Synthesis, copolymerization and peptide-modification of carboxylic acid-functionalized 3,4-ethylenedioxythiophene (EDOTacid) for neural electrode interfaces

Organic Mixed Ionic-Electronic Conductors for Bioelectronic Sensors: Materials and Operation Mechanisms

The field of organic mixed ionic-electronic conductors (OMIECs) has gained significant attention due to their ability to transport both electrons and ions, making them promising candidates for various applications. Initially focused on inorganic materials, the exploration of mixed conduction has expanded to organic materials, especially polymers, owing to their advantages such as solution processability, flexibility, and property tunability. OMIECs, particularly in the form of polymers, possess both electronic and ionic transport functionalities. This review provides an overview of OMIECs in various aspects covering mechanisms of charge transport including electronic transport, ionic transport, and ionic-electronic coupling, as well as conducting/semiconducting conjugated polymers and their applications in organic bioelectronics, including (multi)sensors, neuromorphic devices, and electrochromic devices. OMIECs show promise in organic bioelectronics due to their compatibility with biological systems and the ability to modulate electronic conduction and ionic transport, resembling the principles of biological systems. Organic electrochemical transistors (OECTs) based on OMIECs offer significant potential for bioelectronic applications, responding to external stimuli through modulation of ionic transport. An in-depth review of recent research achievements in organic bioelectronic applications using OMIECs, categorized based on physical and chemical stimuli as well as neuromorphic devices and circuit applications, is presented.

Single-Component Electroactive Polymer Architectures for Non-Enzymatic Glucose Sensing

Organic mixed ionic-electronic conductors (OMIECs) have emerged as promising materials for biological sensing, owing to their electrochemical activity, stability in an aqueous environment, and biocompatibility. Yet, OMIEC-based sensors rely predominantly on the use of composite matrices to enable stimuli-responsive functionality, which can exhibit issues with intercomponent interfacing. In this study, an approach is presented for non-enzymatic glucose detection by harnessing a newly synthesized functionalized monomer, EDOT-PBA. This monomer integrates electrically conducting and receptor moieties within a single organic component, obviating the need for complex composite preparation. By engineering the conditions for electrodeposition, two distinct polymer film architectures are developed: pristine PEDOT-PBA and molecularly imprinted PEDOT-PBA. Both architectures demonstrated proficient glucose binding and signal transduction capabilities. Notably, the molecularly imprinted polymer (MIP) architecture demonstrated faster stabilization upon glucose uptake while it also enabled a lower limit of detection, lower standard deviation, and a broader linear range in the sensor output signal compared to its non-imprinted counterpart. This material design not only provides a robust and efficient platform for glucose detection but also offers a blueprint for developing selective sensors for a diverse array of target molecules, by tuning the receptor units correspondingly.

Carboxyl-Alkyl Functionalized Conjugated Polyelectrolytes for High Performance Organic Electrochemical Transistors

Contemporary design principles for organic mixed ionic electronic conductors (OMIECs) are mostly based on the ethylene glycol moiety, which may not be representative of the OMIEC class as a whole. Furthermore, glycolated polymers can be difficult to synthesize and process effectively. As an emerging alternative, we present a series of polythiophenes functionalized with a hybrid carboxyl-alkyl side chain. By variation of the alkyl spacer length, a comprehensive evaluation of both the impact of carboxylic acid functionalization and alkyl spacer length was conducted. COOH-functionalization endows the polymer with preferential intrinsic low-swelling behavior and water processability to yield solvent-resistant conjugated polyelectrolytes while retaining substantial electroactivity in aqueous environments. Advanced in situ techniques, including time-resolved spectroelectrochemistry and Raman spectroscopy, are used to interrogate the materials' microstructure, ionic-electronic coupling, and operational stability in devices. To compare these materials' performance to state-of-the-art technology for the design of OMIECs, we benchmarked the materials and demonstrated significant application potential in both planar and interdigitated organic electrochemical transistors (OECTs). The polythiophene bearing carboxyl-butyl side chains exhibits greater electrochemical performance and faster doping kinetics within the polymer series, with a record-high OECT performance among conjugated polyelectrolytes ([μC*]pOECT = 107 ± 4 F cm-1 V-1 s-1). The results provide an enhanced understanding of structure-property relationships for conjugated polyelectrolytes operating in aqueous media and expand the materials options for future OMIEC development. Further, this work demonstrates the potential for conjugated polymers bearing alkyl-COOH side chains as a path toward robust OMIEC designs that may facilitate further facile (bio)chemical functionalization for a range of (bio)sensing applications.

Chemically revised conducting polymers with inflammation resistance for intimate bioelectronic electrocoupling

Conducting polymers offer attractive mixed ionic-electronic conductivity, tunable interfacial barrier with metal, tissue matchable softness, and versatile chemical functionalization, making them robust to bridge the gap between brain tissue and electronic circuits. This review focuses on chemically revised conducting polymers, combined with their superior and controllable electrochemical performance, to fabricate long-term bioelectronic implants, addressing chronic immune responses, weak neuron attraction, and long-term electrocommunication instability challenges. Moreover, the promising progress of zwitterionic conducting polymers in bioelectronic implants (≥4 weeks stable implantation) is highlighted, followed by a comment on their current evolution toward selective neural coupling and reimplantable function. Finally, a critical forward look at the future of zwitterionic conducting polymers for in vivo bioelectronic devices is provided.

"Clickable" Organic Electrochemical Transistors

Interfacing the surface of an organic semiconductor with biological elements is a central quest when it comes to the development of efficient organic bioelectronic devices. Here, we present the first example of "clickable" organic electrochemical transistors (OECTs). The synthesis and characterization of an azide-derivatized EDOT monomer (azidomethyl-EDOT, EDOT-N₃) are reported, as well as its deposition on Au-interdigitated electrodes through electropolymerization to yield PEDOT-N₃-OECTs. The electropolymerization protocol allows for a straightforward and reliable tuning of the characteristics of the OECTs, yielding transistors with lower threshold voltages than PEDOT-based state-of-the-art devices and maximum transconductance voltage values close to 0 V, a key feature for the development of efficient organic bioelectronic devices. Subsequently, the azide moieties are employed to click alkyne-bearing molecules such as redox probes and biorecognition elements. The clicking of an alkyne-modified PEG₄-biotin allows for the use of the avidin-biotin interactions to efficiently generate bioconstructs with proteins and enzymes. In addition, a dibenzocyclooctyne-modified thrombin-specific HD22 aptamer is clicked on the PEDOT-N₃-OECTs, showing the application of the devices toward the development of organic transistors-based biosensors. Finally, the clicked OECTs preserve their electronic features after the different clicking procedures, demonstrating the stability and robustness of the fabricated transistors.

Recent Advances and Biomedical Applications of Peptide-Integrated Conducting Polymers

Conducting polymers (CPs) are of great interests to researchers around the world in biomedical applications owing to their unique electrical and mechanical properties. Besides, they are easy to fabricate and have long-term stability. These features make CPs a powerful building block of modern biomaterials. Peptide functionalization has been a versatile tool for the development of CP-based biomaterials. With the aid of peptide modifications, the biocompatibility, target selectivity, and cellular interactions of CPs can be greatly improved. Reflecting these aspects, an increasing number of studies on peptide-integrated conducting polymers have been reported recently. In this review, various kinds of peptide immobilization strategies on CPs are introduced. Moreover, the aims of peptide modification are discussed in three aspects: enhancing the specific selectivity, avoiding nonspecific adhesion, and mimicking the environment of extracellular matrix. We highlighted recent studies in the applications of peptide-integrated CPs in electrochemical sensors, antifouling surfaces, and conductive biointerfaces. These studies have shown great potentials from the integration of peptide and CPs as a versatile platform for advanced biological and clinical applications in the near future.

Organic Bioelectronics for In Vitro Systems

Bioelectronics have made strides in improving clinical diagnostics and precision medicine. The potential of bioelectronics for bidirectional interfacing with biology through continuous, label-free monitoring on one side and precise control of biological activity on the other has extended their application scope to in vitro systems. The advent of microfluidics and the considerable advances in reliability and complexity of in vitro models promise to eventually significantly reduce or replace animal studies, currently the gold standard in drug discovery and toxicology testing. Bioelectronics are anticipated to play a major role in this transition offering a much needed technology to push forward the drug discovery paradigm. Organic electronic materials, notably conjugated polymers, having demonstrated technological maturity in fields such as solar cells and light emitting diodes given their outstanding characteristics and versatility in processing, are the obvious route forward for bioelectronics due to their biomimetic nature, among other merits. This review highlights the advances in conjugated polymers for interfacing with biological tissue in vitro, aiming ultimately to develop next generation in vitro systems. We showcase in vitro interfacing across multiple length scales, involving biological models of varying complexity, from cell components to complex 3D cell cultures. The state of the art, the possibilities, and the challenges of conjugated polymers toward clinical translation of in vitro systems are also discussed throughout.

Functionalization Strategies of PEDOT and PEDOT:PSS Films for Organic Bioelectronics Applications

Organic bioelectronics involves the connection of organic semiconductors with living organisms, organs, tissues, cells, membranes, proteins, and even small molecules. In recent years, this field has received great interest due to the development of all kinds of devices architectures, enabling the detection of several relevant biomarkers, the stimulation and sensing of cells and tissues, and the recording of electrophysiological signals, among others. In this review, we discuss recent functionalization approaches for PEDOT and PEDOT:PSS films with the aim of integrating biomolecules for the fabrication of bioelectronics platforms. As the choice of the strategy is determined by the conducting polymer synthesis method, initially PEDOT and PEDOT:PSS films preparation methods are presented. Later, a wide variety of PEDOT functionalization approaches are discussed, together with bioconjugation techniques to develop efficient organic-biological interfaces. Finally, and by making use of these approaches, the fabrication of different platforms towards organic bioelectronics devices is reviewed.

Bio-PEDOT: Modulating Carboxyl Moieties in Poly(3,4-ethylenedioxythiophene) for Enzyme-Coupled Bioelectronic Interfaces

Modulation of chemical functional groups on conducting polymers (CPs) provides an effective way to tailor the physicochemical properties and electrochemical performance of CPs, as well as serves as a functional interface for stable integration of CPs with biomolecules for organic bioelectronics (OBEs). Herein, we introduced a facile approach to modulate the carboxylate functional groups on the PEDOT interface through a systematic evaluation on the effect of a series of carboxylate-containing molecules as counterion dopant integrated into the PEDOT backbone, including acetate as monocarboxylate (mono-COO^-), malate as dicarboxylate (di-COO^-), citrate as tricarboxylate (tri-COO^-), and poly(acrylamide-co-acrylate) as polycarboxylate (poly-COO^-) bearing different amounts of molecular carboxylate moieties to create tunable PEDOT:COO^- interfaces with improved polymerization efficiency. We demonstrated the modulation of PEDOT:COO^- interfaces with various granulated morphologies from 0.33 to 0.11 μm, tunable surface carboxylate densities from 0.56 to 3.6 μM cm^-2, and with improved electrochemical kinetics and cycling stability. We further demonstrated the effective and stable coupling of an enzyme model lactate dehydrogenase (LDH) with the optimized PEDOT:poly-COO^- interface via simple covalent chemistry to develop biofunctionalized PEDOT (Bio-PEDOT) as a lactate biosensor. The biosensing mechanism is driven by a sequential bioelectrochemical signal transduction between the bio-organic LDH and organic PEDOT toward the concept of all-polymer-based OBEs with a high sensitivity of 8.38 μA mM^-1 cm^-2 and good reproducibility. Moreover, we utilized the LDH-PEDOT biosensor for the detection of lactate in spiked serum samples with a high recovery value of 91-96% and relatively small RSD in the range of 2.1-3.1%. Our findings provide a new insight into the design and optimization of functional CPs, leading to the development of new OBEs for sensing, biosensing, bioengineering, and biofuel cell applications.

Organic Bioelectronics: From Functional Materials to Next-Generation Devices and Power Sources

Conjugated polymers (CPs) possess a unique set of features setting them apart from other materials. These properties make them ideal when interfacing the biological world electronically. Their mixed electronic and ionic conductivity can be used to detect weak biological signals, deliver charged bioactive molecules, and mechanically or electrically stimulate tissues. CPs can be functionalized with various (bio)chemical moieties and blend with other functional materials, with the aim of modulating biological responses or endow specificity toward analytes of interest. They can absorb photons and generate electronic charges that are then used to stimulate cells or produce fuels. These polymers also have catalytic properties allowing them to harvest ambient energy and, along with their high capacitances, are promising materials for next-generation power sources integrated with bioelectronic devices. In this perspective, an overview of the key properties of CPs and examination of operational mechanism of electronic devices that leverage these properties for specific applications in bioelectronics is provided. In addition to discussing the chemical structure-functionality relationships of CPs applied at the biological interface, the development of new chemistries and form factors that would bring forth next-generation sensors, actuators, and their power sources, and, hence, advances in the field of organic bioelectronics is described.

Soft and Ion-Conducting Materials in Bioelectronics: From Conducting Polymers to Hydrogels

Bioelectronics devices that directly interface with cells and tissue have applications in neural and cardiac stimulation and recording, electroceuticals, and brain machine interfaces for prostheses. The interface between bioelectronic devices and biological tissue is inherently challenging due to the mismatch in both mechanical properties (hard vs soft) and charge carriers (electrons vs ions). In addition to conventional metals and silicon, new materials have bridged this interface, including conducting polymers, carbon-based nanomaterials, as well as ion-conducting polymers and hydrogels. This review provides an update on advances in soft bioelectronic materials for current and future therapeutic applications. Specifically, this review focuses on soft materials that can conduct both electrons and ions, and also deliver drugs and small molecules. The future opportunities and emerging challenges in the field are also highlighted.

Thiophene-Based Aldehyde Derivatives for Functionalizable and Adhesive Semiconducting Polymers

The pursuit for novelty in the field of (bio)electronics demands for new and better-performing (semi)conductive materials. Since the discovery of poly(3,4-ethylenedioxythiophene) (PEDOT), the ubiquitous golden standard, many studies have focused on its applications but only few on its structural modification and/or functionalization. This lack of structural variety strongly limits the versatility of PEDOT, thus hampering the development of novel PEDOT-based materials. In this paper, we present a short and simple strategy for introducing an aldehyde functionality in thiophene-based semiconducting polymers. First, through a two-step synthesis, an EDOT-aldehyde derivative was prepared and polymerized, both chemically and electrochemically. Next, to overcome the inability of thiophene-aldehyde to be polymerized by any means, we synthesized a trimer in which thiophene-aldehyde is enclosed between two EDOT groups. The successful chemical and electrochemical polymerization of this new trimer is presented. The polymer suspensions were characterized by ultraviolet-visible-near-infrared spectroscopy, while the corresponding films were characterized by Fourier transform infrared and four-point-probe conductivity measurements. Afterward, insoluble semiconducting films were formed by using ethylenediamine as a cross-linker, demonstrating in this way the suitability of the aldehyde group for the easy chemical modification of our material. The efficient reactivity conferred by aldehyde groups was also exploited for grafting fluorescent polyamine nanoparticles on the film surface, creating a fluorescent semiconducting polymer film. The films prepared by electropolymerization, as shown by means of a sonication test, exhibit strong surface adhesion on pristine indium tin oxide (ITO). This property paves the way for the application of these polymers as conductive electrodes for interfacing with living organisms. Thanks to the high reactivity of the aldehyde group, the aldehyde-bearing thiophene-based polymers prepared herein are extremely valuable for numerous applications requiring the facile incorporation of a functional group on thiophene, such as the functionalization with labile molecules (thermo-, photo-, and electro-labile, pH sensitive, etc.).

Electropolymerized Poly(3,4-ethylenedioxythiophene) (PEDOT) Coatings for Implantable Deep-Brain-Stimulating Microelectrodes

Conducting polymers have been widely explored as coating materials for metal electrodes to improve neural signal recording and stimulation because of their mixed electronic-ionic conduction and biocompatibility. In particular, the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the best candidates for biomedical applications due to its high conductivity and good electrochemical stability. Coating metal electrodes with PEDOT has shown to enhance the electrode's performance by decreasing the impedance and increasing the charge storage capacity. However, PEDOT-coated metal electrodes often have issues with delamination and stability, resulting in decreased device performance and lifetime. In this work, we were able to electropolymerize PEDOT coatings on sharp platinum-iridium recording and stimulating neural electrodes and demonstrated its mechanical and electrochemical stability. Electropolymerization of PEDOT:tetrafluoroborate was carried out in three different solvents: propylene carbonate, acetonitrile, and water. The stability of the coatings was assessed via ultrasonication, phosphate buffer solution soaking test, autoclave sterilization, and electrical pulsing. Coatings prepared with propylene carbonate or acetonitrile possessed excellent electrochemical stability and survived autoclave sterilization, prolonged soaking, and electrical stimulation without major changes in electrochemical properties. Stimulating microelectrodes were implanted in rats and stimulated daily, for 7 and 15 days. The electrochemical properties monitored in vivo demonstrated that the stimulation procedure for both coated and uncoated electrodes decreased the impedance.

Conjugated Polymers in Bioelectronics: Addressing the Interface Challenge

Conjugated polymers are the material of choice for organic bioelectronic interfaces as they combine mechanical flexibility with electric and ionic conductivity. Their attractive properties are largely demonstrated in vitro, while the in vivo applications are limited to the coating of inorganic electrodes, where they are used to improve the intimate electronic contact between the device and the tissue. However, there has not been a commensurate rise in the in vivo applications of entirely organic implantable electronic devices based on conjugated polymers. To date, there is no comprehensive understanding of how these devices will interface with real biological systems. With the push toward increasingly thinner and more flexible next generation medical implants, this limitation remains a major detractor in the translation of conjugated polymers toward biological applications. This research news article examines the few reported in vivo studies and attempts to establish why there is such a dearth in the literature.

Deprotonation-Induced Conductivity Shift of Polyethylenedioxythiophenes in Aqueous Solutions: The Effects of Side-Chain Length and Polymer Composition

Deprotonation-induced conductivity shift of poly(3,4-ethylenedixoythiophene)s (PEDOTs) in aqueous solutions is a promising platform for chemical or biological sensor due to its large signal output and minimum effect from material morphology. Carboxylic acid group functionalized poly(Cn-EDOT-COOH)s are synthesized and electrodeposited on microelectrodes. The microelectrodes are utilized to study the effect of carboxylic acid side-chain length on the conductivity curve profiles in aqueous buffer with different pH. The conductivity shifts due to the buffer pH are effected by the length of the carboxylic acid side-chains. The shifts can be explained by the carboxylic acid dissociation property (pKa) at the solid-liquid interface, self-doping effect, and effective conjugation length. Conductivity profiles of poly(EDOT-OH-co-C₂-EDOT-COOH) copolymers are also studied. The shifts show linear relationship with the feed monomer composition used in electrochemical polymerization.

Facile Synthesis of a 3,4-Ethylene-Dioxythiophene (EDOT) Derivative for Ease of Bio-Functionalization of the Conducting Polymer PEDOT

In the pursuit of conducting polymer based bio-functional devices, a cost-effective and high yield synthesis method for a versatile monomer is desired. We report here a new synthesis strategy for a versatile monomer 2-methylene-2,3-dihydrothieno (3,4-b) (1,4) dioxine, or 3,4-ethylenedioxythiophene with a exomethylene side group (EDOT-EM). Compared to the previously reported synthesis route, the new strategy uses less steps, with faster reaction rate, and higher yield. The presence of EM group opens up endless possibility for derivatization via either hydro-alkoxy addition or thiol-ene click chemistry. EDOT-EM could be polymerized into stable and low impedance PEDOT-EM polymer using electro-polymerization method on different conducting substrates at both macro and micro scales. Facile post-functionalization of PEDOT-EM with molecules of varying size and functionality (from small molecules to DNAs and proteins) was achieved. The new synthetic route of EDOT-EM and the ease of post-functionalization of PEDOT-EM will greatly accelerate the use of conducting polymer in a broad range of organic electronics and bioelectronics applications.

Synthesis of Carboxyl-EDOT as a Versatile Addition and Additive to PEDOT:PSS

Two-step synthesis of EDOT (3,4-ethylenedioxythiophene) derivate bearing a carboxylic acid group (carboxyl-EDOT) is presented. This reactive monomer has been copolymerized with EDOT to afford PEDOT copolymers. Thanks to the most common additives usually added to the PEDOT:PSS dispersion, ethylene glycol and 4-dodecylbenzenesulfonic acid (DBSA), the carboxylic acid has been used to cross-link the material via esterification reactions. This result offers the possibility to produce a polymer network without adding any cross-linking agent. Furthermore, the short synthetic pathway of carboxyl-EDOT offers the possibility to incorporate new functionality either in EDOT monomer or in PEDOT materials with a reasonable chemical effort and background.

Conjugated Polymers in Bioelectronics

The emerging field of organic bioelectronics bridges the electronic world of organic-semiconductor-based devices with the soft, predominantly ionic world of biology. This crosstalk can occur in both directions. For example, a biochemical reaction may change the doping state of an organic material, generating an electronic readout. Conversely, an electronic signal from a device may stimulate a biological event. Cutting-edge research in this field results in the development of a broad variety of meaningful applications, from biosensors and drug delivery systems to health monitoring devices and brain-machine interfaces. Conjugated polymers share similarities in chemical "nature" with biological molecules and can be engineered on various forms, including hydrogels that have Young's moduli similar to those of soft tissues and are ionically conducting. The structure of organic materials can be tuned through synthetic chemistry, and their biological properties can be controlled using a variety of functionalization strategies. Finally, organic electronic materials can be integrated with a variety of mechanical supports, giving rise to devices with form factors that enable integration with biological systems. While these developments are innovative and promising, it is important to note that the field is still in its infancy, with many unknowns and immense scope for exploration and highly collaborative research. The first part of this Account details the unique properties that render conjugated polymers excellent biointerfacing materials. We then offer an overview of the most common conjugated polymers that have been used as active layers in various organic bioelectronics devices, highlighting the importance of developing new materials. These materials are the most popular ethylenedioxythiophene derivatives as well as conjugated polyelectrolytes and ion-free organic semiconductors functionalized for the biological interface. We then discuss several applications and operation principles of state-of-the-art bioelectronics devices. These devices include electrodes applied to sense/trigger electrophysiological activity of cells as well as electrolyte-gated field-effect and electrochemical transistors used for sensing of biochemical markers. Another prime application example of conjugated polymers is cell actuators. External modulation of the redox state of the underlying conjugated polymer films controls the adhesion behavior and viability of cells. These smart surfaces can be also designed in the form of three-dimensional architectures because of the processability of conjugated polymers. As such, cell-loaded scaffolds based on electroactive polymers enable integrated sensing or stimulation within the engineered tissue itself. A last application example is organic neuromorphic devices, an alternative computing architecture that takes inspiration from biology and, in particular, from the way the brain works. Leveraging ion redistribution inside a conjugated polymer upon application of an electrical field and its coupling with electronic charges, conjugated polymers can be engineered to act as artificial neurons or synapses with complex, history-dependent behavior. We conclude this Account by highlighting main factors that need to be considered for the design of a conjugated polymer for applications in bioelectronics-although there can be various figures of merit given the broad range of applications, as emphasized in this Account.

Novel Conducting and Biodegradable Copolymers with Noncytotoxic Properties toward Embryonic Stem Cells

Electroactive biomaterials that are easily processed as scaffolds with good biocompatibility for tissue regeneration are difficult to design. Herein, the synthesis and characterization of a variety of novel electroactive, biodegradable biomaterials based on poly(3,4-ethylenedioxythiphene) copolymerized with poly(d,l lactic acid) (PEDOT-co-PDLLA) are presented. These copolymers were obtained using (2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methanol (EDOT-OH) as an initiator in a lactide ring-opening polymerization reaction, resulting in EDOT-PDLLA macromonomer. Conducting PEDOT-co-PDLLA copolymers (in three different proportions) were achieved by chemical copolymerization with 3,4-ethylenedioxythiophene (EDOT) monomers and persulfate oxidant. The PEDOT-co-PDLLA copolymers were structurally characterized by 1H NMR and Fourier transform infrared spectroscopy. Cyclic voltammetry confirmed the electroactive character of the materials, and conductivity measurements were performed via electrochemical impedance spectroscopy. In vitro biodegradability was evaluated using proteinase K over 35 days, showing 29-46% (w/w) biodegradation. Noncytotoxicity was assessed by adhesion, migration, and proliferation assays using embryonic stem cells (E14.tg2a); excellent neuronal differentiation was observed. These novel electroactive and biodegradable PEDOT-co-PDLLA copolymers present surface chemistry and charge density properties that make them potentially useful as scaffold materials in different fields of applications, especially for neuronal tissue engineering.

Influence of surface treatment on PEDOT coatings: surface and electrochemical corrosion aspects of newly developed Ti alloy

Surface treatment of metallic materials prior to the application of polymer coatings plays an important role in providing improved surface features and enhanced corrosion protection. In the current investigation, we aimed to evaluate the effect of surface treatment of newly developed TiNbZr (TNZ) alloys on the surface characteristics, including the surface topography, morphology, hydrophobicity and adhesion strength of subsequent poly(3,4-ethylenedioxythiophene) (PEDOT) coatings. The surface morphology, chemical composition, and surface roughness of both treated and coated alloys were characterized by scanning electron microscopy, energy dispersive spectroscopy, and optical profilometry, respectively. The adhesion strength of the coating was measured using a micro scratch machine. Furthermore, we also evaluated the performance of electrochemically synthesized PEDOT coatings on surface-treated TNZ alloys in terms of the surface protective performance in simulated body fluid (SBF) and in vitro bioactivity in osteoblast MG63 cells. Surface analysis findings indicated that the nature of the PEDOT coating (surface morphology, topography, wettability and adhesion strength) was intensely altered, while the surface treatment performed before electrodeposition facilitated the overall performance of PEDOT coatings as implant coating materials. The obtained corrosion studies confirmed the enhanced corrosion protection performance of PEDOT coatings on treated TNZ substrates. In vitro cell culture studies validated the improved cell adhesion and proliferation rate, further highlighting the important role of surface treatment before electrodeposition.

Vapor deposition of polyionic nanocoatings for reduction of microglia adhesion

Polyionics have great potential in improving the performance of neural probes by regulating microglial response. With the shrinkage of microelectrode size and increase in device complexity, challenges arise during liquid-based synthesis of polyionic compounds on neural probes. Nanocoatings of polyionics, with highly crosslinked bulk structure and abundant ionic functional groups on the surface, were synthesized using a process combining chemical vapor deposition and free radical polymerization. Both conformal surface engineering of neural microelectrodes and facile tailoring of surface ionic composition was achieved using this single-step vapor-based method. Adhesion of microglia was reduced on all the polyionic modified surfaces after a seven-day in vitro test, and polyionics with mixed charges presented much lower microglial adhesion than surfaces with single charges. Laminin adsorption on polyionics with mixed charges was significantly reduced due to the surface electrical neutrality and the enhanced wettability. These findings provide valuable information towards the development of neural probes with enhanced biocompatibility and signal stability.

Recent Advances in Neural Electrode-Tissue Interfaces

Neurotechnology is facing an exponential growth in the recent decades. Neural electrode-tissue interface research has been well recognized as an instrumental component of neurotechnology development. While satisfactory long-term performance was demonstrated in some applications, such as cochlear implants and deep brain stimulators, more advanced neural electrode devices requiring higher resolution for single unit recording or microstimulation still face significant challenges in reliability and longevity. In this article, we review the most recent findings that contribute to our current understanding of the sources of poor reliability and longevity in neural recording or stimulation, including the material failure, biological tissue response and the interplay between the two. The newly developed characterization tools are introduced from electrophysiology models, molecular and biochemical analysis, material characterization to live imaging. The effective strategies that have been applied to improve the interface are also highlighted. Finally, we discuss the challenges and opportunities in improving the interface and achieving seamless integration between the implanted electrodes and neural tissue both anatomically and functionally.

Poly(3,4-ethylenedioxythiophene) (PEDOT) Derivatives: Innovative Conductive Polymers for Bioelectronics

Poly(3,4-ethylenedioxythiophene)s are the conducting polymers (CP) with the biggest prospects in the field of bioelectronics due to their combination of characteristics (conductivity, stability, transparency and biocompatibility). The gold standard material is the commercially available poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). However, in order to well connect the two fields of biology and electronics, PEDOT:PSS presents some limitations associated with its low (bio)functionality. In this review, we provide an insight into the synthesis and applications of innovative poly(ethylenedioxythiophene)-type materials for bioelectronics. First, we present a detailed analysis of the different synthetic routes to (bio)functional dioxythiophene monomer/polymer derivatives. Second, we focus on the preparation of PEDOT dispersions using different biopolymers and biomolecules as dopants and stabilizers. To finish, we review the applications of innovative PEDOT-type materials such as biocompatible conducting polymer layers, conducting hydrogels, biosensors, selective detachment of cells, scaffolds for tissue engineering, electrodes for electrophysiology, implantable electrodes, stimulation of neuronal cells or pan-bio electronics.

Polymerization of PEDOT/PSS/Chitosan-Coated Electrodes for Electrochemical Bio-Sensing

Poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly (styrene sulfonate) (PSS) has a variety of chemical and biomedical applications. Additionally, chitosan has been extensively used in industrial and medical fields. However, whether chitosan could be incorporated into conducting polymers of PEDOT/PSS is not clear. In this study, the PEDOT/PSS/chitosan coatings were electrochemically polymerized on the surface of 0.5 mm platinum (Pt) electrodes and the properties of electrochemical cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of the PEDOT/PSS/chitosan-coated electrodes were investigated. Furthermore, PEDOT/PSS/chitosan-coated electrodes used for electrochemical bio-sensing, using dexamethasone (Dex) as a model bio-sensing material, were examined. The results demonstrated that PEDOT/PSS/chitosan-coated electrodes were stable in phosphate-buffered saline (PBS) solution. The electrochemical CV curve areas, reflecting the charge delivery capacity, and the EIS of the PEDOT/PSS/chitosan-coated electrodes were sensitive to Dex, and the good linearity can be obtained between CV curve areas, the EIS and the concentration of Dex. In addition, electrochemical sensitivity of the PEDOT/PSS/chitosan-coated electrodes to Dex was much higher than ultraviolet (UV) spectroscopy detection. All these results revealed that the PEDOT/PSS/chitosan-coated electrodes can be electrochemical polymerized and used for electrochemical bio-sensing.

Organic bioelectronics in medicine

A major challenge in the growing field of bioelectronic medicine is the development of tissue interface technologies promoting device integration with biological tissues. Materials based on organic bioelectronics show great promise due to a unique combination of electronic and ionic conductivity properties. In this review, we outline exciting developments in the field of organic bioelectronics and demonstrate the medical importance of these active, electronically controllable materials. Importantly, organic bioelectronics offer a means to control cell-surface attachment as required for many device-tissue applications. Experiments have shown that cells readily attach and proliferate on reduced but not oxidized organic bioelectronic materials. In another application, the active properties of organic bioelectronics were used to develop electronically triggered systems for drug release. After incorporating drugs by advanced loading strategies, small compound drugs were released upon electrochemical trigger, independent of charge. Another type of delivery device was used to achieve well-controlled, spatiotemporal delivery of cationic drugs. Via electrophoretic transport within a polymer, cations were delivered with single-cell precision. Finally, organic bioelectronic materials are commonly used as electrode coatings improving the electrical properties of recording and stimulation electrodes. Because such coatings drastically reduce the electrode impedance, smaller electrodes with improved signal-to-noise ratio can be fabricated. Thus, rapid technological advancement combined with the creation of tiny electronic devices reacting to changes in the tissue environment helps to promote the transition from standard pharmaceutical therapy to treatment based on 'electroceuticals'. Moreover, the widening repertoire of organic bioelectronics will expand the options for true biological interfaces, providing the basis for personalized bioelectronic medicine.

Research trends in biomimetic medical materials for tissue engineering: commentary

We introduce our active experts’ communications and reviews (Part II) of 2015 Korea-China Joint Symposium on Biomimetic Medical Materials in Republic of Korea, which reflect their perspectives on current research trends of biomimetic medical materials for tissue regeneration in both Korea and China. The communications covered three topics of biomimetics, i.e., 1) hydrogel for therapeutics and extracellular matrix environments, 2) design of electrical polymers for communications between electrical sources and biological systems and 3) design of biomaterials for nerve tissue engineering. The reviews in the Part II will cover biomimetics of 3D bioprinting materials, surface modifications, nano/micro-technology as well as clinical aspects of biomaterials for cartilage.

Biofunctionalization of polydioxythiophene derivatives for biomedical applications

It is becoming clear that development of biomedical devices relies on engineering of the interface between the device and the biological component. Improved performance for these sensors and devices can be achieved through biofunctionalization. In this review we focus on highlighting the biofunctionalization of polydioxythiophene sensors.

Conductive hybrid carbon nanotube (CNT)–polythiophene coatings for innovative auditory neuron-multi-electrode array interfacing

Surface modification of platinum electrodes to improve neuron-electrode interface and electrode conductive properties in cochlear implants.

Significant enhancement of PEDOT thin film adhesion to inorganic solid substrates with EDOT-acid

With its high conductivity, tunable surface morphology, relatively soft mechanical response, high chemical stability, and excellent biocompatibility, poly(3,4-ethylenedioxythiophene) (PEDOT) has become a promising coating material for a variety of electronic biomedical devices. However, the relatively poor adhesion of PEDOT to inorganic metallic and semiconducting substrates still poses challenges for long-term applications. Here, we report that 2,3-dihydrothieno(3,4-b)(1,4)dioxine-2-carboxylic acid (EDOT-acid) significantly improves the adhesion between PEDOT thin films and inorganic solid electrodes. EDOT-acid molecules were chemically bonded onto activated oxide substrates via the chemisorption of the carboxylic groups. PEDOT was then polymerized onto the EDOT-acid modified substrates, forming covalently bonded coatings. The adsorption of EDOT-acid onto the electrode surfaces was characterized by cyclic voltammetry (CV), contact angle measurements, atomic force microscopy, and X-ray photoelectron spectroscopy. The electrical properties of the subsequently coated PEDOT films were studied by electrochemical impedance spectroscopy and CV. An aggressive ultrasonication test confirmed the significantly improved adhesion and mechanical stability of the PEDOT films on electrodes with EDOT-acid treatment over those without treatment.

A lysinated thiophene-based semiconductor as a multifunctional neural bioorganic interface

Lysinated molecular organic semiconductors are introduced as valuable multifunctional platforms for neural cells growth and interfacing. Cast films of quaterthiophene (T4) semiconductor covalently modified with lysine-end moieties (T4Lys) are fabricated and their stability, morphology, optical/electrical, and biocompatibility properties are characterized. T4Lys films exhibit fluorescence and electronic transport as generally observed for unsubstituted oligothiophenes combined to humidity-activated ionic conduction promoted by the charged lysine-end moieties. The Lys insertion in T4 enables adhesion of primary culture of rat dorsal root ganglion (DRG), which is not achievable by plating cells on T4. Notably, on T4Lys, the number on adhering neurons/area is higher and displays a twofold longer neurite length than neurons plated on glass coated with poly-l-lysine. Finally, by whole-cell patch-clamp, it is shown that the biofunctionality of neurons cultured on T4Lys is preserved. The present study introduces an innovative concept for organic material neural interface that combines optical and iono-electronic functionalities with improved biocompatibility and neuron affinity promoted by Lys linkage and the softness of organic semiconductors. Lysinated organic semiconductors could set the scene for the fabrication of simplified bioorganic devices geometry for cells bidirectional communication or optoelectronic control of neural cells biofunctionality.

Decellular biological scaffold polymerized with PEDOT for improving peripheral nerve interface charge transfer

Regenerative peripheral nerve interfaces (RPNIs) are for signal transfer between peripheral nerves inside the body to controllers for motorized prosthetics external to the body. Within the residual limb of an amputee, surgical construction of a RPNI connects a remaining peripheral nerve and spare muscle. Nerve signals become concentrated within the RPNI. Currently metal electrodes implanted on the RPNI muscle transfer signals but scarring around metal electrodes progressively diminishes charge transfer. Engineered materials may benefit RPNI signal transfer across the neural interface if they lower the power and charge density of the biologically meaningful signals. Poly3,4-ethylenedioxythiophene (PEDOT) is known to mediate ionic potentials allowing excitation across a critical nerve gap. We hypothesize that the capacity of an interface material to conduct electron mediated current is significantly increased by polymerized coating of PEDOT. SIS was either used plain or after PEDOT coating by electrochemical polymerization. Muscle forces are a direct representation of stimulating current distribution within an RPNI. In situ muscle forces were measured for the same muscle by electrically stimulating: a) the muscle's innervating nerve, b) directly on the muscle, c) on plain SIS laid on the muscle, and d) on SIS polymerized with PEDOT laid on the muscle. Electro-chemically coating PEDOT on SIS resulted in a thin, flexible material. PEDOT coated SIS distributed electrical stimulation more efficiently than SIS alone. Conductive polymer containing biological material allowed ionic signal distribution within the RPNI like muscle at lower charge density.

Post-polymerization functionalization of poly(3,4-propylenedioxythiophene) (PProDOT) via thiol–ene “click” chemistry

Post-polymerization functionalization using thiol–ene “click” chemistry provides an effective, convenient means to modify the surface properties of conjugated polymers such as poly(3,4-propylenedioxythiophene) (PProDOT).

A simple approach for protein covalent grafting on conducting polymer films

By mixing a PEDOT:PSS suspension with the modified biopolymer carboxymethylated dextran (CMD), we obtain conductive films displaying carboxyl (–COOH) groups allowing for covalent grafting of proteins via amide bonds.

In vivo polymerization of poly(3,4-ethylenedioxythiophene) in the living rat hippocampus does not cause a significant loss of performance in a delayed alternation task

After extended implantation times, traditional intracortical neural probes exhibit a foreign-body reaction characterized by a reactive glial sheath that has been associated with increased system impedance and signal deterioration. Previously, we have proposed that the local in vivo polymerization of an electronically and ionically conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), might help to rebuild charge transport pathways across the glial scar between the device and surrounding parenchyma (Richardson-Burns et al 2007 J. Neural Eng. 4 L6-13). The EDOT monomer can be delivered via a microcannula/electrode system into the brain tissue of living animals followed by direct electrochemical polymerization, using the electrode itself as a source of oxidative current. In this study, we investigated the long-term effect of local in vivo PEDOT deposition on hippocampal neural function and histology. Rodent subjects were trained on a hippocampus-dependent task, delayed alternation (DA), and implanted with the microcannula/electrode system in the hippocampus. The animals were divided into four groups with different delay times between the initial surgery and the electrochemical polymerization: (1) control (no polymerization), (2) immediate (polymerization within 5 min of device implantation), (3) early (polymerization within 3-4 weeks after implantation) and (4) late (polymerization 7-8 weeks after polymerization). System impedance at 1 kHz was recorded and the tissue reactions were evaluated by immunohistochemistry. We found that under our deposition conditions, PEDOT typically grew at the tip of the electrode, forming an ∼500 µm cloud in the tissue. This is much larger than the typical width of the glial scar (∼150 µm). After polymerization, the impedance amplitude near the neurologically important frequency of 1 kHz dropped for all the groups; however, there was a time window of 3-4 weeks for an optimal decrease in impedance. For all surgery-polymerization time intervals, the polymerization did not cause significant deficits in performance of the DA task, suggesting that hippocampal function was not impaired by PEDOT deposition. However, GFAP+ and ED-1+ cells were also found at the deposition two weeks after the polymerization, suggesting potential secondary scarring. Therefore, less extensive deposition or milder deposition conditions may be desirable to minimize this scarring while maintaining decreased system impedance.

A facile biofunctionalisation route for solution processable conducting polymer devices

For the majority of biosensors or biomedical devices, immobilization of the biorecognition element is a critical step for device function.

Biofunctionalisation of electrically conducting polymers

During a single decade of research, evidence has emerged that glial scar formation around the electro-tissue interface drives neural loss and increases the signal impedance of the electrodes, compromising the efficiency of the stimulating systems. Studies with conducting polymers (CPs) as electrode coatings have shown enhanced tissue integration and electrode performance in situ through biochemical and physicomechanical functionalisation. In this review, recent findings on CP modifications are provided in the context of neurospecific biomaterials, shedding light on the valuable impact of multifunctionalised strategies for biomedical applications.