Xiaolong Luo - Academia.edu (original) (raw)

Papers by Xiaolong Luo

RSC Advances, 2018

Fluid viscosity proportional to pressure drop in a capillary (L) was reflected by the air–fluid i... more Fluid viscosity proportional to pressure drop in a capillary (L) was reflected by the air–fluid interface displacement (ΔL) to enclosed air.

Journal of Bioengineering and Biomedical Sciences, 2012

Antibiotic resistance is an increasing public health concern and few new drugs for bacterial path... more Antibiotic resistance is an increasing public health concern and few new drugs for bacterial pathogenesis have been obtained without addressing this resistance. Quorum sensing (QS) is a newly-discovered system mediated by extracellular chemical signals known as "autoinducers", which can coordinate population-scale changes in gene regulation when the number of cells reaches a "quorum" level. The capability to intercept

Biofabrication, 2011

Home > Biofabrication of chitosan-silver composite SERS substrates enabling quantification of ade... more Home > Biofabrication of chitosan-silver composite SERS substrates enabling quantification of adenine by a spectroscopic shift. Biofabrication of chitosan-silver composite SERS substrates enabling quantification of adenine by a spectroscopic shift.

We report an in situ biofabrication strategy that partitions a microfluidic network into multiple... more We report an in situ biofabrication strategy that partitions a microfluidic network into multiple microchannels with semi-permeable biopolymer membranes. The biofabrication of parallel biopolymer membranes was initiated with trapped air bubbles in hydrophobic polydimethylsiloxane (PDMS) microfluidic devices, followed by tunable membrane growth with time. Static gradients were generated and well maintained in the partitioned microfluidic channels by pure diffusion through the semipermeable membranes. The in situ biofabrication provides a simple approach to generate static gradients and an ideal platform for biological applications where flow-free static gradients are indispensable.

Journal of Materials Chemistry B, 2020

Glutaraldehyde crosslinking significantly enhances the mechanical robustness of the originally co... more Glutaraldehyde crosslinking significantly enhances the mechanical robustness of the originally compromised flow-assembled chitosan membranes after Pluronic passivation in microfluidics.

Materials Advances, 2020

Tuning the membrane porosity in microfluidics with co-assembled nanoparticles as templates for en... more Tuning the membrane porosity in microfluidics with co-assembled nanoparticles as templates for enhanced mass transport and biomacromolecule gradient generation.

Biomicrofluidics, 2016

Chemotaxis is a phenomenon which enables cells to sense concentrations of certain chemical specie... more Chemotaxis is a phenomenon which enables cells to sense concentrations of certain chemical species in their microenvironment and move towards chemically favorable regions. Recent advances in microbiology have engineered the chemotactic properties of bacteria to perform novel functions, but traditional methods of characterizing chemotaxis do not fully capture the associated cell motion, making it difficult to infer mechanisms that link the motion to the microbiology which induces it. Microfluidics offers a potential solution in the form of gradient generators. Many of the gradient generators studied to date for this application are flow-based, where a chemical species diffuses across the laminar flow interface between two solutions moving through a microchannel. Despite significant research efforts, flow-based gradient generators have achieved mixed success at accurately capturing the highly subtle chemotactic responses exhibited by bacteria. Here we present an analysis encompassing ...

Biomicrofluidics, 2017

We have developed a user-friendly microfluidic device for the study of gradient-mediated bacteria... more We have developed a user-friendly microfluidic device for the study of gradient-mediated bacterial behaviors, including chemotaxis. This device rapidly establishes linear concentration gradients by exploiting solute diffusion through porous membranes in the absence of convective flows. As such, the gradients are created rapidly and can be sustained for long time periods (e.g., hrs); sufficient to evaluate cell phenotype. The device exploits a unique simple bilayer configuration that enables rapid setup and quick, reproducible introduction of cells. Its reusability represents an additional advantage in that it need not be limited to settings with microfluidics expertise. We have successfully demonstrated the applicability of this tool in studying the chemotactic response of Escherichia coli to glucose. When coupled with our recent Python program, quantified metrics such as speed, ratio of tumble to run, and effective diffusivity can be obtained from slow frame rate videos. Moreover, we introduce a chemotaxis partition coefficient that conveniently scores swimming behavior on the single-cell level.

Biotechnology and bioengineering, 2017

Cover Legend The cover image, by William Bentley et al., is based on the Article Conferring biolo... more Cover Legend The cover image, by William Bentley et al., is based on the Article Conferring biological activity to native spider silk: A biofunctionalized protein-based microfiber, DOI: 10.1002/bit.26065.

Biotechnology and bioengineering, 2016

Spider silk is an extraordinary material with physical properties comparable to the best scaffold... more Spider silk is an extraordinary material with physical properties comparable to the best scaffolding/structural materials, and as a fiber it can be manipulated with ease into a variety of configurations. Our work here demonstrates that natural spider silk fibers can also be used to organize biological components on and in devices through rapid and simple means. Micron scale spider silk fibers (5-10 µm in diameter) were surface modified with a variety of biological entities engineered with pentaglutamine tags via microbial transglutaminase (mTG). Enzymes, enzyme pathways, antibodies, and fluorescent proteins were all assembled onto spider silk fibers using this biomolecular engineering/biofabrication process. Additionally, arrangement of biofunctionalized fiber should in of itself generate a secondary level of biomolecular organization. Toward this end, as proofs of principle, spatially defined arrangement of biofunctionalized spider silk fiber was shown to generate effects specific ...

Lab on a chip, Jan 31, 2015

The human gut is over a meter in length, liquid residence times span several hours. Recapitulatin... more The human gut is over a meter in length, liquid residence times span several hours. Recapitulating the human gut microbiome "on chip" holds promise to revolutionize therapeutic strategies for a variety of diseases, as well as for maintaining homeostasis in healthy individuals. A more refined understanding of bacterial-bacterial and bacterial-epithelial cell signalling is envisioned and such a device is a key enabler. Indeed, significant advances in the study of bacterial cell-cell signalling have been reported, including at length and time scales of the cells and their responses. Few reports exist, however, where signalling events that span physiologically relevant time scales are monitored and coordinated. Here, we employ principles of biofabrication to assemble, in situ, cell communities that are (i) spatially adjacent within partitioned microchannels for studying near communication and (ii) distally connected within longitudinal microfluidic networks so as to mimic long...

Journal of visualized experiments : JoVE, 2012

Advancements in lab-on-a-chip technology promise to revolutionize both research and medicine thro... more Advancements in lab-on-a-chip technology promise to revolutionize both research and medicine through lower costs, better sensitivity, portability, and higher throughput. The incorporation of biological components onto biological microelectromechanical systems (bioMEMS) has shown great potential for achieving these goals. Microfabricated electronic chips allow for micrometer-scale features as well as an electrical connection for sensing and actuation. Functional biological components give the system the capacity for specific detection of analytes, enzymatic functions, and whole-cell capabilities. Standard microfabrication processes and bio-analytical techniques have been successfully utilized for decades in the computer and biological industries, respectively. Their combination and interfacing in a lab-on-a-chip environment, however, brings forth new challenges. There is a call for techniques that can build an interface between the electrode and biological component that is mild and is easy to fabricate and pattern. Biofabrication, described here, is one such approach that has shown great promise for its easy-to-assemble incorporation of biological components with versatility in the on-chip functions that are enabled. Biofabrication uses biological materials and biological mechanisms (selfassembly, enzymatic assembly) for bottom-up hierarchical assembly. While our labs have demonstrated these concepts in many formats 1,2,3 , here we demonstrate the assembly process based on electrodeposition followed by multiple applications of signal-based interactions. The assembly process consists of the electrodeposition of biocompatible stimuli-responsive polymer films on electrodes and their subsequent functionalization with biological components such as DNA, enzymes, or live cells 4,5. Electrodeposition takes advantage of the pH gradient created at the surface of a biased electrode from the electrolysis of water 6,7 ,. Chitosan and alginate are stimuli-responsive biological polymers that can be triggered to self-assemble into hydrogel films in response to imposed electrical signals 8

Soft Matter, 2010

We report the first in situ quantitative visualization and characterization of electro-induced ch... more We report the first in situ quantitative visualization and characterization of electro-induced chitosan hydrogel growth in an aqueous environment. This was enabled with a pair of sidewall electrodes within a transparent fluidic system, which allowed us to resolve the electrogelling mechanism and interpret the dominant causes responsible for the formation and density distribution of the deposited hydrogel. The pH and the time-dependent growth profiles of the chitosan hydrogel were directly visualized, analyzed, and characterized. The results indicate that the gelation and immobilization of chitosan onto the cathode at a pH above its pK a value ($6.3) are due to the electrochemically generated concentration gradient of reactant OH À ions, and their subsequent neutralization of the NH 3 + groups of chitosan chains in solution near the cathode. The increased gel density around the fringes of the electrodes was demonstrated and correlated with the electrophoretic migration of chitosan cations during deposition. Simulation of the electric potential/field distribution, together with the corresponding dry film topography confirmed the non-uniform, electric field-dependent density distribution of deposited hydrogel. This report provides fundamental understanding towards the mechanism and the kinetics of the electro-induced chitosan gel formation. It also provides important guidelines for pursuing its application in bio-components integrated microsystems. The method in use exemplifies a simple, effective and non-destructive approach for in situ characterization of electro-responsive biopolymers in an aqueous environment.

Lab on a Chip, 2008

We report a biofunctionalization strategy for the assembly of catalytically active enzymes within... more We report a biofunctionalization strategy for the assembly of catalytically active enzymes within a completely packaged bioMEMS device, through the programmed generation of electrical signals at spatially and temporally defined sites. The enzyme of a bacterial metabolic pathway, S-adenosylhomocysteine nucleosidase (Pfs), is genetically fused with a pentatyrosine "pro-tag" at its C-terminus. Signal responsive assembly is based on covalent conjugation of Pfs to the aminopolysaccharide, chitosan, upon biochemical activation of the pro-tag, followed by electrodeposition of the enzyme-chitosan conjugate onto readily addressable sites in microfluidic channels. Compared to traditional physical entrapment and surface immobilization approaches in microfluidic environments, our signal-guided electrochemical assembly is unique in that the enzymes are assembled under mild aqueous conditions with spatial and temporal programmability and orientational control. Significantly, the chitosan-mediated enzyme assembly can be reversed, making the bioMEMS reusable for repeated assembly and catalytic activity. Additionally, the assembled enzymes retain catalytic activity over multiple days, demonstrating enhanced enzyme stability. We envision that this assembly strategy can be applied to rebuild metabolic pathways in microfluidic environments for antimicrobial drug discovery.

Lab Chip, 2010

We report the in situ generation of pH gradients in microfluidic devices for biofabrication of fr... more We report the in situ generation of pH gradients in microfluidic devices for biofabrication of freestanding, semi-permeable chitosan membranes. The pH-stimuli-responsive polysaccharide chitosan was enlisted to form a freestanding hydrophilic membrane structure in microfluidic networks where pH gradients are generated at the converging interface between a slightly acidic chitosan solution and a slightly basic buffer solution. A simple and effective pumping strategy was devised to realize a stable flow interface thereby generating a stable, well-controlled and localized pH gradient. Chitosan molecules were deprotonated at the flow interface, causing gelation and solidification of a freestanding chitosan membrane from a nucleation point at the junction of two converging flow streams to an anchoring point where the two flow streams diverge to two output channels. The fabricated chitosan membranes were about 30-60 mm thick and uniform throughout the flow interface inside the microchannels. A T-shaped membrane formed by sequentially fabricating orthogonal membranes demonstrates flexibility of the assembly process. The membranes are permeable to aqueous solutions and are removed by mildly acidic solutions. Permeability tests suggested that the membrane pore size was a few nanometres, i.e., the size range of antibodies. Building on the widely reported use of chitosan as a soft interconnect for biological components and microfabricated devices and the broad applications of membrane functionalities in microsystems, we believe that the facile, rapid biofabrication of freestanding chitosan membranes can be applied to many biochemical, bioanalytical, biosensing applications and cellular studies.

Lab on a Chip, 2006

We report facile in situ biomolecule assembly at readily addressable sites in microfluidic channe... more We report facile in situ biomolecule assembly at readily addressable sites in microfluidic channels after complete fabrication and packaging of the microfluidic device. Aminopolysaccharide chitosan's pH responsive and chemically reactive properties allow electric signal-guided biomolecule assembly onto conductive inorganic surfaces from the aqueous environment, preserving the activity of the biomolecules. A transparent and nonpermanently packaged device allows consistently leak-free sealing, simple in situ and ex situ examination of the assembly procedures, fluidic input/outputs for transport of aqueous solutions, and electrical ports to guide the assembly onto the patterned gold electrode sites within the channel. Both in situ fluorescence and ex situ profilometer results confirm chitosan-mediated in situ biomolecule assembly, demonstrating a simple approach to direct the assembly of biological components into a completely fabricated device. We believe that this strategy holds significant potential as a simple and generic biomolecule assembly approach for future applications in complex biomolecular or biosensing analyses as well as in sophisticated microfluidic networks as anticipated for future lab-on-a-chip devices.

Lab on a Chip, 2010

The emergence of bacteria that evade antibiotics has accelerated research on alternative approach... more The emergence of bacteria that evade antibiotics has accelerated research on alternative approaches that do not target cell viability. One such approach targets cell-cell communication networks mediated by small molecule signaling. In this report, we assemble biological nanofactories within a bioMEMS device to capture and manipulate the behavior of quorum sensing (QS) bacteria as a step toward modifying small molecule signaling. Biological nanofactories are bio-inspired nanoscale constructs which can include modules with different functionalities, such as cell targeting, molecular sensing, product synthesis, and ultimately self-destruction. The biological nanofactories reported here consist of targeting, sensing, synthesis and, importantly, assembly modules. A bacteria-specific antibody constitutes the targeting module while a genetically engineered fusion protein contains the sensing, synthesis and assembly modules. The nanofactories are assembled on chitosan electrodeposited within a microchannel of the bioMEMS device; they capture QS bacteria in a spatially selective manner and locally synthesize and deliver the ''universal'' small signaling molecule autoinducer-2 (AI-2) at the captured cell surface. The nanofactory based AI-2 delivery is demonstrated to alter the progression of the native AI-2 based QS response of the captured bacteria. Prospects are envisioned for utilizing our technique as a test-bed for understanding the AI-2 based QS response of bacteria as a means for developing the next generation of antimicrobials.