Journal of Physics D: Applied Physics

Ping Du, Xi Lin, Xin Zhang

Volume 46, Number 19, Page 195303

doi:10.1088/0022-3727/46/19/195303

A generic experimental procedure is presented in this work to enhance
the electrical responses of polydimethylsiloxane (PDMS) through
incorporation of conducting polymer nanowires, while maintaining the
desirable mechanical flexibility of PDMS. The conducting polypyrrole
(PPy) nanowires are synthesized using a template method. The dielectric
constants of the composites are characterized by impedance measurements,
and the effect of nanowire concentration is investigated by the
percolation theory. Using a continuous hyperbolic tangent function,
critical volume fraction is estimated to be 9.6 vol%, at which an
85-fold enhancement in the dielectric constants is achieved. The
viscoelastic properties of the composites are characterized by the
stress relaxation nanoindentation tests, and the effect of nanowire
concentration on the elastic modulus of composites is found to deviate
significantly from the Wang–Pyrz model at the critical volume fraction.
The tunable multifunctionality of PDMS composites that possess
significantly enhanced electrical and moderate viscoelastic responses is
desirable for many sensing and actuation applications.

Journal of Microelectromechanical Systems
Ping Du; Chen Cheng; Hongbing Lu; Xin Zhang
Volume: 22, Issue: 1, Page(s): 44 - 53

DOI: 10.1109/JMEMS.2012.2213070

Polydimethylsiloxane (PDMS) micropillar-based biotransducers are
extensively used in cellular force measurements. The accuracy of these
devices relies on the appropriate material characterization of PDMS and
modeling to convert the micropillar deformations into the corresponding
forces. Cellular contraction is often accompanied by oscillatory motion,
the frequency of which ranges in several hertz. In this paper, we
developed a methodology to calculate the cellular contraction forces in
the frequency domain with improved accuracy. The contraction data were
first expressed as a Fourier series. Subsequently, we measured the
complex modulus of PDMS using a dynamic nanoindentation technique. An
improved method for the measurement of complex modulus was developed
with the use of a flat punch indenter. The instrument dynamics was
characterized, and the full contact region was identified. By
incorporating both the Fourier series of contraction data and the
complex modulus function, the cellular contraction force was calculated
by finite-element analysis (FEA). The difference between the Euler beam
formula and the viscoelastic FEA was discussed. The methodology
presented in this work is anticipated to benefit the material
characterization of other soft polymers and complex biological behavior
in the frequency domain.

Sensors and Actuators A: Physical

Vol 176, Page 90-98.

http://dx.doi.org/10.1016/j.sna.2012.01.002

There is an increasing trend to incorporate silicon carbide (SiC) into silicon oxides to improve the mechanical properties, thermal stability, and chemical resistance. In this work the silicon oxycarbide (SiOC) films were deposited by RF magnetron co-sputtering from silicon dioxide and silicon carbide targets. Subsequently rapid thermal annealing was applied to the as-deposited films to tune the mechanical properties. Energy dispersive spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and ellipsometry were employed to characterize the compositions and microstructure of the films. The residual stress of the films was calculated from the film–substrate curvature measurement using Stoney's equation. The film stress changed from compressive to tensile after annealing, and it generally increased with carbon contents. The Young's modulus and hardness were investigated by the depth-sensing nanoindentation, which were found to increase with the carbon content and annealing temperature. A thorough microstructural analysis was conducted to investigate the effect of carbon content and annealing temperature on the mechanical properties of SiOC films.

Applied Physics Letters 99(8): 083701.

http://link.aip.org/link/doi/10.1063/1.3628456

Polymeric deformable sensor arrays have been employed to measure cellular forces and offered insights into the study of cellular mechanics. Previous studies have been focused on using transducers in static domain and assumed elastic beam theory as the force conversion model. Neglecting the inherent viscoelastic behavior of polydimethylsiloxane and low aspect ratios of the sensor arrays compromised the accuracy of these devices. In this work, a more in-depth viscoelastic Timoshenko beam model was developed incorporating dynamic cellular forces. We studied chemically stimulated contractions of cardiac myocytes and found that the loading rate has a considerable influence on the sensitivity of the sensor arrays.

Journal of Micromechanics and Microengineering

Volume 20, Number 9, 095016

http://dx.doi.org/10.1088/0960-1317/20/9/095016

Polydimethylsiloxane (PDMS)-based micropillars (or microcantilevers) have been used as bio-transducers for measuring cellular forces on the order of pN to uN. The measurement accuracy of these sensitive devices depends on appropriate modeling to convert the micropillar deformations into the corresponding reaction forces. The traditional approach to calculating the reaction force is based on the Euler beam theory with consideration of a linear elastic slender beam for the micropillar. However, the low aspect ratio in geometry of PDMS micropillars does not satisfy the slender beam requirement. Consequently, the Timoshenko beam theory, appropriate for a beam with a low aspect ratio, should be used. In addition, the inherently time-dependent behavior in PDMS has to be considered for accurate force conversion. In this paper, the Timoshenko beam theory, along with the consideration of viscoelastic behavior of PDMS, was used to model the mechanical response of micropillars. The viscoelastic behavior of PDMS was characterized by stress relaxation nanoindentation using a circular flat punch. A correction procedure was developed to determine the load-displacement relationship with consideration of ramp loading. The relaxation function was extracted and described by a generalized Maxwell model. The bending of rectangular micropillars was performed by a wedge indenter tip. The viscoelastic Timoshenko beam formula was used to calculate the mechanical response of the micropillar, and the results were compared with measurement data. The calculated reaction forces agreed well with the experimental data at three different loading rates. A parametric study was conducted to evaluate the accuracy of the viscoelastic Timoshenko beam model by comparing the reaction forces calculated from the elastic Euler beam, elastic Timoshenko beam and viscoelastic Euler beam models at various aspect ratios and loading rates. The extension of modeling from the elastic Euler beam theory to the viscoelastic Timoshenko beam theory has improved the accuracy for the conversion of the PDMS micropillar deformations to forces, which will benefit the polymer-based micro bio-transducer applications.

Sensors and Actuators A: Physical

Volume 163, Issue 1, September 2010, Pages 240–246

http://dx.doi.org/10.1016/j.sna.2010.06.002

Electroactive conducting polymers (CPs) have been frequently used for fabricating bending actuators. To model this type of actuation, the traditional double-layer beam bending theory was implemented by neglecting the thickness of the thin intermediate metal layers for the sake of simplification. However, this common assumption has not been carefully validated and the associated errors have not been well acknowledged. In this work, a generic multilayer bending model was introduced to account for the actuators consisting of an arbitrary number of layers. Our model found the bending curvature, strain, stress, and in particular work density of the multilayer actuator as explicit functions of the thickness and modulus of each individual layer. The thickness of metals and conducting polymers were controlled in thermal evaporation and electrochemical synthesis, respectively. The modulus of polypyrrole (PPy), the conducting polymer used in this work, was determined within our model by the bending curvature measured using the charge-coupled device (CCD). This gave a modulus of our electrochemically synthesized PPy of 80 MPa, corresponding to an actuation strain of 2% in our model. It was concluded that neglecting the intermediate metal layers would lead to substantial errors. For instance, using a PPy/Au/Kapton trilayer actuator, a 5% error or below in strain can only be found if the Au layer is one thousand times thinner than Kapton. To enhance the actuation, a PPy/Pt/PVDF/Pt/PPy five-layer actuator has been often used. In this case, even if the Pt layer was reduced to 10 nm, our predicted error of neglecting the two metal layers would be 12.59%. Our results showed that the work density, chosen to measure the overall performance of the actuator, was highly sensitive to the modulus of the substrate polymer layer so that it was generally desirable of using a soft polymer substrate. With the multilayer bending model, we intend to provide an accurate and reliable tool for systematically analyzing the bending behavior and performance of the CP-based actuators.