Carolyn L Ren

• PhD
• Professor at University of Waterloo

Viscosity is a critical fluid property that significantly influences fluid behavior and performance across various systems. Most commercial viscometers require relatively large sample volumes (on the order of milliliters), which restricts their utility in scenarios where only limited sample volumes are available. For instance, human tear fluid—esse...

Free‐flow counterflow gradient focusing (FF‐CGF) is a promising tool for protein fractionation. This work establishes guidelines to construct an FF‐CGF system suitable for preparative applications. Numerical and analytical computations are undertaken to investigate design parameters, including device geometry, flow conditions, sample concentration,...

Despite the exponentially expanding capabilities of robotic systems with the introduction of soft robotics, the lack of practical considerations in designing and integrating soft robotic components hinders the widespread application of newly developed technology in real life. This study investigates the development and performance evaluation of sof...

This paper introduces an hands-on laboratory exercise focused on assembling and testing a hybrid soft-rigid active finger prosthetic for biomechanical and biomedical engineering education. This hands-on laboratory activity focuses on the design of a myoelectric finger prosthesis, integrating mechanical, electrical, sensor (i.e., inertial measuremen...

We present a novel electronics-free soft robotic knee brace which employs a closed-loop fluidic regenerative (CLFR) system for dynamic unloading in unicompartmental tibiofemoral osteoarthritis (OA). The existing dynamic unloaders are bulky and heavy largely and have low compliance likely due to the use of electrical control box, which is eliminated...

Background Vertical displacement of the residual limb within transtibial prosthetic socket, often known as “pistoning” or downward movement, may lead to skin breakdowns and ulcers. Downward movement is particularly difficult to self-manage for diabetic individuals living with amputation because of diminished sensation in the residual limb from peri...

Polydimethylsiloxane (PDMS) elastomers have been extensively used in the development of microfluidic devices, capable of miniaturizing biomolecular and cellular assays to the microlitre and nanolitre range, thereby increasing the throughput of experimentation. PDMS has been widely used due to its optical clarity and biocompatibility, among other de...

Inaccurate prediction of droplet hydrodynamic resistance has a profound impact on droplet chip performance and lengthens the iterative design process. Previous studies measuring droplet resistance use various approaches such as interface comparison to quantify flow rate, and pressure taps; all these methods are classified as passive. Although each...

Wearable technology and soft robotics have experienced significant growth in the past decade. In these applications, flexible and highly sensitive pressure and bending sensors are essential for providing feedback to control and operate devices. Despite the development of many pressure and bending sensors, the challenge remains to develop one single...

Free‐flow electrophoresis (FFE) has the ability to continuously separate charged solutes from complex biological mixtures. Recently, a free‐flow counterflow gradient focusing (FF‐CGF) mechanism has been introduced to FFE, and it offers the potential for improved resolution and versatility. However, further investigation is needed to understand the...

Heavy metal pollution on earth has evolved into a global issue causing serious risks to human health and other living entities and having an impact on sustainability. Accurate identification of metal contamination is often carried out in centralized facilities involving sampling, transportation, and the need for highly trained personnel, which beco...

Non-invasive contactless simultaneous sensing and heating of individual droplets would allow droplet microfluidics to empower a wide range of applications. However, it is challenging to realize simultaneous sensing and heating of individual droplets as the resonance frequency of the droplet fluid, which is decided by its permittivity, must be known...

Residual limb volume fluctuation and the resulting contact pressures are some of the key factors leading to skin ulcerations, suboptimal prosthetic functioning, pain, and diminishing quality of life of transtibial amputees. Self-management of socket fit is complicated by peripheral neuropathy, reducing the perception of pressure and pain in the res...

Polydimethylsiloxane (PDMS) is a widely used material for microfluidic devices due to its low cost, superior optical properties and fast iterative design process. Its softness however creates challenges for the device design and operation because part of the applied pressures contributes to deform chips instead of controlling the flow. The resultin...

A proof of concept of a novel air microfluidics-enabled soft robotic sleeve to enable lymphedema treatment is presented. Compression sleeves represent the current, suboptimal standard of care, and stationary pumps assist with lymph drainage; however, effective systems that are truly wearable while performing daily activities are very scarce. This p...

The global COVID-19 pandemic caused by SARS-CoV-2 has resulted in an unprecedented economic and societal impact. Developing simple and accurate testing methods for point-of-care (POC) diagnosis is crucial not only for the control of COVID-19, but also for better response to similar outbreaks in the future. In this work, we present a novel proof-of-...

Purpose: To evaluate the shear viscosity of contemporary, commercially available ocular lubricants at various shear rates and temperatures and to derive relevant mathematical viscosity models that are impactful for prescribing and developing eye drops to treat dry eye disease. Methods: The shear viscosity of 12 ocular lubricants was measured usi...

Microfluidics has progressed tremendously as a field over the last two decades. Various areas of microfluidics developed in fully-fledged domains of their own such as organ-on-a-chip, digital and paper microfluidics. Nevertheless, the technological advancement of microfluidics as a field has not yet reached end-users for independent use. This is th...

This work represents the second part of a two-part series on the dynamics of droplet formation in a T-junction generator under the squeezing regime when using solutions of red blood cells as the dispersed phase. Solutions containing red blood cells are non-Newtonian; however, these solutions do not behave in the same way as other non-Newtonian flui...

The extension of microfluidics to many bioassay applications requires the ability to work with non-Newtonian fluids. One case in point is the use of microfluidics with blood having different hematocrit levels. This work is the first part of a two-part study and presents the formation dynamics of blood droplets in a T-junction generator under the sq...

The usability and robustness of droplet microfluidic devices constitute significant challenges to the widespread adoption of droplet microfluidics despite the field’s immense promise. Active droplet manipulation via visual feedback-based pressure driven control has addressed some of the concerns; however, the use of large and costly optical microsc...

Free‐flow electrophoresis (FFE) enables the continuous separation and collection of charged solutes, and as a result it has drawn interest as both a preparative and an analytical tool for biological applications. Recently, a free‐flow counterflow gradient focusing (FF‐CGF) mechanism has been proposed with the goal of improving the resolution and ve...

Microfluidics is a science that deals with small amounts of fluids at the microscale. The confinement brings in many advantages such as reduced reagent consumption, shortened analysis time, increased throughput and potential for portable systems. These advantages and others make microfluidic systems ideal enabling platform technologies that can be...

Soft robotics has gone through a decade of tremendous progress in advancing both fundamentals and technologies. It has also seen a wide range of applications such as surgery assistance, handling of delicate foods, and wearable assistive systems driven by its soft nature that is more human friendly than traditional hard robotics. The rapid growth of...

Microfluidics is an enabling technology that leverages smaller dimensions. The development of droplet microfluidics by introducing a second phase aimed to address challenges arising when using a single phase. The advantages of droplet microfluidics are used for various biomedical applications. Biomaterial applications are enabled by the complex str...

In this note, we report a simple, new method for droplet generation in microfluidic systems using integrated microwave heating. This method enables droplet generation on-demand by using microwave heating to induce Laplace pressure change at the interface of the two fluids. The distance between the interface and junction and microwave excitation pow...

Simple yet powerful clustered regularly-interspaced short palindromic repeats (CRISPR) technology has led to the advent of numerous developments in life sciences, biotechnology, therapeutics, and molecular diagnostics, enabled by gene editing capability. By exploiting the CRISPR-Cas system’s nucleic acid sequence detection abilities, CRISPR-based m...

A review on microfluidic technology for antibacterial resistance study and antibiotic susceptibility testing (AST) is presented here. Antibiotic resistance has become a global health crisis in recent decades, severely threatening public health, patient care, economic growth, and even national security. It is extremely urgent that antibiotic resista...

Microfluidics typically uses either a syringe pump that regulates the flow rate in microchannels or a pressure pump that controls the inlet pressures to drive the flow. In the context of pressure-driven flow, a reservoir holder containing liquid samples is normally used to interface the pressure pump with the microfluidic chip via soft tubing. The...

Microfluidic platform is an enabling technology for a wide range of applications such as life science research, material synthesis, drug discovery and environmental monitoring and protection. Most applications require separation or filtration of samples such as cell or particle separation which can be achieved by manipulating flow and channel geome...

A dynamic and low-profile unloader tibiofemoral knee brace is designed and prototyped by synergizing concepts from the fields of microfluidics and soft robotics. Microfluidics provides strategies for miniaturization and multiplexing while soft robotics afford the tools to create soft fluidic actuators and allow compliant and inherently safe robotic...

Microfluidics is a science that deals with small amounts of fluids at the microscale. The confinement brings in many advantages such as reduced reagent consumption, shortened analysis time, increased throughput and potential for portable systems. These advantages and others make microfluidic systems ideal enabling platform technologies that can be...

Microwave resonator offers simultaneous sensing and heating of individual droplets in microchannels presenting unique advantages for applications requiring well-controlled reaction time. This study numerically investigates the effects of the temperature-dependent fluid properties including viscosity, density, diffusivity, and interfacial tension, o...

With its ability to continuously separate and collect charged analytes, free-flow electrophoresis (FFE) has become a useful tool for the purification and real-time analysis of biological mixtures. This work presents a new free-flow counterflow gradient focusing (FF-CGF) mechanism that uses a novel velocity gradient to counterbalance electrophoretic...

An open-source precision pressure pump system and control software is presented, primarily designed for the experimental microfluidics community, although others may find additional uses for this precision pressure source. This mechatronic system is coined ‘µPump,’ and its performance rivals that of commercially available systems, at a fraction of...

Off-the-shelf planar strain gauges are ubiquitous and are generally designed for materials with a large elastic modulus such as steel or aluminum. Correspondingly, the strain gauges themselves are stiff and do not deform substantially under applied stress. Pairs of this type of strain gauge are typically used in a Wheatstone bridge circuit allowing...

Active control of individual picolitre- to nanoliter sized droplets in a network of microchannels is vital to make droplet microfluidic platform an enabling technology for single-cell or single particle analysis, which has found application in areas such as advanced manufacturing, material synthesis, life science research and personalized medicine....

One key factor that plays a significant role in the efficiency of on-chip electrophoresis separation is the initial shape of the sample plug at injection. For microchips involved in separation, injection is a key step in efficient analysis, however, detailed analysis of the effect of sample plug shape and size at injection on separation efficiency...

Electrophoretic deposition (EPD) is useful for conductive substrates, where a requisite electrical path can be formed. In order to make EPD more broadly applicable for semi-/non-conducting substrates, e.g. EPD of carbon nanotubes (CNTs) on silicon and rubber, we proposed and demonstrated a simple modified EPD set-up. The substrate was directly atta...

Microfluidics is the science of miniaturization of conventional lab devices to microchips; electrophoresis is one of the techniques for on-chip DNA and protein separation. In this article, nondimensionalization of the problem governing equations and boundary conditions is presented as preliminary preparation for performing computational and physica...

In the setup for oil/water separation, a piece of sample was clamped under the tube and a bowl was used to collect the filtrated water. For better visualization, oil was dyed by oil red. When the sample was prewetted by water, it separated oil/water, allowing water to filtrate and resisting the oil (holding the oil on top the sample).

Automated control of individual droplets in microfluidic channels offers tremendous potential for applications requiring high accuracy and minimal user involvement. The feasibility of active droplet control has been previously demonstrated with pressure-driven flow control and visual feedback, but the manual operation required to perform droplet ma...

This work investigated a two-step surface modification of polyamide meshes and nonwoven fabrics for oil/water separation and looked into the durability of such modified polyamide. The two-step modification included 1) pre-etching the polyamide surface using plasma treatment and 2) coating the pre-etched surface by eco-friendly polydopamine (PDA)/ce...

Counter‐flow gradient electrofocusing uses the bulk flow of a liquid solution to counterbalance the electrophoretic migration of an analyte. When either the bulk velocity or the electrophoretic velocity of the analyte is made to vary across the length of the channel, there exists a unique zero‐velocity point for the analyte. This focusing method en...

With the advent of microfluidics and lab-on-chip systems, DNA and protein separation technologies are being developed for biology, diagnostics, and health purposes. Fully realizing these applications requires developing numerical models for sample transport. In this paper, a thorough investigation of electrokinetics and microfluidics transport phen...

The functional performance of passively operated droplet microfluidics is sensitive with respect to the dimensions of the channel network, the fabrication precision as well as the applied pressure because the entire network is coupled together. Especially, the local and global hydrodynamic resistance changes caused by droplets make the task to deve...

Porous polyamide functionalized by plasma or various coatings has been investigated for oil/water separation. In literature, polyamide has rarely been studied for oil removal, and this work investigated the performance of bare polyamide 6.6 (nylon 6.6) in terms of the oil/water separation efficiency and the intrusion pressure, inspiring cost-effect...

We report an experimental study on the hydrodynamic shrinkage of supercritical carbon dioxide (scCO2) microdroplets during a nonequilibrium process. After scCO2 microdroplets are generated by water shearing upon a scCO2 flow in a micro T-junction, they are further visualized and characterized at the midpoint and the ending point of a straight recta...

Manipulation of light transmission/absorbance and reflection/emission has a great significance in smart windows and displaying media like liquid crystal. Here, we report the usage of an external electric field to reversibly switch the molecular spectra of a model molecule based on its interaction with an electro-responsible polymer brush. Both the...

Quantum dots (QDs) provide opportunities for the development of bioassays, biosensors, and drug delivery strategies. Decoration of the surface of QDs offers unique functions such as resistance to non-specific adsorption, selective binding to target molecules, and cellular uptake. The quality of decoration has substantial impact on the functionality...

Taylor segments, as a common feature in two- or multi-phase microflows, are a strong flow pattern candidate for applications when enhanced heat or mass transfer is particularly considered. A thin film that separates these segments from touching the solid channel and the flow fields near and inside the segment are two key factors that influence (eit...

The process of detection and separation of yeast cells based on their morphological characteristics is critical to the understanding of cell division cycles, which is of vital importance to the understanding of some diseases such as cancer. The traditional process of manual detection is usually tedious and inconsistent. This paper presents a microf...

Hydrodynamic shrinkage of liquid CO2 drops in water under a Taylor flow regime is studied using a straight microchannel (length/width ~ 100). A general form of a mathematical model of the solvent-side mass transfer coefficient (ks) is developed first. Based on formulations of the surface area (A) and the volume (V) of a general Taylor drop in a rec...

In this study, mono-dispersed poly (acrylamide-co-sodium acrylate) hydrogel micro-particles with the controlled water absorbance capacity were synthesized in a droplet microfluidic device which can be used for enhanced oil recovery application. The experimental method and the microfluidic device were optimized to produce a well-spaced mono-disperse...

This is the first part of a two-part study on a partially miscible liquid-liquid flow (liquid carbon dioxide and deionized water) which is highly pressurized and confined in a microfluidic T-junction. Our main focuses are to understand the flow regimes as a result of varying flow conditions and investigate the characteristics of drop flow distinct...

This is the second part of a two-part study on a partially miscible liquid-liquid flow (carbon dioxide and deionized water) that is highly pressurized and confined in a microfluidic T-junction. In the first part of this study, we reported experimental observations of the development of flow regimes under various flow conditions and the quantitative...

Microfluidic flow-focusing devices offer excellent control over fluid flow, enabling formation of drops with a narrow size distribution. However, the throughput of microfluidic flow-focusing devices is limited and scale-up through operation of multiple drop makers in parallel often compromises the robustness of their operation. We demonstrate that...

Developing a microfluidic chip with multiple functions is highly demanded for practical applications, such as chemical analysis, diagnostics, particles synthesis and drug screening. This work demonstrates a microfluidic chip integrated with a series of functions including droplet generation, pairing, trapping, merging, mixing and releasing, and con...

The nanoparticle seed layer (a film) and vertically aligned nanorods of zinc oxide (ZnO) with different lengths were fabricated within a novel chamber-based microfluidic (microchamber) reactor with a varying height of 0.127-5 mm and characterized with their microstructures, photocatalytic performances as well as qualitative reaction kinetics. The Z...

Droplet microfluidics has been proven to be a viable platform for a variety of reactions by using uniform droplets well encapsulated by an immiscible carrier medium as reaction vesicles. Merging as one of the most basic forms of droplet manipulation is highly demanded in controlling the concentration of reagents for reactions. After preliminary tes...

High throughput analysis is highly demanded in a variety of chemical reactions. Droplet microfluidics offers unique advantages over traditional multi-well plate systems for high throughput analysis such as providing a confined and more controllable environment for single particle or cell analysis. Driven by the need to improve the efficiency of enc...

In this study, we present a microwave based microfluidic mixer that allows rapid mixing within individual droplets efficiently. The designed microwave mixer is a coplanar design with a small footprint, which is fabricated on a glass substrate and integrated with a microfluidic chip. The mixer works essentially as a resonator that accumulates intens...

96-well plate has been the traditional method used for screening drug compounds libraries for potential bioactivity. Although this method has been proven successful in testing dose-response analysis, the microliter consumption of expensive reagents and hours of reaction and analysis time call for innovative methods for improvements. This work demon...

We combine image processing and feedback controls to regulate droplet movements. A general modelling approach is provided to describe droplet motion in a pressure-driven microfluidic channel network. A state space model is derived from electric circuit analogy and validated with experimental data. We then design simple decentralized controllers to...

In spite of various existing thermometry methods for microfluidic applications, it remains challenging to measure the temperature of individual droplets in segmented flow since fast moving droplets do not allow sufficient exposure time demanded by both fluorescence based techniques and resistance temperature detectors. In this contribution, we pres...

It is widely believed that media surface roughness enhances particle deposition-numerous, but inconsistent, examples of this effect have been reported. Here, a new mathematical framework describing the effects of hydrodynamics and interaction forces on particle deposition on rough spherical collectors in absence of an energy barrier was developed a...

Deposition of colloidal- and nano-scale particles on surfaces is critical to numerous natural and engineered environmental, health, and industrial applications ranging from drinking water treatment to semi-conductor manufacturing. Nano-scale surface roughness-induced hydrodynamic impacts on particle deposition were evaluated in the absence of an en...

This study reports a microwave-microfluidics integrated approach capable of performing droplet detection at high-throughput as well as content sensing of individual droplets without chemical or physical intrusion. The sensing system consists of a custom microwave circuitry and a spiral-shaped microwave resonator that is integrated with microfluidic...

This study reports a new method for establishing an open tubular immobilized pH gradient (IPG) in a microchip coupled with a whole column image detection (WCID) system for protein separation applications. This method allows a wider range of immobilized pH (2.6-9.5) to be established in a PDMS/quartz channel by controlling the diffusion of acidic an...

A two-dimensional separation system is presented combining scanning temperature gradient focusing (TGF) and sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis (PAGE) in a PDMS/glass microfluidic chip. Denatured proteins are first focused and separated in a 15mm long channel via TGF with a temperature range of 16-47°C and a pressure sca...

Counter-flow gradient electrofocusing allows the simultaneous concentration and separation of analytes by generating a gradient in the total velocity of each analyte which is the sum of its electrophoretic velocity and the bulk counter-flow velocity. In the scanning format, the bulk counter-flow velocity is varying with time so that a number of ana...

This work presents a simple method to fabricate controllable microscale wave structures on the top of regular interdigitated electrode (IDE) arrays using electrically-assisted lithography techniques. Smooth wave structures are extremely difficult, if not impossible, to fabricate using traditional multilayer photolithography technology. The fabricat...

Flow focusing generators have been widely used to generate droplets for many applications which call for accurate physical models that describe the droplet formation process in such configurations for design and operation purposes. Most existing models are empirical correlations obtained based on extensive experimental results and thus very sensiti...

This work presents a robust method for whole chip temperature mapping in microfluidic devices using a photostable fluorescent-polymer thin film that can be incorporated during the bonding stage. Temperature measurements are based on the ratio of two bands in the fluorescence spectrum of N,N-bis(2,5-di-tertbutylphenyl)-3,4,9,10-perylenedi carboximid...

Droplet-based microfluidics is an emerging high-throughput screening technology finding applications in a variety of areas such as life science research, drug discovery and material synthesis. In this paper we present a cost-effective, scalable microwave system that can be integrated with microfluidic devices enabling remote, simultaneous sensing a...

Isoelectric focusing (IEF) is considered as an attractive separation technique for biologically amphoteric compounds (e.g., proteins and peptides) based on their isoelectric point (pI). With the advancement in micromachining technology, microchip format IEF has attracted significant attention. Both single-point and whole column imaging detection (W...

Motivated by stabilizing a hydrophilic surface condition of polydimethylsiloxane (PDMS) and improving its zeta potential, several reported methods for treating PDMS surface were evaluated using two or more of the following techniques: capillary flow in dryly stored channels, static contact angle measurement, current monitoring technique and attenua...

Micellar affinity gradient focusing (MAGF) is a microfluidic counterflow gradient focusing technique that combines the favorable features of MEKC and temperature gradient focusing. MAGF separates analytes on the basis of a combination of electrophoretic mobility and partitioning with the micellar phase. A temperature gradient is produced along the...

This study investigates the influence of both local generator design and global network architecture in improving the stability and operational performance of microfluidic droplet generators. We identify naturally occurring short-term and long-term oscillations that are related to changes in the flow of the two phases. Short-term oscillations are r...