Mechanics of controlled release of insulin entrapped in polyacrylic acid gels via variable electrical stimuli.

Controlled release insulin delivery systems possess multiple advantages over conventional ones, including maintaining desired blood glucose levels for prolonged periods and minimizing complications due to insulin overdose. Compared to other controlled-release mechanisms, electro-responsive polymers present the advantages of high controllability and ability to be coupled with microelectronics. This paper reports the possibility of using electro-responsive polyacrylic acid (PAA) and polymethacrylic acid (PMA) hydrogels for controlled delivery of insulin using intermittent electrical signals via matrix deformation. PAA hydrogels showed very good electrical responsivity under both constant and step current inputs, releasing up to 80% of protein at 10 V stimulus, compared to 20% release in the absence of stimulus. Analysis of spatial variation under electrical stimuli suggested that release of protein is a combined effect of deformation of the hydrogel and electrophoresis of protein molecules. Binding interaction analysis revealed that insulin entrapment is largely due to hydrogen bonding between the polymer matrix and insulin, and flooding the matrix with electrical charge likely disrupts the attractive forces that kept protein in place helping the release of the proteins. Understanding the molecular interactions affecting insulin retention and release mechanisms of PAA hydrogels is useful for developing and optimizing hydrogel-based controlled drug release systems.

Inorganic iron-sulfur clusters enhance electron transport when used for wiring the NAD-glucose dehydrogenase based redox system.

Wiring the active site of an enzyme directly to an electrode is the key to ensuring efficient electron transfer for the proper performance of enzyme-based bioelectronic systems. Iron-sulfur complexes, the first link between proteins and mediating molecules in the biological electron transport chain(s), possess an intrinsic electron transport capability. The authors demonstrate the application of inorganic iron-sulfur clusters (Fe-S) viz. FeS, FeS2, Fe2S3, and Fe3S4, as molecular wires to mediate electron transport between a glucose-selective redox enzyme and the gold electrode. It is shown that Fe-S can emulate the functionality of the natural electron transport chain. Voltammetric studies indicate a significant improvement in electron transport, surface coverage, and resilience achieved by the Fe-S-based glucose anodes when compared to a conventional pyrroloquinoline quinone (PQQ)-based electrode. The Fe-S-based glucose anodes showed glucose oxidation at a potential of +0.5 V vs. Ag/AgCl with Tris-HCl buffer (pH 8) acting as a carrier. The current densities positively correlated with the concentrations of glucose in the range 0.1-100 mM displaying detection limits of 0.77 mM (FeS), 1.22 mM (FeS2), 2.95 mM (Fe2S3), and 14.57 mM (Fe3S4). The metal-anchorable sulfur atom, the strong π-coordinating iron atom, the favorable redox properties, low cost, and natural abundance make Fe-S an excellent electron-mediating relay capable of wiring redox active sites to electrode surfaces. Graphical abstract Schematic representation of inorganic iron-sulfur clusters used as molecular wires to facilitate direct electron transfer between NAD-glucose dehydrogenase and the gold electrode. The iron-sulfur based glucose anodes improve current response to selectively sense glucose concentrations in the range 0.1-100 mM.

An improved glycerol biosensor with an Au-FeS-NAD-glycerol-dehydrogenase anode.

An improved glycerol biosensor was developed via direct attachment of NAD+-glycerol dehydrogenase coenzyme-apoenzyme complex onto supporting gold electrodes, using novel inorganic iron (II) sulfide (FeS)-based single molecular wires. Sensing performance factors, i.e., sensitivity, a detection limit and response time of the FeS and conventional pyrroloquinoline quinone (PQQ)-based biosensor were evaluated by dynamic constant potential amperometry at 1.3V under non-buffered conditions. For glycerol concentrations ranging from 1 to 25mM, a 77% increase in sensitivity and a 53% decrease in detection limit were observed for the FeS-based biosensor when compared to the conventional PQQ-based counterpart. The electrochemical behavior of the FeS-based glycerol biosensor was analyzed at different concentrations of glycerol, accompanied by an investigation into the effects of applied potential and scan rate on the current response. Effects of enzyme stimulants ((NH4)2SO4 and MnCl2·4H2O) concentrations and buffers/pH (potassium phosphate buffer pH 6-8, Tris buffer pH 8-10) on the current responses generated by the FeS-based glycerol biosensor were also studied. The optimal detection conditions were 0.03M (NH4)2SO4 and 0.3µm MnCl2·4H2O in non-buffered aqueous electrolyte under stirring whereas under non-stirring, Tris buffer at pH 10 with 0.03M (NH4)2SO4 and 30µm MnCl2·4H2O were found to be optimal detection conditions. Interference by glucose, fructose, ethanol, and acetic acid in glycerol detection was studied. The observations indicated a promising enhancement in glycerol detection using the novel FeS-based glycerol sensing electrode compared to the conventional PQQ-based one. These findings support the premise that FeS-based bioanodes are capable of biosensing glycerol successfully and may be applicable for other enzymatic biosensors.

Iron-sulfur-based single molecular wires for enhancing charge transport in enzyme-based bioelectronic systems.

When redox enzymes are wired to electrodes outside a living cell (ex vivo), their ability to produce a sufficiently powerful electrical current diminishes significantly due to the thermodynamic and kinetic limitations associated with the wiring systems. Therefore, we are yet to harness the full potential of redox enzymes for the development of self-powering bioelectronics devices (such as sensors and fuel cells). Interestingly, nature uses iron-sulfur complexes ([Fe-S]), to circumvent these issues in vivo. Yet, we have not been able to utilize [Fe-S]-based chains ex vivo, primarily due to their instability in aqueous media. Here, a simple technique to attach iron (II) sulfide (FeS) to a gold surface in ethanol media and then complete the attachment of the enzyme in aqueous media is reported. Cyclic voltammetry and spectroscopy techniques confirmed the concatenation of FeS and glycerol-dehydrogenase/nicotinamide-adenine-dinucleotide (GlDH-NAD(+)) apoenzyme-coenzyme molecular wiring system on the base gold electrode. The resultant FeS-based enzyme electrode reached an open circuit voltage closer to its standard potential under a wide range of glycerol concentrations (0.001-1M). When probed under constant potential conditions, the FeS-based electrode was able to amplify current by over 10 fold as compared to electrodes fabricated with the conventional pyrroloquinoline quinone-based composite molecular wiring system. These improvements in current/voltage responses open up a wide range of possibilities for fabricating self-powering, bio-electronic devices.