Yu-Hsiang Hsu

A microfluidic platform for generating large-scale nearly identical human microphysiological vascularized tissue arrays

This paper reports a polydimethylsiloxane microfluidic model system that can develop an array of nearly identical human microtissues with interconnected vascular networks. The microfluidic system design is based on an analogy with an electric circuit, applying resistive circuit concepts to design pressure dividers in serially-connected microtissue chambers. A long microchannel (550, 620 and 775 mm) creates a resistive circuit with a large hydraulic resistance. Two media reservoirs with a large cross-sectional area and of different heights are connected to the entrance and exit of the long microchannel to serve as a pressure source, and create a near constant pressure drop along the long microchannel. Microtissue chambers (0.12 μl) serve as a two-terminal resistive component with an input impedance >50-fold larger than the long microchannel. Connecting each microtissue chamber to two different positions along the long microchannel creates a series of pressure dividers. Each microtissue chamber enables a controlled pressure drop of a segment of the microchannel without altering the hydrodynamic behaviour of the microchannel. The result is a controlled and predictable microphysiological environment within the microchamber. Interstitial flow, a mechanical cue for stimulating vasculogenesis, was verified by finite element simulation and experiments. The simplicity of this design enabled the development of multiple microtissue arrays (5, 12, and 30 microtissues) by co-culturing endothelial cells, stromal cells, and fibrin within the microchambers over two and three week periods. This methodology enables the culturing of a large array of microtissues with interconnected vascular networks for biological studies and applications such as drug development.

Electrical and mechanical fully coupled theory and experimental verification of Rosen-type piezoelectric transformers

A piezoelectric transformer is a power transfer device that converts its input and output voltage as well as current by effectively using electrical and mechanical coupling effects of piezoelectric materials. Equivalent-circuit models, which are traditionally used to analyze piezoelectric transformers, merge each mechanical resonance effect into a series of ordinary differential equations. Because of using ordinary differential equations, equivalent circuit models are insufficient to reflect the mechanical behavior of piezoelectric plates. Electromechanically, fully coupled governing equations of Rosen-type piezoelectric transformers, which are partial differential equations in nature, can be derived to address the deficiencies of the equivalent circuit models. It can be shown that the modal actuator concept can be adopted to optimize the electromechanical coupling effect of the driving section once the added spatial domain design parameters are taken into account, which are three-dimensional spatial dependencies of electromechanical properties. The maximum power transfer condition for a Rosen-type piezoelectric transformer is detailed. Experimental results, which lead us to a series of new design rules, also are presented to prove the validity and effectiveness of the theoretical predictions.

Optimizing piezoelectric transformer for maximum power transfer

The piezoelectric transformer should be considered as not only a mechanical system but also an electrical fully coupled system, which uses finite structure resonance to perform voltage conversion. The traditional piezoelectric transformer uses an equivalent circuit concept to simulate its mechanical motion. Since the field equation of a piezoelectric transformer is a partial differential equation, an equivalent circuit developed based on an ordinary differential equation cannot express its performance thoroughly. Since the motion of the piezoelectric transformer is determined by both the mechanical and the electrical boundary conditions, the transfer function and electrical impedance of a piezoelectric transformer will vary with different electrical and mechanical conditions. However, it has been discovered that once an equivalent circuit is used to simplify the mechanical motion of the piezoelectric transformer, the coupling effects between the mechanical and the electrical properties will be averaged out and all spatial information will be lost. A fully coupled field equation is derived in this paper to examine the motion of the piezoelectric transformer by taking into account both the mechanical and electrical boundary conditions. It can be shown that by using this newly developed field equation, a new concept using a weighting function in the spatial domain can be adopted to optimize the electro-mechanical coupling effects. In this paper, both impedance and power transfer optimization of the source as well as the sink of the piezoelectric transformer, and the corresponding interface circuits, are verified theoretically and experimentally.

Microplatform for intercellular communication

A microplatform was designed, fabricated, and tested for demonstrating the propagation of molecular signals through a line of patterned HeLa cells expressing gap junction channels (HeLa Cx43 cells). The microplatform was capable of patterning cells onto a predefined design with lithography and surface chemical treatment. Lucifer Yellow was first used as a fluorescent marker to demonstrate the formation of functional gap junction channels between patterned HeLa Cx43 cells. The cells at one end of the cell line were next chemically stimulated to induce the propagation of intercellular calcium waves along the cell line, which was successfully monitored with Fluo4. The designed microplatform allowed intercellular communication over an arbitrary network topology of cells, which may provide new insight into mechanisms of intercellular communication.

Targeted origin placement for the autonomous gain-phase tailoring of piezoelectric sensors

Targeted origin placement can be shown to compliment mathematical tools such as the image method, window functions and two-sided Laplace transformation for designing a whole new class of piezoelectric sensors that can be used to tailor the gain and phase portions of the sensor transfer functions independently. It can be demonstrated that by properly applying the boundary conditions and the method of imaging, the targeted origin of this new class of sensor design can be traversed anywhere along the surface of the testing structure. In addition, the active sensor concept that integrates classical control theory with piezoelectric sensors can be adapted to further advance sensor technology. The design concept, theoretical derivations, numerical calculations and experimental verifications of implementing this class of new sensor design to influence flexible structure control and point sensor design methodologies are detailed.

Performance Evaluation of Leader–Follower-Based Mobile Molecular Communication Networks for Target Detection Applications

This paper proposes a leader–follower-based model of mobile molecular communication networks for target detection applications. The proposed model divides the application functionalities of molecular communication networks into two types of mobile bio-nanomachine: leader and follower bio-nanomachines. Leader bio-nanomachines distribute in the environment to detect a target and create an attractant gradient around the target. Follower bio-nanomachines move according to the attractant gradient established by leader bio-nanomachines; they approach the target and perform necessary functionalities, such as releasing drug molecules. This paper develops mathematical expressions for the proposed model, describes wet laboratory experiments designed to estimate model parameters, and performs biologically realistic computer simulation experiments to evaluate the performance of the proposed model. The main contributions of this paper are to demonstrate the functional division of molecular communication networks, which will facilitate the design and development of molecular communication networks. Furthermore, insight into the application-level performance of molecular communication networks will be provided based on the proposed model.

A microfabricated piezoelectric transducer platform for mechanical characterization of cellular events

During the last decade, it was discovered that the mechanical properties and interactions of cells and their surrounding extra-cellular matrix play important roles in cellular activities. Substantial efforts have been made to develop various methodologies and tools to study cell mechanics. In this paper, we report an ongoing study on integrating the concept of a smart structure with a microfabricated thin film piezoelectric transducer for characterizing the various changes in mechanical properties associated with cellular events. A microbridge sensor integrated with a thin film piezoelectric transducer was created from silicon dioxide and zinc oxide sandwiched between two gold electrodes. The cells to be tested were cultured on the microbridge surface. The surface tractions exerted by the cells on the microbridge directly modulated the selected resonant behaviors, which were detected with the custom designed effective surface electrode. Our theory and simulation results showed, for the first time, that the application and changes in these surface tractions resulted in resonant and anti-resonant frequency shifts in the impedance response of the piezoelectric transducer. Both spatial and temporal information of dynamic cellular activities could be inferred from the changes in the impedance spectra. The design, theory, finite-element simulation, microfabrication techniques, and preliminary test results are discussed.

Electrical and mechanical field interactions of piezoelectric systems: foundation of smart structures-based piezoelectric sensors and actuators, and free-fall sensors*

Based on an idea to fully integrate the advantages of both distributed and point sensors, we adopted mathematical tools such as method of image, linear superposition, and window functions to develop a fundamental approach towards creating no-phase delay filters for finite sensor structures. Through an expanded design freedom for piezoelectric sensors and actuators as a result of adding in situ, we can obtain a no-phase delay signal filtering capability by properly designing an effective surface electrode onto the piezoelectric layer. An inertia-based free-fall sensor that can measure the start of free-fall motion in addition to creating a piezoelectric sensor and actuator pair that possesses a spatial dependent transfer function and demonstrates the wide range applicability of the new concepts are disclosed. The fundamental perspective of pursuing signal processing through an electromechanical interaction of wave modes and of piezoelectric materials is examined in detail. It can be shown that the fundamental design criteria of electromechanical-coupled systems are highly related to the temporal and spatial characteristics of the mechanical structure and its electrical interaction between interface circuits. After pursuing the physical characteristics of these systems, the possibilities of extending the no-phase delay signal processing capabilities to other physical fields are discussed as well.

Visible-Light Modulation on Lattice Dielectric Responses of a Piezo-Phototronic Soft Material

In situ synchrotron X-ray diffraction is used to investigate a three-way piezo-phototronic soft material. This new system is composed of a semi-crystalline poly(vinylidene fluoride-co-trifluoroethylene) piezoelectric polymer and titanium oxide nanoparticles. Under light illumination, photon-induced piezoelectric responses are nearly two times higher at both the lattice-structure and the macroscopic level than under conditions without light illumination. A mechanistic model is proposed.

A light-activated optopiezoelectric thin-film actuator for microfluidic applications

In this paper, we present a new type of piezoelectric composite material, optopiezoelectric thin-film, to serve as a lightactivated micropump for integrating with a microfluidic device. By using a photoconductive material (titanium oxide phthalocyanine) to serve as one of the electrodes of a piezoelectric polyvinylidene fluoride (PVDF) polymer, multiple locations of this optopiezoelectric thin-film can be actuated independently with one driving voltage source and a programmable light mask. Integrating this optopiezoelectric thin-film to a microfluidic device, complex operations of a multi-functioned microfluidic device can potentially be simplified and scaled up. Here, we present our preliminary result to demonstrate the feasibility of using one optopiezoelectric thin-film to serve as two microfluidic micropumps controlled by a light mask.