Microengineered platforms for cell mechanobiology

Biophysical assessment of single cell cytotoxicity: diesel exhaust particle-treated human aortic endothelial cells

Exposure to diesel exhaust particles (DEPs), a major source of traffic-related air pollution, has become a serious health concern due to its adverse influences on human health including cardiovascular and respiratory disorders. To elucidate the relationship between biophysical properties (cell topography, cytoskeleton organizations, and cell mechanics) and functions of endothelial cells exposed to DEPs, atomic force microscope (AFM) was applied to analyze the toxic effects of DEPs on a model cell line from human aortic endothelial cells (HAECs). Fluorescence microscopy and flow cytometry were also applied to further explore DEP-induced cytotoxicity in HAECs. Results revealed that DEPs could negatively impair cell viability and alter membrane nanostructures and cytoskeleton components in a dosage- and a time-dependent manner; and analyses suggested that DEPs-induced hyperpolarization in HAECs appeared in a time-dependent manner, implying DEP treatment would lead to vasodilation, which could be supported by down-regulation of cell biophysical properties (e.g., cell elasticity). These findings are consistent with the conclusion that DEP exposure triggers important biochemical and biophysical changes that would negatively impact the pathological development of cardiovascular diseases. For example, DEP intervention would be one cause of vasodilation, which will expand understanding of biophysical aspects associated with DEP cytotoxicity in HAECs.

Hydrodynamic stretching of single cells for large population mechanical phenotyping

Cell state is often assayed through measurement of biochemical and biophysical markers. Although biochemical markers have been widely used, intrinsic biophysical markers, such as the ability to mechanically deform under a load, are advantageous in that they do not require costly labeling or sample preparation. However, current techniques that assay cell mechanical properties have had limited adoption in clinical and cell biology research applications. Here, we demonstrate an automated microfluidic technology capable of probing single-cell deformability at approximately 2,000 cells/s. The method uses inertial focusing to uniformly deliver cells to a stretching extensional flow where cells are deformed at high strain rates, imaged with a high-speed camera, and computationally analyzed to extract quantitative parameters. This approach allows us to analyze cells at throughputs orders of magnitude faster than previously reported biophysical flow cytometers and single-cell mechanics tools, while creating easily observable larger strains and limiting user time commitment and bias through automation. Using this approach we rapidly assay the deformability of native populations of leukocytes and malignant cells in pleural effusions and accurately predict disease state in patients with cancer and immune activation with a sensitivity of 91% and a specificity of 86%. As a tool for biological research, we show the deformability we measure is an early biomarker for pluripotent stem cell differentiation and is likely linked to nuclear structural changes. Microfluidic deformability cytometry brings the statistical accuracy of traditional flow cytometric techniques to label-free biophysical biomarkers, enabling applications in clinical diagnostics, stem cell characterization, and single-cell biophysics.

Microfluidic approaches for cancer cell detection, characterization, and separation

This article reviews the recent developments in microfluidic technologies for in vitro cancer diagnosis. We summarize the working principles and experimental results of key microfluidic platforms for cancer cell detection, characterization, and separation based on cell-affinity micro-chromatography, magnetic activated micro-sorting, and cellular biophysics (e.g., cell size and mechanical and electrical properties). We examine the advantages and limitations of each technique and discuss future research opportunities for improving device throughput and purity, and for enabling on-chip analysis of captured cancer cells.

Design of pressure-driven microfluidic networks using electric circuit analogy

This article reviews the application of electric circuit methods for the analysis of pressure-driven microfluidic networks with an emphasis on concentration- and flow-dependent systems. The application of circuit methods to microfluidics is based on the analogous behaviour of hydraulic and electric circuits with correlations of pressure to voltage, volumetric flow rate to current, and hydraulic to electric resistance. Circuit analysis enables rapid predictions of pressure-driven laminar flow in microchannels and is very useful for designing complex microfluidic networks in advance of fabrication. This article provides a comprehensive overview of the physics of pressure-driven laminar flow, the formal analogy between electric and hydraulic circuits, applications of circuit theory to microfluidic network-based devices, recent development and applications of concentration- and flow-dependent microfluidic networks, and promising future applications. The lab-on-a-chip (LOC) and microfluidics community will gain insightful ideas and practical design strategies for developing unique microfluidic network-based devices to address a broad range of biological, chemical, pharmaceutical, and other scientific and technical challenges.

How changes in cell mechanical properties induce cancerous behavior

Tumor growth and metastasis are ultimately mechanical processes involving cell migration and uncontrolled division. Using a 3D discrete model of cells, we show that increased compliance as observed for cancer cells causes them to grow at a much faster rate compared to surrounding healthy cells. We also show how changes in intercellular binding influence tumor malignancy and metastatic potential. These findings suggest that changes in the mechanical properties of cancer cells is the proximate cause of uncontrolled division and migration and various biochemical factors drive cancer progression via this mechanism.

Cellular self-organization by autocatalytic alignment feedback

Myoblasts aggregate, differentiate and fuse to form skeletal muscle during both embryogenesis and tissue regeneration. For proper muscle function, long-range self-organization of myoblasts is required to create organized muscle architecture globally aligned to neighboring tissue. However, how the cells process geometric information over distances considerably longer than individual cells to self-organize into well-ordered, aligned and multinucleated myofibers remains a central question in developmental biology and regenerative medicine. Using plasma lithography micropatterning to create spatial cues for cell guidance, we show a physical mechanism by which orientation information can propagate for a long distance from a geometric boundary to guide development of muscle tissue. This long-range alignment occurs only in differentiating myoblasts, but not in non-fusing myoblasts perturbed by microfluidic disturbances or other non-fusing cell types. Computational cellular automata analysis of the spatiotemporal evolution of the self-organization process reveals that myogenic fusion in conjunction with rotational inertia functions in a self-reinforcing manner to enhance long-range propagation of alignment information. With this autocatalytic alignment feedback, well-ordered alignment of muscle could reinforce existing orientations and help promote proper arrangement with neighboring tissue and overall organization. Such physical self-enhancement might represent a fundamental mechanism for long-range pattern formation during tissue morphogenesis.

Effect of nanogroove geometry on adipogenic differentiation

We present the effect of nanotopographically defined surfaces on adipocyte differentiation using various nanogroove patterns. Parallel nanogroove arrays with equal inter-groove distance (400, 550, 800 nm width) and varying distances (550 nm width with three different spacings of 550, 1100, and 2750 nm) were fabricated by UV-assisted capillary force lithography (CFL) on 18 mm diameter glass coverslips using biocompatible polyurethane (PU)-based material. After coating with fibronectin and subsequent culture of 3T3-L1 preadipocytes, the degree of adipocyte differentiation was determined by Oil Red O staining and adipogenic gene expression. We observed that adipocyte differentiation was slightly but substantially affected by culture on various nanogrooved surfaces. In particular, the cell crawling into nanogrooves contributed substantially to an enhanced level of differentiation with higher contact guidance, suggesting that cell-to-surface interactions would play a role for the adipocyte differentiation.

Tissue engineering bone-ligament complexes using fiber-guiding scaffolds

Regeneration of bone-ligament complexes destroyed due to disease or injury is a clinical challenge due to complex topologies and tissue integration required for functional restoration. Attempts to reconstruct soft-hard tissue interfaces have met with limited clinical success. In this investigation, we manufactured biomimetic fiber-guiding scaffolds using solid free-form fabrication methods that custom fit complex anatomical defects to guide functionally-oriented ligamentous fibers in vivo. Compared to traditional, amorphous or random-porous polymeric scaffolds, the use of perpendicularly oriented micro-channels provides better guidance for cellular processes anchoring ligaments between two distinct mineralized structures. These structures withstood biomechanical loading to restore large osseous defects. Cell transplantation using hybrid scaffolding constructs with guidance channels resulted in predictable oriented fiber architecture, greater control of tissue infiltration, and better organization of ligament interface than random scaffold architectures. These findings demonstrate that fiber-guiding scaffolds drive neogenesis of triphasic bone-ligament integration for a variety of clinical scenarios.

(Micro)managing the mechanical microenvironment

Mechanical forces are critical components of the cellular microenvironment and play a pivotal role in driving cellular processes in vivo. Dissecting cellular responses to mechanical forces is challenging, as even "simple" mechanical stimulation in vitro can cause multiple interdependent changes in the cellular microenvironment. These stimuli include solid deformation, fluid flows, altered physical and chemical surface features, and a complex transfer of loads between the various interacting components of a biological culture system. The active mechanical and biochemical responses of cells to these stimuli in generating internal forces, reorganizing cellular structures, and initiating intracellular signals that specify cell fate and remodel the surrounding environment further complicates cellular response to mechanical forces. Moreover, cells present a non-linear response to combinations of mechanical forces, materials, chemicals, surface features, matrix properties and other effectors. Microtechnology-based approaches to these challenges can yield key insights into the mechanical nature of cellular behaviour, by decoupling stimulation parameters; enabling multimodal control over combinations of stimuli; and increasing experimental throughput to systematically probe cellular response. In this critical review, we briefly discuss the complexities inherent in the mechanical stimulation of cells; survey and critically assess the applications of present microtechnologies in the field of experimental mechanobiology; and explore opportunities and possibilities to use these tools to obtain a deeper understanding of mechanical interactions between cells and their environment.

Quantitative photochemical immobilization of biomolecules on planar and corrugated substrates: a versatile strategy for creating functional biointerfaces

Methods for the generation of substratespresenting biomolecules in a spatially controlled manner are enabling tools for applications in biosensor systems, microarray technologies, fundamental biological studies and biointerface science. We have implemented a method to create biomolecular patterns by using light to control the direct covalent immobilization of biomolecules onto benzophenone-modified glass substrates. We have generated substrates presenting up to three different biomolecules patterned in sequence, and demonstrate biomolecular photopatterning on corrugated substrates. The chemistry of the underlying monolayer was optimized to incorporate poly(ethylene glycol) to enable adhesive cell adhesion onto patterned extracellular matrix proteins. Substrates were characterized with contact angle goniometry, AFM, and immunofluorescence microscopy. Importantly, radioimmunoassays were performed to quantify the site density of immobilized biomolecules on photopatterned substrates. Retained function of photopatterned proteins was demonstrated both by native ligand recognition and cell adhesion to photopatterned substrates, revealing that substrates generated with this method are suitable for probing specific cell receptor-ligand interactions. This molecularly general photochemical patterning method is an enabling tool for the creation of substrates presenting both biochemical and topographical variation, which is an important feature of many native biointerfaces.

Micro- and nanoengineering for stem cell biology: the promise with a caution

Current techniques used in stem cell research only crudely mimic the physiological complexity of the stem cell niches. Recent advances in the field of micro- and nanoengineering have brought an array of in vitro cell culture models that have enabled development of novel, highly precise and standardized tools that capture physiological details in a single platform, with greater control, consistency, and throughput. In this review, we describe the micro- and nanotechnology-driven modern toolkit for stem cell biologists to design novel experiments in more physiological microenvironments with increased precision and standardization, and caution them against potential challenges that the modern technologies might present.

Paper-based piezoresistive MEMS sensors

This paper describes the development of MEMS force sensors constructed using paper as the structural material. The working principle on which these paper-based sensors are based is the piezoresistive effect generated by conductive materials patterned on a paper substrate. The device is inexpensive (∼$0.04 per device for materials), simple to fabricate, lightweight, and disposable. Paper can be readily folded into three-dimensional structures to increase the stiffness of the sensor while keeping it light in weight. The entire fabrication process can be completed within one hour without expensive cleanroom facilities using simple tools (e.g., a paper cutter and a painting knife). We demonstrated that the paper-based sensor can measure forces with moderate performance (i.e., resolution: 120 μN, measurement range: ±16 mN, and sensitivity: 0.84 mV mN(-1)). We applied this sensor to characterizing the mechanical properties of a soft material. Leveraging the same sensing concept, we also developed a paper-based balance with a measurement range of 15 g, and a resolution of 0.39 g.

Transport and shear in a microfluidic membrane bilayer device for cell culture

Microfluidic devices have been established as useful platforms for cell culture for a broad range of applications, but challenges associated with controlling gradients of oxygen and other soluble factors and hemodynamic shear forces in small, confined channels have emerged. For instance, simple microfluidic constructs comprising a single cell culture compartment in a dynamic flow condition must handle tradeoffs between sustaining oxygen delivery and limiting hemodynamic shear forces imparted to the cells. These tradeoffs present significant difficulties in the culture of mesenchymal stem cells (MSCs), where shear is known to regulate signaling, proliferation, and expression. Several approaches designed to shield cells in microfluidic devices from excessive shear while maintaining sufficient oxygen concentrations and transport have been reported. Here we present the relationship between oxygen transport and shear in a "membrane bilayer" microfluidic device, in which soluble factors are delivered to a cell population by means of flow through a proximate channel separated from the culture channel by a membrane. We present an analytical model that describes the characteristics of this device and its ability to independently modulate oxygen delivery and hemodynamic shear imparted to the cultured cells. This bilayer configuration provides a more uniform oxygen concentration profile that is possible in a single-channel system, and it enables independent tuning of oxygen transport and shear parameters to meet requirements for MSCs and other cells known to be sensitive to hemodynamic shear stresses.

System Integration - A Major Step toward Lab on a Chip

Microfluidics holds great promise to revolutionize various areas of biological engineering, such as single cell analysis, environmental monitoring, regenerative medicine, and point-of-care diagnostics. Despite the fact that intensive efforts have been devoted into the field in the past decades, microfluidics has not yet been adopted widely. It is increasingly realized that an effective system integration strategy that is low cost and broadly applicable to various biological engineering situations is required to fully realize the potential of microfluidics. In this article, we review several promising system integration approaches for microfluidics and discuss their advantages, limitations, and applications. Future advancements of these microfluidic strategies will lead toward translational lab-on-a-chip systems for a wide spectrum of biological engineering applications.

Integrated strain array for cellular mechanobiology studies

We have developed an integrated strain array for cell culture enabling high-throughput mechano-transduction studies. Biocompatible cell culture chambers were integrated with an acrylic pneumatic compartment and microprocessor-based control system. Each element of the array consists of a deformable membrane supported by a cylindrical pillar within a well. For user-prescribed waveforms, the annular region of the deformable membrane is pulled into the well around the pillar under vacuum, causing the pillar-supported region with cultured cells to be stretched biaxially. The optically clear device and pillar-based mechanism of operation enables imaging on standard laboratory microscopes. Straightforward fabrication utilizes off-the-shelf components, soft lithography techniques in polydimethylsiloxane, and laser ablation of acrylic sheets. Proof of compatibility with basic biological assays and standard imaging equipment were accomplished by straining C2C12 skeletal myoblast cells on the device for 6 hours. At higher strains, cells and actin stress fibers realign with a circumferential preference.

Actuation means for the mechanical stimulation of living cells via microelectromechanical systems: A critical review

Within a living body, cells are constantly exposed to various mechanical constraints. As a matter of fact, these mechanical factors play a vital role in the regulation of the cell state. It is widely recognized that cells can sense, react and adapt themselves to mechanical stimulation. However, investigations aimed at studying cell mechanics directly in vivo remain elusive. An alternative solution is to study cell mechanics via in vitro experiments. Nevertheless, this requires implementing means to mimic the stresses that cells naturally undergo in their physiological environment. In this paper, we survey various microelectromechanical systems (MEMS) dedicated to the mechanical stimulation of living cells. In particular, we focus on their actuation means as well as their inherent capabilities to stimulate a given amount of cells. Thereby, we report actuation means dependent upon the fact they can provide stimulation to a single cell, target a maximum of a hundred cells, or deal with thousands of cells. Intrinsic performances, strengths and limitations are summarized for each type of actuator. We also discuss recent achievements as well as future challenges of cell mechanostimulation.

Transfection of molecular beacons in microchannels for single-cell gene-expression analysis

BACKGROUND

Efficient transfection of molecular beacons has to be performed in the microscale in order to fully utilize the potential of molecular beacons and microfluidics for studying the real-time gene-expression dynamics in living cells. Nevertheless, there has been relatively little study on transfection of molecular beacons in microfluidic channels.

RESULTS

In this work, the differences between transfection in conventional cell culture systems and in microfluidic cell culture systems were investigated systematically with a combination of computational and experimental methods. Comparison between a no-flow microchannel and a 96-well plate revealed that the scale-dependence of reaction-diffusion kinetics contributes to the reduced transfection efficiency in the no-flow microchannel. Study on transfection in the microfluidic system under flow conditions suggested that the fluid flow enhances mass transfer, while the fluid shear stress can reduce the transfection efficiency.

CONCLUSION

The results of this study will provide useful guidelines in optimizing molecular beacon transfection efficiency in microfluidic systems for studying gene-expression dynamics in living cells.

MEMS Sensors and Microsystems for Cell Mechanobiology

Forces generated by cells play a vital role in many cellular processes like cell spreading, motility, differentiation and apoptosis. Understanding the mechanics of single cells is essential to delineate the link between cellular force generation/sensing and function. MEMS sensors, because of their small size and fine force/displacement resolution, are ideal for force and displacement sensing at the single cell level. In addition, the amenability of MEMS sensors to batch fabrication methods allows the study of large cell populations simultaneously, leading to robust statistical studies. In this review, we discuss various microsystems used for studying cell mechanics and the insights on cell mechanical behavior that have resulted from their use. The advantages and limitations of these microsystems for biological studies are also outlined.

Probing cell migration in confined environments by plasma lithography

Cellular processes are regulated by various mechanical and physical factors in their local microenvironment such as geometric confinements, cell-substrate interactions, and cell-cell contact. Systematic elucidation of these regulatory mechanisms is crucial for fundamental understanding of cell biology and for rational design of biomedical devices and regenerative medicine. Here, we report a generally applicable plasma lithography technique, which performs selective surface functionalization on large substrate areas, for achieving long-term, stable confinements with length scales from 100 nm to 1 cm toward the investigation of cell-microenvironment interactions. In particular, we applied plasma lithography for cellular confinement of neuroblastomas, myoblasts, endothelial cells, and mammary gland epithelial cells, and examined the motion of mouse embryonic fibroblasts in directionality-confined environments for studying the effect of confinements on migratory behavior. In conjunction with live cell imaging, the distance traveled, velocity, and angular motion of individual cells and collective cell migration behaviors were measured in confined environments with dimensions comparable to a cell. A critical length scale that a cell could conceivably occupy and migrate to was also identified by investigating the behaviors of cells using confined environments with subcellular length scales.

Optofluidic tunable microlens by manipulating the liquid meniscus using a flared microfluidic structure

We have designed, demonstrated, and characterized a simple, novel in-plane tunable optofluidic microlens. The microlens is realized by utilizing the interface properties between two different fluids: CaCl(2)solution and air. A constant contact angle of ∼90° is the pivotal factor resulting in the outward bowing and convex shape of the CaCl(2) solution-air interface. The contact angle at the CaCl(2) solution-air interface is maintained by a flared structure in the polydimethylsiloxane channel. The resulting bowing interface, coupled with the refractive index difference between the two fluids, results in effective in-plane focusing. The versatility of such a design is confirmed by characterizing the intensity of a traced beam experimentally and comparing the observed focal points with those obtained via ray-tracing simulations. With the radius of curvature conveniently controlled via fluid injection, the resulting microlens has a readily tunable focal length. This ease of operation, outstandingly low fluid usage, large range tunable focal length, and in-plane focusing ability make this lens suitable for many potential lab-on-a-chip applications such as particle manipulation, flow cytometry, and in-plane optical trapping.

Synergistically enhanced osteogenic differentiation of human mesenchymal stem cells by culture on nanostructured surfaces with induction media

We have examined the effects of surface nanotopography on in vitro osteogenesis of human mesenchymal stem cells (hMSCs). UV-assisted capillary force lithography was employed to fabricate a scalable (4x5 cm), well-defined nanostructured substrate of a UV curable polyurethane polymer with dots (150, 400, 600 nm diameter) and lines (150, 400, 600 nm width). The influence of osteogenic differentiation of hMSCs was characterized at day 8 by alkaline phosphatase (ALP) assay, RT-PCR, and real-time PCR analysis. We found that hMSCs cultured on the nanostructured surfaces in osteogenic induction media showed significantly higher ALP activity compared to unpatterned PUA surface (control group). In particular, the hMSCs on the 400 nm dot pattern showed the highest level of ALP activity. Further investigation with real-time quantitative RT-PCR analysis demonstrated significantly higher expression of core binding factor 1 (Cbfa1), osteopontin (OP), and osteocalcin (OC) levels in hMSCs cultured on the 400 nm dot pattern in osteogenic induction media. These findings suggest that surface nanotopography can enhance osteogenic differentiation synergistically with biochemical induction substance.

Electrothermal Fluid Manipulation of High-Conductivity Samples for Laboratory Automation Applications

Electrothermal flow is a promising technique in microfluidic manipulation toward laboratory automation applications, such as clinical diagnostics and high throughput drug screening. Despite the potential of electrothermal flow in biomedical applications, relative little is known about electrothermal manipulation of highly conductive samples, such as physiological fluids and buffer solutions. In this study, the characteristics and challenges of electrothermal manipulation of fluid samples with different conductivities were investigated systematically. Electrothermal flow was shown to create fluid motion for samples with a wide range of conductivity when the driving frequency was above 100 kHz. For samples with low conductivities (below 1 S/m), the characteristics of the electrothermal fluid motions were in quantitative agreement with the theory. For samples with high conductivities (above 1 S/m), the fluid motion appeared to deviate from the model as a result of potential electrochemical reactions and other electrothermal effects. These effects should be taken into consideration for electrothermal manipulation of biological samples with high conductivities. This study will provide insights in designing microfluidic devices for electrokinetic manipulation of biological samples toward laboratory automation applications in the future.

Biomimetic nanopatterns as enabling tools for analysis and control of live cells

It is becoming increasingly evident that cell biology research can be considerably advanced through the use of bioengineered tools enabled by nanoscale technologies. Recent advances in nanopatterning techniques pave the way for engineering biomaterial surfaces that control cellular interactions from the nano- to the microscale, allowing more precise quantitative experimentation capturing multi-scale aspects of complex tissue physiology in vitro. The spatially and temporally controlled display of extracellular signaling cues on nanopatterned surfaces (e. g., cues in the form of chemical ligands, controlled stiffness, texture, etc.) that can now be achieved on biologically relevant length scales is particularly attractive enabling experimental platform for investigating fundamental mechanisms of adhesion-mediated cell signaling. Here, we present an overview of bio-nanopatterning methods, with the particular focus on the recent advances on the use of nanofabrication techniques as enabling tools for studying the effects of cell adhesion and signaling on cell function. We also highlight the impact of nanoscale engineering in controlling cell-material interfaces, which can have profound implications for future development of tissue engineering and regenerative medicine.

In situ mechanical characterization of mouse oocytes using a cell holding device

This paper presents a cellular force measurement technique that allows for mechanical characterization of mouse oocytes during microinjection (i.e., in situ) without requiring a separate characterization process. The technique employs an elastic cell holding device and a sub-pixel computer vision tracking algorithm to resolve cellular forces in real time with a nanonewton force measurement resolution (2 nN at 30 Hz). Mechanical properties (i.e., stiffness) of both healthy and defective mouse oocytes are characterized. The experimental results suggest that the in situ obtained force-deformation data are useful for distinguishing healthy mouse oocytes from those with aging-induced cellular defects, promising an approach for oocyte quality assessment during microinjection. Biomembrane and cytoskeleton structures of the healthy and defective oocytes are also investigated in an attempt to correlate the measured subtle mechanical difference to cellular structure changes.

Lab-on-a-chip devices as an emerging platform for stem cell biology

The advent of stem cell based therapies has brought regenerative medicine into an increased focus as a part of the modern medicine practice, with a potential to treat a myriad of intractable diseases in the future. Stem cells reside in a complex microenvironment presenting them with a multitude of potential cues that are chemical, physical, and mechanical in nature. Conventional techniques used for experiments involving stem cells can only poorly mimic the physiological context, and suffer from imprecise spatial and temporal control, low throughput, lack of scalability and reproducibility, and poor representation of the mechanical and physical cell microenvironment. Novel lab-on-a-chip platforms, on the other hand, can much better mimic the complexity of in vivo tissue milieu and provide a greater control of the parameter variation in a high throughput and scalable manner. This capability may be especially important for understanding the biology and cementing the clinical potential of stem cell based therapies. Here we review microfabrication- and microfluidics-based approaches to investigating the complex biology of stem cell responses to changes in the local microenvironment. In particular, we categorize each method based on the types of controlled inputs it can have on stem cells, including soluble biochemical factors, extracellular matrix interactions, homotypic and heterotypic cell-cell signaling, physical cues (e.g. oxygen tension, pH, temperature), and mechanical forces (e.g. shear, topography, rigidity). Finally, we outline the methods to perform large scale observations of stem cell phenotypes and high-throughput screening of cellular responses to a combination of stimuli, and many new emerging technologies that are becoming available specifically for stem cell applications.

Simple haptotactic gradient generation within a triangular microfluidic channel

Most microfluidic devices developed to date for the analysis of live cells incorporate channels with relatively simple constant rectangular or semi-circular cross-sections, relying on complex channel network geometries rather than alteration of the shapes of the channels themselves for development of diverse functional fluidic controls, e.g., spatial gradients of bioactive ligands. In this study we describe a simple alternative method to create highly defined and predictable gradients of surface bound molecules. This method relies on the generation of a considerable variation in the spatial distribution of flow velocities within a channel with a triangular cross-section. The triangular shape can be easily implemented by using bulk wet etching and polydimethylsiloxane (PDMS) replica molding techniques. By analytical modeling and simulation, we predict that the deposition of the solute onto a channel boundary depends on the local flow rate values, yielding gradient spanning the whole width of the channel. This prediction was validated by direct visualization of the flow rate and fibronectin-rhodamine deposition in a fabricated microchannel. Using this experimental platform, we assessed cell migration in response to a fibronectin gradient deposited in the microchannels. We find that this gradient could induce robust haptotaxis of Chinese Hamster Ovary (CHO) cells towards the areas of higher fibronectin surface density. We propose that the described simple gradient generation method can help to avoid complexity present in many current device designs, allowing to introduce more easily other potentially useful design features.

Antimicrobial susceptibility testing using high surface-to-volume ratio microchannels

This study reports the use of microfluidics, which intrinsically has a large surface-to-volume ratio, toward rapid antimicrobial susceptibility testing at the point of care. By observing the growth of uropathogenic Escherichia coli in gas permeable polymeric microchannels with different dimensions, we demonstrate that the large surface-to-volume ratio of microfluidic systems facilitates rapid growth of bacteria. For microchannels with 250 microm or less in depth, the effective oxygenation can sustain the growth of E. coli to over 10(9) cfu/mL without external agitation or oxygenation, which eliminates the requirement of bulky instrumentation and facilitates rapid bacterial growth for antimicrobial susceptibility testing at the point of care. The applicability of microfluidic rapid antimicrobial susceptibility testing is demonstrated in culture media and in urine with clinical bacterial isolates that have different antimicrobial resistance profiles. The antimicrobial resistance pattern can be determined as rapidly as 2 h compared to days in standard clinical procedures facilitating diagnostics at the point of care.

Computational simulation of a magnetic microactuator for tissue engineering applications

The next generation of tissue engineered constructs (TECs) requires the incorporation of a controllable and optimized microstructure if they are to chemically, mechanically, and biologically mimic tissue function. In order to obtain TECs with optimized microstructures, a combination of spatiotemporally regulated mechanical and biochemical stimuli is necessary during the formation of the construct. While numerous efforts have been made to create functional tissue constructs, there are few techniques available to stimulate TECs in a localized manner. We herein describe the design of a microdevice which can stimulate TECs in a localized, inhomogeneous, and predefined anisotropic fashion using ferromagnetically doped polydimethylsiloxane microflaps (MFs). Specifically, a sequential magneto-structural finite element model of the proposed microdevice is constructed and utilized to understand how changes in magnetic and geometrical properties of the device affect MF deflection. Our study indicates that a relatively small density of ferromagnetic material is required to result in adequate force and MF defection (175 microm approximately 7% TEC strain). We also demonstrate that MF to magnet distance is more important than inherent MF magnetic permeability in determining resulting MF deflection. An experimental validation test setup was used to validate the computational solutions. The comparison shows reasonable agreement indicating a 5.9% difference between experimentally measured and computationally predicted MF displacement. Correspondingly, an apparatus with two MFs and two magnets has been made and is currently undergoing construct testing. The current study presents the design of a novel magnetic microactuator for tissue engineering applications. The computational results reported here will form the foundation in the design and optimization of a functional microdevice with multiple MFs and magnets capable of stimulating TECs in nonhomogenous and preferred directions with relevant spatial resolution.

Mechanical induction of gene expression in connective tissue cells

The extracellular matrices of mammals undergo coordinated synthesis and degradation, dynamic remodeling processes that enable tissue adaptations to a broad range of environmental factors, including applied mechanical forces. The soft and mineralized connective tissues of mammals also exhibit a wide repertoire of mechanical properties, which enable their tissue-specific functions and modulate cellular responses to forces. The expression of genes in response to applied forces are important for maintaining the support, attachment, and function of various organs including kidney, heart, liver, lung, joint, and periodontium. Several high-prevalence diseases of extracellular matrices including arthritis, heart failure, and periodontal diseases involve pathological levels of mechanical forces that impact the gene expression repertoires and function of bone, cartilage, and soft connective tissues. Recent work on the application of mechanical forces to cultured connective tissue cells and various in vivo force models have enabled study of the regulatory networks that control mechanically induced gene expression in connective tissue cells. In addition to the influence of mechanical forces on the expression of type 1 collagen, which is the most abundant protein of mammals, new work has shown that the expression of a wide range of matrix, signaling, and cytoskeletal proteins are regulated by exogenous mechanical forces and by the forces generated by cells themselves. In this chapter, we first discuss the fundamental nature of the extracellular matrix in health and the impact of mechanical forces. Next we consider the utilization of several, widely employed model systems for mechanical stimulation of cells. Finally, we consider in detail how application of tensile forces to cultured cardiac fibroblasts can be used for the characterization of the signaling systems by which mechanical forces regulate myofibroblast differentiation that is seen in cardiac pressure overload.

Mechanosensitivity of fibroblast cell shape and movement to anisotropic substratum topography gradients

In this report, we describe using ultraviolet (UV)-assisted capillary force lithography (CFL) to create a model substratum of anisotropic micro- and nanotopographic pattern arrays with variable local density for the analysis of cell-substratum interactions. A single cell adhesion substratum with the constant ridge width (1 microm), and depth (400 nm) and variable groove widths (1-9.1 microm) allowed us to characterize the dependence of cellular responses, including cell shape, orientation, and migration, on the anisotropy and local density of the variable micro- and nanotopographic pattern. We found that fibroblasts adhering to the denser pattern areas aligned and elongated more strongly along the direction of ridges, vs. those on the sparser areas, exhibiting a biphasic dependence of the migration speed on the pattern density. In addition, cells responded to local variations in topography by altering morphology and migrating along the direction of grooves biased by the direction of pattern orientation (short term) and pattern density (long term), suggesting that single cells can sense the topography gradient. Molecular dynamic live cell imaging and immunocytochemical analysis of focal adhesions and actin cytoskeleton suggest that variable substratum topography can result in distinct types of cytoskeleton reorganization. We also demonstrate that fibroblasts cultured as monolayers on the same substratum retain most of the properties displayed by single cells. This result, in addition to demonstrating a more sophisticated method to study aspects of wound healing processes, strongly suggests that even in the presence of adhesive cell-cell interactions, the cues provided by the underlying substratum topography continue to exercise substantial influence on cell behavior. The described experimental platform might not only further our understanding of biomechanical regulation of cell-matrix interactions, but also contribute to bioengineering of devices with the optimally structured design of cell-material interface.