This PDF file contains the front matter associated with SPIE Proceedings Volume 11637 including the Title Page, Copyright information, and Table of Contents.

Welcome and Introduction to SPIE Conference 11637: Microfluidics, BioMEMS, and Medical Microsystems XIX

Microstructured fused silica glass is of high demand for many applications in microsystems engineering, microfluidics and microoptics. However, structuring of fused silica glass is extremely difficult needing either very high temperatures for melt processing, hazardous chemicals for wet etching or time-consuming mechanical post-processing steps. The lack of feasible high-throughput manufacturing techniques prevent the usage of fused silica for many applications so far. Recently, soft replication and 3D printing of fused silica were introduced using silica nanocomposites that are converted into fused silica glass in a subsequent heat treatment. While these processes allow facile structuring of silica glasses on the laboratory scale, it is not suited for high-throughput manufacturing. In this work, we present a process towards rapid manufacturing of microstructured fused silica glass by hot embossing a thermoplastic silica nanocomposite. The structured nanocomposite is converted to high quality fused silica glass by subsequent debinding and sintering at 1320°C. We show hot embossing of microoptical structures as well as microfluidic channels with an aspect ratio of up to 6. Further, we have developed facile solvent and thermal bonding procedures allowing the fabrication of embedded and fully functional microfluidic chips in fused silica.

Structuring polymeric materials is important for almost all applications in microsystems engineering, microfluidics and microoptics. Especially rapid prototyping using direct optical printing methods has gained great importance, also for facilitating product development for microfluidic applications. However, the choice of materials is still limited. Polystyrene (PS) is the material of choice for medical, biological and biochemical applications due to its biocompatibility, optical transparency, surface properties and low costs. However, PS is usually structured using industrial polymer replication techniques like injection molding or hot embossing. So far, only little work has been done on rapid prototyping and direct printing of microfluidic chips in PS. In this work, we present a novel liquid polystyrene prepolymer, which can be photocured and structured on the microscale using direct lithography printing. Using this method microchannels with a minimum channel width of 500 µm have been fabricated. The cured PS shows material properties comparable to those of commercially available polystyrene.

Digital microfluidics (DMF) represent an important subfield of microfluidics due to the ability to resolve complex processes into a sequence of programmable discrete steps with small liquid volumes. The most commonly used surfaces for liquid actuation have an unstructured hydrophobic layer as the top-coat having biofouling as a main obstacle. In this work we present a novel material, Fluoropor a nanofoamed Fluoropolymer that can be easily fabricated, coated and utilized for DMF. Fluoropor-films were fabricated with different porosities simply by adjusting the ratio of added porogens. The fabricated films ware integrated in a commercial DMF-device and it was shown that droplets can be moved and merged on top of a 22 μm thick Fluoropor-coating.

Digital microfluidics (DMF) is an emerging technology for liquid-handling of picoliter- to microliter-sized droplets. It enables individual control over droplets by applying electrical fields to an array of electrodes. Standard DMF devices include four key components: substrates, electrodes, a dielectric layer and hydrophobic layers. This work outlines the fabrication of dielectric layers with a high relative permittivity by inkjet printing. The layers consists of OrmoComp, silver nanoparticles and different solvents. OrmoComp has a relative permittivity of about 2.5. By adding 24.2 vol% of silver nanoparticles the relative permittivity rises to 76. Thereby the operating voltage can be reduced drastically.

Using bioinks for 3D bioprinting of cellular constructs remains a challenge due to factors including viscosity, fluid dynamics and shear stress. The encapsulation of cells within the bioinks directly affects the quality of 3D bioprinting and microfluidic pumping is a commonly used supporting approach. The accuracy of microfluidic pumps can be further improved by introducing various mixing techniques. However, many of these techniques introduce complex geometries or external fields. In this study, we used a simple control technique of time-dependent pulsing for instant gelation of the peptide bioinks and observed its effect during the bioprinting process. Various time-dependent periodic signals are imposed on to a stable flow cycle and the effects are analyzed. The microfluidic pumps are programmed with different flow patterns represented by low frequency sinusoidal pulses, ramp inputs, and duty cycle pulses. Different combinations of these pulses are tested to achieve an optimal pulse for improved quality of printed constructs. Time-varied pulsing of microfluidic pumps, particularly as square waveforms, is found to provide better continuous flow and avoid material buildup within the extruder unit when compared to pumping at a constant flow rate with manual tuning. Clogging is avoided since the gelation rate is periodically reduced which avoids gel clumps in the printed constructs. This study substantially improves the use of suitable peptide bioinks, standardizes the 3D bioprinting process, and reduces clogging and clumping during printing. Our findings allow for printing of more accurate and complex constructs for applications in tissue engineering, such as skin grafting, and other regenerative medical applications.

We present a library of resin formulations for projection micro-stereolithography (PµSL) consisting of 4-hydroxybutyl acrylate (HBA), poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) and poly(ethylene glycol) diacrylate (PEGDA), diluted with aqueous solutions of the photoinitiator lithium phenyl(2,4,6-trimethylbenzoyl)phosphinate (LAP) and the photoabsorber tartrazine. By varying the concentration and molecular weight of PEGMEMA and PEGDA, the swelling ratios of as-PµSL-printed hydrogel microstructures in water are well tunable with a reversible volume increase ranging from 13% to 86%. Furthermore, we illustrate the influence of exposure time per 3D-printed layer on the swelling ratio of the hydrogels, as well as the swelling time. The minimum feature size of rectangular void structures achieved with an exemplary resin from our material library is approx. 71 μm, while rectangular microchannels at the surface of a PµSLprinted hydrogel made from the same photopolymer formulation exhibit cross-sectional dimensions designed at 54 μm x 50 μm. Based on this initial characterization, microfluidic devices are fabricated to elucidate dimensional changes of microchannels under different swelling conditions (e.g., free swelling and confined swelling inside a chamber or microfluidic device). In addition, we PµSL-print microscopic parts with tailored geometries (cylindrical, pyramidal) that are capable of completely closing microfluidic chambers made from commercially available Perfactory R11 resin in a time-dependent fashion. Our resin library provides 3D-printed hydrogels with micron-scale feature size combined with tunable water uptake, rendering them suitable for designing functional microfluidic units such as membranes, valves and pumps.

We present our progress on hybrid additive and subtractive 3D femtosecond laser direct writing (fs-3D-LDW) of protein bovine serum albumin (BSA). We combine the additive multiphoton cross-linking with consecutive subtractive fs-LDW ablation to tailor geometries of proteinaceous microstructures. In this paper, we perform restructuring of the proteinaceous solid structures to show 3D treatment capability.

Microfluidic systems are growing increasingly prevalent as the future for modern medicine. With the transition to miniaturized systems, comes a growing need for equally miniaturized fluid delivery and control mechanisms. Electrokinetic pumping systems are uniquely suited to this task due to low power requirements and ease of scalability. Electrothermal micropumps in particular are efficient at manipulating high conductivity fluids, such as biofluids. This work describes methods by which electrothermal pumps are effectively simulated, fabricated, and tested with unique improvements designed to improve efficiency and adoptability in microfluidic systems.

Colorimetric analysis is being broadly applied in chemical sensing today; however, detection ranges and resolution limits are typically modest. In this paper, we introduce a methodology to quantify the colorimetric chemical response on a paper-based microfluidic device that enables high-resolution colorimetric detection over a broad pH range. We have achieved this by combining data from various indicators displaying sensitivity on partially overlapping small pH ranges and training machine learning classification models to the colorimetric output. The training dataset consists of images taken from the colorimetric response of three different pH indicators previously deposited on circular spots of a multilayer paper-based device, captured with a reference lab-grade camera. Instead of restricting the use of each pH indicator to their linear response regime within the RGB space, the models are trained against data spanning the entire range of pH values, from 3 to 9, in increments of 0.1, exploring the optimum combination of feature engineering and classification model to maximize the overall model accuracy. The combined analysis of image data captured simultaneously with the three indicators resulted in a pH detection accuracy above 85% with over the entire pH range with resolution down to 0.2 pH points. The demonstrated detection range and resolution are well-suited to support various applications in environmental and industrial analysis.

The integration of membranes in microfluidic devices is a topic of growing interest in various fields, most commonly particle filtrations and water-oil separations. Yet, choosing the suitable material and properties of the membrane is of major importance as it influences the performance and determines the application of integrated microfluidic device. Polymeric porous membranes with special wetting properties, such as superhydrophobicity/superoleophilicity and superhydrophilicity/superoleophobicity are of high interest due to ability to efficiently separate water and oil by absorbing one of the liquids and repelling the other one. We have previously introduced Fluoropor, a superhydrophobic fluorinated polymer foam that ca be prepared in a simple one-step process. Fluoropor possesses an inherent nano‐/microstructure throughout the whole bulk material making its superhydrophobic properties insensitive to abrasion. Within this work, we report a facile fabrication of a membrane-based microfluidic device that can be used for the separation of water-oil mixtures. The separation chip and the membrane are 3D printed from perfluoropolyethers methacrylate (PFPE-MA) resin and Fluoropor. Due to its hydrophobicity and porous nature, the Fluoropor membrane can efficiently be used for the separation of water-oil mixtures by selectively absorbing the oil. This way effective separation of a water-chloroform mixture was achieved.

In this work, we present numerical simulation results of a designed microfluidic probe for liquid extraction and local drug delivery. The designed probe ensures three different operating modes which are, direct sampling, direct drug delivery and diffusion based sampling. The reported results of finite elements simulation shows that using diffusion phenomena a molecular sample can be extracted from an aqueous sample with this probe. In addition, the best operational parameters of the probe were a tilt angle of 25° and an inlet/outlet velocity of 2 μm/s in order to get the higher concentration of extracted molecule in the buffer solution.

Superhydrophobic surfaces and coatings are of high interest for numerous applications. Inspired from the lotus effect in nature, where droplets easily slide off a surface, due to the formation of an air layer (Salvinia layer) between droplet and surface, superhydrophobic surfaces can be fabricated. For water-immersed structures, especially under dynamic conditions, the stabilization of the Salvinia layer on the superhydrophobic surfaces is of great importance. Due to the shear applied on such structures, the long-term stability of these surfaces necessitates precise measurement of the IDT electrode coated by a 50 μm Fluoropor layer, a transparent, fluorinated polymer foam. We measured Salvinia layer decomposition under low and high shear stress imposed in an aquatic flow cell. Moreover, the current sensor detects the degradation of Salvinia layer and regaining of that by pumping air when shear is applied.

High Q whispering gallery mode optical resonators are capable of rapid and ultra-sensitive biological detection at attomolar concentrations in under 30 seconds. One main question in the field is how these sensors detect such low concentrations of molecules so quickly. Calculations based on diffusion alone suggest that transport to these sensors should take hours to days. Here, we show using bromothymol blue dye flow visualization methods that transport to a microtoroid optical resonator can take place in seconds. We reconcile these results with finite element simulations.

Suspended microchannel resonators (SMRs) are sensitive biosensors for microgravimetric analysis. In this work, an innovative glass SMR was monolithically manufactured by a femtosecond laser technique coupled with a wet etching. The SMR was used to analyze liquids with different density, thanks to a 1.04·10⁻³ kg/m³ density resolution, comparable with the state-of-art silicon-based SMRs. The effective biosensing capability was demonstrated by evaluating the microbial load of different concentrations of P. fluorescens. This innovative and completely transparent SMR can be exploited for real time biosensing, associated with a microscope analysis. Rapid and cost-effective 3D FEMTOPRINT technology can sustain industrial production for point-of-care devices.

Recently, interest in the biophysics of cells has been stimulated by evidence from many studies that external force applied to a cell generates signals that are as potent as those of biochemical stimuli for cell growth, differentiation, migration and function. Furthermore, living cells as open systems maintain their homeostasis, i.e. the internal condition necessary for physiological functioning, by exchanging substances with their environments including energy substrate, ions and water across their membrane. Consequently, the objective is to develop milli/microfluidic assays to measure biophysical properties of human cells with a multi-modality imaging system combining digital holographic microscopy and fluorescence microscopy.

Epigenetics, the study of inheritable mechanisms that regulate gene expression, has clinical ramifications from cancer to autoimmune disorders to psychiatric pathologies. The main tool to study epigenetics is chromatin immunoprecipitation (ChIP), which probes the relationship between DNA and its structural nucleosome-forming histone proteins. Standard benchtop ChIP has three major drawbacks: (1) it requires a large input volume of cells, (2) it is very time consuming and work intensive, and (3) it is low throughput. Digital microfluidic biochips (DMFB) have proven to be successful at utilizing small volumes of reagents and samples to perform high throughput bioanalyses and assays of macromolecules. Their ease of configurability, automation, and high sensitivity make them an ideal platform for ChIP adaptation, addressing the three biggest issues facing epigenetic study and workflow. Herein, we demonstrate the first step towards ChIP implementation on a DMFB by detecting specifically modified nucleosomes, the building blocks of chromatin, in a nucleosome immunoprecipitation assay. Using magnetic beads to capture the nucleosomes with magnetic fields generated by embedded current wires and fluorescent conjugated antibodies for detection, this DMFB system allows complete on-chip isolation and detection without the need for external magnets or specialized fluoroscopy equipment. This assay design can be adapted to probe for multiple specific nucleosome modifications, thus establishing a rapid screening method for antibody specificity and sensitivity. Most importantly, this novel confirmatory checkpoint, currently unavailable when running ChIP, ensures that the target analyte has been isolated prior to intensive downstream analyses such as PCR and sequencing.

We report the development of an opto-acousto-fluidic platform by combining an illumination source in the form of a pulsed laser, a microfluidic channel, and an ultrasound transducer to detect photoacoustic signals generated from the fluid sample inside the channel. We study the effect of the channel dimensions on the emitted acoustic signals using methylene blue solution, a dye of immense interest in processing industry, as a target fluid and select an appropriate channel for further studies. We vary the concentration of the methylene blue dye and collect the corresponding photoacoustic signals. We find that the measured acoustic signal strength varies linearly with the increasing dye concentration, thus making this measurement scheme a potential dye concentration detector. This is a significant finding as it paves the way for developing a miniaturized photoacoustic detector for onsite sensing of dye concentration and perhaps even an online monitoring system which will be radical departure for current analysis methods using bench top bulky and expensive analytical tools.

Micro-physiological platforms (organ-on-chip, multi-organ-chip) have an enormous potential to strengthen research in drug development, toxicological screening, personalized medicine and disease modeling. The design of such microphysiological systems (MPS) is an interdisciplinary challenge. We designed a modular plug and play construction kit for the development of MPS. The modular system provides a large number of functional, miniaturized modules such as pumps, oxygenators, reservoirs and cell culture compartments whose fluidic interfaces comply with the luer-lock standard. This allows the modules to be combined quickly and easily with each other, according to the intended application. Depending on their functionality, the modules are implemented using the multilayer technology established at the IWS or modern 3D printing processes. The construction kit also provides a universal control platform. This consists of a basic controller for micro pumps and valves which can be combined with numerous additional modules like gas mixers, oxygenators and pH sensors. Thus, the financial and time expenditure of an MPS development can be drastically reduced with the help of the modular system.

While optical forces may provide very efficient techniques to manipulate and sort nanomaterials according to their optical properties, experimental realizations remain challenging. In most cases, the key issue lies in the imbalance between the fast Brownian diffusion of nanoscale objects and the weak, localized effect of optical forces. We propose here a new approach based on tapered glass capillaries with only few-μm²-large cross-section areas. The transparent pipe-like structure of tapered glass capillaries allows for simultaneously confining and guiding both the light and the liquid solution in a narrow, few-mm-long optofluidic channel. In this work, a tapered glass capillary is filled with a liquid dispersion of fluorescent nanodiamonds, cleaved, and sealed. Light from a green laser source is then coupled to one end of the capillary. Optical transport of nanodiamonds is observed by fluorescence microscopy. Velocities of nanodiamonds reaching few tens of micrometers per second are measured at the waist of the tapered capillary. In the presence of a liquid flow inside the optofluidic channel, size-dependent sorting of a large ensemble of nanodiamonds is demonstrated. Based on an analytical model, we evaluate the influence of the nanodiamonds’ size on both the optical and the hydrodynamic drag forces acting on the nanoparticles. Our results show that tapered glass capillaries provide a suitable optofluidic platform to achieve efficient optical sorting of nanoparticles by exploiting optical forces as weak as few femtonewtons.

The extension of imaging flow cytometry (IFC) with snapshot-mosaic cameras is a novel approach for the in-flow characterization of bioparticles. With this innovative approach, cells at the single cell level as well as whole cell populations can be analyzed and counted. Spatial and spectrally resolving snapshot-mosaic cameras and real time capable multivariate data processing extend the possibilities for automated and in-flow characterization and analysis of mixed bioparticle populations. Cells move through a microfluidic chip for analysis and measurement as well as classification. For consistent imaging quality the cells passing an innovative 3-step microfluidic focusing unit. For the investigations we used the microalgae Haematococcus pluvialis (HP). These microalgae are used commercially to produce the red carotenoid pigment astaxanthin. Therefore, HP is suitable to practically demonstrate the usability of the developed system.

Dysfunctions in the endothelial cell lining of the vascular endothelium are linked with human pathogenesis of atherosclerosis, venous thrombosis, and several human viral infections. These diseases typically originate from abnormalities resulting from poor structural integrity of the tunica intima of the vascular endothelium. In this report, impulsive stimulated Brillouin scattering spectroscopy was used to assess viscoelastic properties of cells in a microfluidic chip which was designed to mimic the vascular endothelium tunica intima. Brillouin spectroscopy method enabled non-invasive data acquisition of viscoelastic measurements to understand the role of collagen type I on the anchoring of endothelial cells to the extracellular matrix.

We have developed hybrid femtosecond laser processing for fabrication of 3D microfluidic SERS chips. The resulting chip can perform the real-time SERS detection of Cd2+ ions at concentrations as low as 10 ppb. We have further proposed a novel strategy by taking advantage of the microfluidic configuration. The microfluidic chip can produce an interface of analyte solution and air on the SERS substrate in the microfluidic channel, which locally aggregates the analyte molecules at the interface during the measurements (LI-SERS). The LI-SERS technique achieves the analytical enhancement factor reaching 1.5 × 10^14 with a detection limit below 10 aM.