Newton resolution. The overall device performance is demonstrated by testing freestanding cofabricated polysilicon films and multiwalled carbon nanotubes.

Design and Operation of a MEMS-Based Material Testing System for Nanomechanical Characterization

This review surveys micromachined gyroscope structure and circuitry technology The principle of micromachined gyroscopes is first introduced Then, different kinds of MEMS gyroscope structures, materials and fabrication technologies are illustrated Micromachined gyroscopes are mainly categorized into micromachined vibrating gyroscopes (MVGs), piezoelectric vibrating gyroscopes (PVGs), surface acoustic wave (SAW) gyroscopes, bulk acoustic wave (BAW) gyroscopes, micromachined electrostatically suspended gyroscopes (MESGs), magnetically suspended gyroscopes (MSGs), micro fiber optic gyroscopes (MFOGs), micro fluid gyroscopes (MFGs), micro atom gyroscopes (MAGs), and special micromachined gyroscopes Next, the control electronics of micromachined gyroscopes are analyzed The control circuits are categorized into typical circuitry and special circuitry technologies The typical circuitry technologies include typical analog circuitry and digital circuitry, while the special circuitry consists of sigma delta, mode matching, temperature/quadrature compensation and novel special technologies Finally, the characteristics of various typical gyroscopes and their development tendency are discussed and investigated in detail

The Development of Micromachined Gyroscope Structure and Circuitry Technology

Micropumps and biomedical applications – A review

Force and power characteristics are experimentally measured for three types of toggling microthermal actuators: (i) standard two-arm thermal actuators, (ii) chevron or 'V' type actuators and (iii) chevron actuators with a toggle amplifier. Micro-Newton scale forces and micron scale displacements are experimentally measured using an off-chip probe. For each type of actuator, force versus deflection curves are measured to determine mechanical advantage and maximum ranges. For each type of actuator, work output versus input power curves are measured to determine actuator effectiveness. Different actuator designs are compared in terms of force, displacement, power and effectiveness.

https://iopscience.iop.org/article/10.1088/0960-1317/14/1/307/pdf

Force, deflection and power measurements of toggled microthermal actuators

Micropumps are finding more applications in biomedical fields. One application that holds great potential for micropumps is the treatment of glaucoma. Shunts are currently used in the treatment glaucoma. They lower intraocular pressure by passively increasing the outflow of aqueous humour from the anterior chamber in the eye. Nozzle/diffuser micropumps are an attractive alternative to shunts. They would provide both passive outflow and variable active outflow of aqueous humour as necessary. They would be able to overcome increases in flow resistance caused by biofouling and scar tissue growth. This study presents a proof of concept of a 1-mm-thick electromagnetic PDMS nozzle/diffuser micropump for biomedical applications. The pump is composed of a cast PDMS body, a spin coated PDMS membrane, and commercial silicone tubing, all bonded together with PDMS. Micromachining a pump cast enables multiple pumps to be produced quickly and reliably. An in-plane design is used to attach the inlet and outlet tubes. The pump produces a peak flow rate of 135 μL/min and a maximum backpressure of 25 mmH2O at an actuation frequency of 12 Hz and a duty cycle of 25%. The membrane thickness, actuator type, and duty cycle were shown to have a significant impact on the performance of the pump.

A thin PDMS nozzle/diffuser micropump for biomedical applications

The present paper describes a novel concept that employs a thermal-based approach for in-situ displacement sensing of electro-thermal actuators. A device encompassing an in-plane electro-thermal actuator and a thermal sensor was monolithically fabricated using the MetalMUMPS process. Analytical models were developed for both the actuator and the thermal sensor. Simulation and experimental results demonstrated good agreement. The experimental results indicated that the sensor achieved high linearity and sensitivity (∼4.5 nm/Ω).

Displacement sensing of a micro-electro-thermal actuator using a monolithically integrated thermal sensor

Background Glaucoma is the second leading cause of blindness in the world and the first leading cause of irreversible vision loss. Currently, the primary methodology of testing for the intraocular pressure (IOP) is during clinical office hours, which only provide a limited amount of information on the trends and fluctuations of the IOP. Therefore, a continuous monitoring system is required to properly determine the peaks of pressure and to negate any false results obtained by sparse, clinic hour testing. The objective of this study is to determine the ability of a newly designed contact lens with an embedded microchannel, to accurately measure the fluctuations in the IOP. Methods Experimentation was completed on fresh enucleated porcine eyes. The contact lens was placed on the porcine eye and utilising a camera the fluid movement, within the microchannel in the contact lens, was recorded. A micro-pressure catheter, threaded into the centre of the vitreous chamber, recorded the true IOP and was compared with the displacement of the indicator fluid within the microchannel. Results The contact lenses showed a consistent linear responsiveness to changes in IOP and robust to the effects of anatomical differences among eyes. The indicator fluid had an average fluid movement of 28 um/mm Hg between all the trials. Additionally, the devices showed the ability to measure both increases and decreases in IOP during cyclical fluctuations. Conclusion The described inexpensive and non-invasive sensor is able to reliably monitor the IOP changes based on porcine eye model.

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Non-invasive Intraocular pressure monitoring with contact lens

A cantilever biosensor with integrated electrodes for piezoelectric actuation and electrokinetic capture is used to detect cells in real time. The sandwich electrodes used for electrokinetic capture present advantages over side-by-side electrodes in terms of reducing the gap size between electrodes and the structural mass added to the cantilever. The 5th resonant mode (≈617 kHz) of the biosensor provided a signal-to-noise ratio of 82 within 10 min when E. coli was measured from a stagnant sample with a concentration of 107 cells/ml. The 7th resonant mode (≈1160 kHz) provided a signal-to-noise ratio of 26 within 10 min when E. coli was measured from a flowing sample (400 μl/min) with a concentration of 105 cells/ml. The estimated sensitivity of the 7th resonant mode is 326 fg/Hz based on images of captured cells.

Yongjun Lai

Papers

Force, deflection and power measurements of toggled microthermal actuators

A thin PDMS nozzle/diffuser micropump for biomedical applications

Displacement sensing of a micro-electro-thermal actuator using a monolithically integrated thermal sensor

Non-invasive Intraocular pressure monitoring with contact lens

A cantilever biosensor exploiting electrokinetic capture to detect Escherichia coli in real time