Fusing Sensor Paradigms to Acquire Chemical Information: An Integrative Role for Smart Biopolymeric Hydrogels

Diffusion- and Chemometric-Based Separation of Complex Electrochemical Signals That Originated from Multiple Redox-Active Molecules

In situ analysis of multiple biomarkers in the body provides better diagnosis and enables personalized health management. Since many of these biomarkers are redox-active, electrochemical sensors have shown promising analytical capabilities to measure multiple redox-active molecules. However, the analytical performance of electrochemical sensors rapidly decreases in the presence of multicomponent biofluids due to their limited ability to separate overlapping electrochemical signals generated by multiple molecules. Here we report a novel approach to use charged chitosan-modified electrodes to alter the diffusion of ascorbic acid, clozapine, L-homocysteine, and uric acid—test molecules with various molecular charges and molecular weights. Moreover, we present a complementary approach to use chemometrics to decipher the complex set of overlapping signals generated from a mixture of differentially charged redox molecules. The partial least square regression model predicted three out of four redox-active molecules with root mean square error, Pearson correlation coefficient, and R-squared values of 125 µM, 0.947, and 0.894; 51.8 µM, 0.877, and 0.753; 55.7 µM, 0.903, and 0.809, respectively. By further enhancing our understanding of the diffusion of redox-active molecules in chitosan, the in-situ separation of multiple molecules can be enabled, which will be used to establish guidelines for the effective separation of biomarkers.

Biofabrication of controllable alginate hydrogel cell scaffolds based on bipolar electrochemistry

Bipolar electrochemistry successfully realized the electrodeposition of calcium alginate hydrogels in specific target areas in tissue engineering. However, the shape and quantity of three-dimensional cannot be accurately controlled. We presented a novel growth model for fabricating hydrogels based on bipolar electrochemical by patterned bipolar electrodes using photolithography. This work highlights pattern customization and quantitative control of hydrogels in cell culture platforms. Furthermore, alginate hydrogels with different heights can be controlled by adjusting the key parameters of the growth model. This strategy exhibits promising potential for cell-oriented scaffolds in tissue engineering.

Fabrication of three-dimensional calcium alginate hydrogels using sacrificial templates of sugar

Hydrogels are receiving increasing attention in bioapplications. Among hydrogels, calcium alginate (Ca-alginate) hydrogels are widely used for their biocompatibility, low toxicity, low cost, and rapid fabrication by simple mixing of Ca²⁺ and sodium alginate (Na-alginate). For bioapplications using hydrogels, it is necessary to construct designed hydrogel structures. Although several methods have been proposed for fabricating designed hydrogels, a simple and low-cost method is desirable. Therefore, we developed a new method using sacrificial templates of sugar structures to fabricate three-dimensional (3D) designed Ca-alginate hydrogels. In this method, Na-alginate solution is mixed with molten sugar, and the resulting highly viscous material used to mold 3D sugar structures as sacrificial templates. Since sugar constructs are easily handled compared to hydrogels, sugar templates are useful for preparing 3D constructs. Finally, the sugar and Na-alginate structure is immersed in a CaCl₂ solution to simultaneously dissolve the template and form the Ca-alginate hydrogel. The resulting hydrogel takes the shape of the sugar template. By stacking and fusing various sugar structures, such as fibers and blocks, 3D designed Ca-alginate hydrogels can be successfully fabricated. This simple and low-cost method shows excellent potential for application to a variety of bioapplications.

Underpinning transport phenomena for the patterning of biomolecules

Surface-based assays are increasingly being used in biology and medicine, which in turn demand increasing quantitation and reproducibility. This translates into more stringent requirements on the patterning of biological entities on surfaces (also referred to as biopatterning). This tutorial focuses on mass transport in the context of existing and emerging biopatterning technologies. We here develop a step-by-step analysis of how analyte transport affects surface kinetics, and of the advantages and limitations this entails in major categories of patterning methods, including evaporating sessile droplets, laminar flows in microfluidics or electrochemistry. Understanding these concepts is key to obtaining the desired pattern uniformity, coverage, analyte usage or processing time, and equally applicable to surface assays. A representative technological review accompanies each section, highlighting the technical progress enabled by transport control in e.g. microcontact printing, inkjet printing, dip-pen nanolithography and microfluidic probes. We believe this tutorial will serve researchers to better understand available patterning methods/principles, optimize conditions and to help design protocols/assays. By highlighting fundamental challenges and available approaches, we wish to trigger the development of new surface patterning methods and assays.

Hydrogel electrodeposition based on bipolar electrochemistry

Bipolar electrochemistry has attracted great interest for applications based on sensing, electrografting, and electrodeposition, because the technique enables electrochemical reactions to be induced at multiple bipolar electrodes (BPEs) with only a single power supply. However, there are only a few reports on the biofabrication of hydrogels using BPEs. In this study, we applied bipolar electrochemistry to achieve the electrodeposition of calcium-alginate hydrogels at specified target areas, which is possible because of the use of water electrolysis to obtain acidification at the anodic pole. This scheme was used to successfully fabricate an array of hydrogel deposits at a BPE array. In addition, hydrogels were successfully fabricated either at only the target BPEs or only the target areas of BPEs by repositioning the driving electrodes. Furthermore, a hydrogel was drawn on a large BPE as a canvas by using small driving electrodes. As a demonstration of the electrodeposited hydrogels for bioapplications, mammal cells were cultured in the hydrogels. Because the amount and shape of the hydrogel deposits can be controlled by using the bipolar system, the system we developed can be used for biosensors and cell culture platforms.

Catechol-chitosan redox capacitor for added amplification in electrochemical immunoanalysis

Antibodies are common recognition elements for molecular detection but often the signals generated by their stoichiometric binding must be amplified to enhance sensitivity. Here, we report that an electrode coated with a catechol-chitosan redox capacitor can amplify the electrochemical signal generated from an alkaline phosphatase (AP) linked immunoassay. Specifically, the AP product p-aminophenol (PAP) undergoes redox-cycling in the redox capacitor to generate amplified oxidation currents. We estimate an 8-fold amplification associated with this redox-cycling in the capacitor (compared to detection by a bare electrode). Importantly, this capacitor-based amplification is generic and can be coupled to existing amplification approaches based on enzyme-linked catalysis or magnetic nanoparticle-based collection/concentration. Thus, the capacitor should enhance sensitivities in conventional immunoassays and also provide chemical to electrical signal transduction for emerging applications in molecular communication.

Connecting Biology to Electronics: Molecular Communication via Redox Modality

Biology and electronics are both expert at for accessing, analyzing, and responding to information. Biology uses ions, small molecules, and macromolecules to receive, analyze, store, and transmit information, whereas electronic devices receive input in the form of electromagnetic radiation, process the information using electrons, and then transmit output as electromagnetic waves. Generating the capabilities to connect biology-electronic modalities offers exciting opportunities to shape the future of biosensors, point-of-care medicine, and wearable/implantable devices. Redox reactions offer unique opportunities for bio-device communication that spans the molecular modalities of biology and electrical modality of devices. Here, an approach to search for redox information through an interactive electrochemical probing that is analogous to sonar is adopted. The capabilities of this approach to access global chemical information as well as information of specific redox-active chemical entities are illustrated using recent examples. An example of the use of synthetic biology to recognize external molecular information, process this information through intracellular signal transduction pathways, and generate output responses that can be detected by electrical modalities is also provided. Finally, exciting results in the use of redox reactions to actuate biology are provided to illustrate that synthetic biology offers the potential to guide biological response through electrical cues.

Catechol-Based Hydrogel for Chemical Information Processing

Catechols offer diverse properties and are used in biology to perform various functions that range from adhesion (e.g., mussel proteins) to neurotransmission (e.g., dopamine), and mimicking the capabilities of biological catechols have yielded important new materials (e.g., polydopamine). It is well known that catechols are also redox-active and we have observed that biomimetic catechol-modified chitosan films are redox-active and possess interesting molecular electronic properties. In particular, these films can accept, store and donate electrons, and thus offer redox-capacitor capabilities. We are enlisting these capabilities to bridge communication between biology and electronics. Specifically, we are investigating an interactive redox-probing approach to access redox-based chemical information and convert this information into an electrical modality that facilitates analysis by methods from signal processing. In this review, we describe the broad vision and then cite recent examples in which the catechol–chitosan redox-capacitor can assist in accessing and understanding chemical information. Further, this redox-capacitor can be coupled with synthetic biology to enhance the power of chemical information processing. Potentially, the progress with this biomimetic catechol–chitosan film may even help in understanding how biology uses the redox properties of catechols for redox signaling.

Electrochemical reverse engineering: A systems-level tool to probe the redox-based molecular communication of biology

The intestine is the site of digestion and forms a critical interface between the host and the outside world. This interface is composed of host epithelium and a complex microbiota which is "connected" through an extensive web of chemical and biological interactions that determine the balance between health and disease for the host. This biology and the associated chemical dialogues occur within a context of a steep oxygen gradient that provides the driving force for a variety of reduction and oxidation (redox) reactions. While some redox couples (e.g., catecholics) can spontaneously exchange electrons, many others are kinetically "insulated" (e.g., biothiols) allowing the biology to set and control their redox states far from equilibrium. It is well known that within cells, such non-equilibrated redox couples are poised to transfer electrons to perform reactions essential to immune defense (e.g., transfer from NADH to O₂ for reactive oxygen species, ROS, generation) and protection from such oxidative stresses (e.g., glutathione-based reduction of ROS). More recently, it has been recognized that some of these redox-active species (e.g., H₂O₂) cross membranes and diffuse into the extracellular environment including lumen to transmit redox information that is received by atomically-specific receptors (e.g., cysteine-based sulfur switches) that regulate biological functions. Thus, redox has emerged as an important modality in the chemical signaling that occurs in the intestine and there have been emerging efforts to develop the experimental tools needed to probe this modality. We suggest that electrochemistry provides a unique tool to experimentally probe redox interactions at a systems level. Importantly, electrochemistry offers the potential to enlist the extensive theories established in signal processing in an effort to "reverse engineer" the molecular communication occurring in this complex biological system. Here, we review our efforts to develop this electrochemical tool for in vitro redox-probing.

Redox Probing for Chemical Information of Oxidative Stress

Oxidative stress is implicated in many diseases yet no simple, rapid, and robust measurement is available at the point-of-care to assist clinicians in detecting oxidative stress. Here, we report results from a discovery-based research approach in which a redox mediator is used to probe serum samples for chemical information relevant to oxidative stress. Specifically, we use an iridium salt (K₂IrCl₆) to probe serum for reducing activities that can transfer electrons to iridium and thus generate detectable optical and electrochemical signals. We show that this Ir-reducing assay can detect various biological reductants and is especially sensitive to glutathione (GSH) compared to alternative assays. We performed an initial clinical evaluation using serum from 10 people diagnosed with schizophrenia, a mental health disorder that is increasingly linked to oxidative stress. The measured Ir-reducing capacity was able to discriminate people with schizophrenia from healthy controls (p < 0.005), and correlations were observed between Ir-reducing capacity and independent measures of symptom severity.