Duplex Electrochemical Microfluidic Sensor for COVID-19 Antibody Detection: Natural versus Vaccine-Induced Humoral Response

The rapid transmission and resilience of coronavirus disease 2019 (COVID-19) have led to urgent demands in monitoring humoral response for effective vaccine development, thus a multiplex co-detection platform to discriminate infection-induced from vaccine-induced antibodies is needed. Here a duplex electrochemical immunosensor for co-detection of anti-nucleocapsid IgG (N-IgG) and anti-spike IgG (S-IgG) is developed by using a two-working electrode system, via an indirect immunoassay, with antibody quantification obtained by differential pulse voltammetry. The screen-printed electrodes (SPEs) are modified by carbon black and electrodeposited gold nanoflowers for maximized surface areas, enabling the construction of an immunological chain for S-IgG and N-IgG electrochemical detection with enhanced performance. Using an optimized immunoassay protocol, a wide linear range between 30-750 and 20-1000 ng mL-1 , and a limit of detection of 28 and 15 ng mL-1 are achieved to detect N-IgG and S-IgG simultaneously in serum samples. This duplex immunosensor is then integrated in a microfluidic device to obtain significantly reduced detection time (≤ 7 min) while maintaining its analytical performance. The duplex microfluidic immunosensor can be easily expanded into multiplex format to achieve high throughput screening for the sero-surveillance of COVID-19  and other infectious diseases.

Alignment-Rheology Relationship of Biosourced Rod-Like Colloids and Polymers under Flow

Fluids composed of biosourced rod-like colloids (RC) and rod-like polymers (RP) have been extensively studied due to various promising applications relying on their flow-induced orientation (e.g., fiber spinning). However, the relationship between RC and RP alignment and the resulting rheological properties is unclear due to experimental challenges. We investigate the alignment-rheology relationship for a variety of biosourced RC and RP, including cellulose-based particles, filamentous viruses, and xanthan gum, by simultaneous measurements of the shear viscosity and fluid anisotropy under rheometric shear flows. For each system, the RC and RP contribution to the fluid viscosity, captured by the specific viscosity η_sp, follows a universal trend with the extent of the RC and RP alignment independent of concentration. We further exploit this unique rheological-structural link to retrieve a dimensionless parameter (β) directly proportional to η_sp at zero shear rate (η_0,sp), a parameter often difficult to access from experimental rheometry for RC and RP with relatively long contour lengths. Our results highlight the unique link between the flow-induced structural and rheological changes occurring in RC and RP fluids. We envision that our findings will be relevant in building and testing microstructural constitutive models to predict the flow-induced structural and rheological evolution of fluids containing RC and RP.

Microfluidics as a tool to assess and induce emulsion destabilization

Microfluidic technology enables judicious control of the process parameters on a small length scale, which in turn allows speeding up the destabilization of emulsion droplets interface in microfluidic devices. In this light, microfluidic channels can be used as an efficient tool to assess emulsion stability and to observe the behavior of the droplets immediately after their formation, enabling to determine whether or not they are prone to re-coalescence. Observation of the droplets after emulsifier adsorption also allows the investigation of emulsion stability over time. Both evaluations would contribute to determine emulsion stability aiming at specific applications in food and pharmaceutical industries. Furthermore, emulsion coalescence can also be performed under extremely controlled conditions within the microfluidic devices in order to explore emulsion droplets as micro-reactors (for regulated biological and chemical assays). Such microfluidic procedures can be performed either in confined environments or under dynamic flow conditions. Under confined environments, droplets are observed in fixed positions simulating different environmental conditions. On the other hand, with the scrutiny of emulsions under dynamic flow processes, it is possible to determine the behavior of the droplets when subjected to shear forces, comparable to those experienced in conventional emulsification techniques or even in pumping operations. Given the above, this paper reviews different microfluidic techniques (such as changing channel geometry or wettability) hitherto used to destabilize emulsions, mainly focusing on the specificities of each study, whether the droplets are destabilized in confined or dynamic flow processes. Thereby, by going deeper into this review, readers will be able to identify different strategies for emulsion destabilization (in order to understand stabilizing mechanisms or even to apply these droplets as micro-reactors), as this paper shows the particularities of the most recent studies and elucidates the current state-of-the-art of this microfluidic-related application.

Microfluidics-based production of chitosan-gellan nanocomplexes encapsulating caffeine

Electrostatic complexes produced by interactions between polysaccharides have promising applications in the medical, pharmaceutical and food fields. In this light, for the development of such particles, microfluidics emerges as a promising technique in which processes occur at a strict laminar flow regime, allowing diffusion-dominated transport and particle formation in highly-controlled conditions. As a proof of concept, we compared bulk versus microfluidic (different devices simulating a range of residence times) processes for the production of electrostatic complexes of gellan with either chitosan (molecular weight ∼ 28 kDa) or hydrolyzed chitosan (molecular weight ∼ 3 kDa). Regardless of the process, polysaccharide solutions (pH 4.5) were mixed in pre-defined concentrations (polysaccharide ratios) to form electrostatic complexes that were used to encapsulate caffeine. These complexes were characterized by zeta potential measurements and particle size distribution. Overall, microfluidics produced complexes with improved characteristics such as lower polydispersity index (PDI ∼ 0.1) and mean size (∼200 nm) when compared to the conventional bulk process (PDI ∼ 0.3 and mean size ∼ 400 nm). Moreover, hydrolyzed chitosan (HC) contributed to an even smaller size and PDI value of the complexes. Such outcome is associated with the lower molecular weight and higher solubility of HC when comparing to conventional chitosan, which in turn improves electrostatic complexation. Caffeine could also be encapsulated in all complexes, but the highest encapsulation efficiency was achieved using microfluidics (70%) and with the geometry that provided a longer residence time. Therefore, we were able to demonstrate that microfluidics is clearly an effective strategy for generating electrostatic complexes with improved properties. Ultimately, this technique demonstrated a high potential for the production of vehicles of bioactive compounds.

Unraveling driving regimes for destabilizing concentrated emulsions within microchannels

Coalescence is the most widely demonstrated mechanism for destabilizing emulsion droplets in microfluidic chambers. However, we find that depending on the channel wall surface functionalization, surface zeta potential, type of surfactant, characteristics of the oil as a dispersed phase, or even the presence of externally-induced stress, other different destabilization mechanisms can occur in subtle ways. In general, we observe four regimes leading to destabilization of concentrated emulsions: (i) coalescence, (ii) emulsion bursts, (iii) a combination of the two first mechanisms, attributed to the simultaneous occurrence of coalescence and emulsion bursts; and (iv) compaction of the droplet network that eventually destabilizes to fracture-like behavior. We correlate various physico-chemical properties (zeta potential, contact angle, interfacial tension) to understand their respective influence on the destabilization mechanisms. This work provides insights into possible ways to control or inflict emulsion droplet destabilization for different applications.

Modulating properties of polysaccharides nanocomplexes from enzymatic hydrolysis of chitosan

Synthesis of nanocomplexes is a simple and low-cost technique for the production of encapsulation systems aiming industrial applications, based on the interaction of at least two oppositely charged molecules. Gellan gum (anionic) is a water-soluble biopolymer resistant to stomach pH conditions, therefore an interesting alternative as an encapsulating matrix. Chitosan (cationic) is also widely used due to its biocompatibility and mucoadhesive properties, although its low water solubility is an important step to be overcome for the production of the complexes. To improve this property, many techniques have been employed, but most of them use unsustainable techniques and chemical agents. The enzymatic hydrolysis of chitosan using proteases emerges as an alternative to these drawbacks and, therefore, this study aimed to evaluate the electrostatic nanocomplexation of native (C) or hydrolyzed (HC) chitosan (by porcine pepsin protease) with gellan gum (G). Polysaccharides and nanocomplexes formed with different G:C or G:HC ratio were evaluated by zeta potential measurements, particle size distribution, X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Transmission Electron Microscopy (STEM), intrinsic viscosity and turbidity analyses. Chitosan hydrolysis allowed the formation of a smaller (445.3 nm in pH 4.5) and more soluble structure (3 kDa), which positively influenced the formation of the complexes. The ratios G:HC of 7:3 and 8:2 formed complexes with lower values of zeta potential (13.9 mV and -5.0 mV, respectively), particle size (635.8 nm and 533.6 nm, respectively) and polydispersity (0.28 and 0.23) compared to complexes formed with native chitosan. Overall, our results show that enzymatic hydrolysis of chitosan favored the formation of electrostatic complexes with reduced size and low polydispersity, which can be used as efficient encapsulating matrices for improved targeted delivery and controlled release of bioactive compounds.

Colloidal particle deposition on microchannel walls, for attractive and repulsive surface potentials

When both surfaces possess opposite charges, particle deposition increases at low ionic strengths due to van der Waals forces assisted by electrostatic attraction.

Role of process variables on the formation and in vitro digestion of gellan gels

Gellan gels were produced using different approaches forming structures with varied potential applications. Gels were characterized from appearance, mechanical properties, water holding capacity (WHC) and microstructure. In addition, in vitro digestibility of these gels was evaluated to understand the effect of gastrointestinal environment on their structure stability. All gels presented high WHC (>80% w/w) but gels were stronger with salt or acid addition, which was associated to the reduction of double helices repulsion of the negatively charged carboxyl groups of gellan. Moreover, low gelation rate induced a more controlled gellan helices aggregation, strengthening even more gels structure. Gellan gels presented resistance to digestion conditions but hardness of these gels during digestion mainly depended on the gelation rate. Based on these findings it would be possible to tune gel properties for a specific application as texture modifier or even as a faecal bulk formation assistant and an "ileal break" inducer.