Biocompatibility of a polymer based on Off-Stoichiometry Thiol-Enes + Epoxy (OSTE+) for neural implants

Performance and biocompatibility of OSTEMER 322 in cell-based microfluidic applications

The Off-Stoichiometry Thiol-ene and Epoxy (OSTE+) polymer technology has been increasingly utilised in the field of microfluidics and lab-on-a-chip applications. However, the impact of OSTEMER polymers, specifically the OSTEMER 322 formulation, on cell viability has remained limited. In this work, we thoroughly explored the biocompatibility of this commercial OSTEMER formulation, along with various surface modifications, through a broad range of cell types, from fibroblasts to epithelial cells. We employed cell viability and confluence assays to evaluate the performance of the material and its modified variants in cell culturing. The properties of the pristine and modified OSTEMER were also investigated using surface characterization methods including contact angle, zeta potential, and X-ray photoelectron spectroscopy. Mass spectrometry analysis confirmed the absence of leaching constituents from OSTEMER, indicating its safety for cell-based applications. Our findings demonstrated that cell viability on OSTEMER surfaces is sufficient for typical cell culture experiments, suggesting OSTEMER 322 is a suitable material for a variety of cell-based assays in microfluidic devices.

A Rapid Prototyping Approach for Multi-Material, Reversibly Sealed Microfluidics

Microfluidic organ-on-chip models recapitulate increasingly complex physiological phenomena to study tissue development and disease mechanisms, where there is a growing interest in retrieving delicate biological structures from these devices for downstream analysis. Standard bonding techniques, however, often utilize irreversible sealing, making sample retrieval unfeasible or necessitating destructive methods for disassembly. To address this, several commercial devices employ reversible sealing techniques, though integrating these techniques into early-stage prototyping workflows is often ignored because of the variation and complexity of microfluidic designs. Here, we demonstrate the concerted use of rapid prototyping techniques, including 3D printing and laser cutting, to produce multi-material microfluidic devices that can be reversibly sealed. This is enhanced via the incorporation of acrylic components directly into polydimethylsiloxane channel layers to enhance stability, sealing, and handling. These acrylic components act as a rigid surface separating the multiple mechanical seals created between the bottom substrate, the microfluidic features in the device, and the fluidic interconnect to external tubing, allowing for greater design flexibility. We demonstrate that these devices can be produced reproducibly outside of a cleanroom environment and that they can withstand ~1 bar pressures that are appropriate for a wide range of biological applications. By presenting an accessible and low-cost method, we hope to enable microfluidic prototyping for a broad range of biomedical research applications.

Conformable neural interface based on off-stoichiometry thiol-ene-epoxy thermosets

Off-stoichiometry thiol-ene-epoxy (OSTE+) thermosets show low permeability to gases and little absorption of dissolved molecules, allow direct low-temperature dry bonding without surface treatments, have a low Young's modulus, and can be manufactured via UV polymerisation. For these reasons, OSTE+ thermosets have recently gained attention for the rapid prototyping of microfluidic chips. Moreover, their compatibility with standard clean-room processes and outstanding mechanical properties make OSTE+ an excellent candidate as a novel material for neural implants. Here we exploit OSTE+ to manufacture a conformable multilayer micro-electrocorticography array with 16 platinum electrodes coated with platinum black. The mechanical properties allow conformability to curved surfaces such as the brain. The low permeability and strong adhesion between layers improve the stability of the device. Acute experiments in mice show the multimodal capacity of the array to record and stimulate the neural tissue by smoothly conforming to the mouse cortex. Devices are not cytotoxic, and immunohistochemistry stainings reveal only modest foreign body reaction after two and six weeks of chronic implantation. This work introduces OSTE+ as a promising material for implantable neural interfaces.

Stereochemistry and Stoichiometry in Aliphatic Polyester Photopolymers for 3D Printing Tailored Biomaterial Scaffolds

Stereoselective aliphatic polyesters were synthesized through the ring opening copolymerization of cyclic anhydrides and epoxides using a tin catalyst to yield Mn ~ 10-13 kDa macromolecules (Đ < 1.6). Isomerization...

Lung on a Chip Development from Off-Stoichiometry Thiol-Ene Polymer

Current in vitro models have significant limitations for new respiratory disease research and rapid drug repurposing. Lung on a chip (LOAC) technology offers a potential solution to these problems. However, these devices typically are fabricated from polydimethylsiloxane (PDMS), which has small hydrophobic molecule absorption, which hinders the application of this technology in drug repurposing for respiratory diseases. Off-stoichiometry thiol–ene (OSTE) is a promising alternative material class to PDMS. Therefore, this study aimed to test OSTE as an alternative material for LOAC prototype development and compare it to PDMS. We tested OSTE material for light transmission, small molecule absorption, inhibition of enzymatic reactions, membrane particle, and fluorescent dye absorption. Next, we microfabricated LOAC devices from PDMS and OSTE, functionalized with human umbilical vein endothelial cell (HUVEC) and A549 cell lines, and analyzed them with immunofluorescence. We demonstrated that compared to PDMS, OSTE has similar absorption of membrane particles and effect on enzymatic reactions, significantly lower small molecule absorption, and lower light transmission. Consequently, the immunofluorescence of OSTE LOAC was significantly impaired by OSTE optical properties. In conclusion, OSTE is a promising material for LOAC, but optical issues should be addressed in future LOAC prototypes to benefit from the material properties.

The material-enabled oxygen control in thiol-ene microfluidic channels and its feasibility for subcellular drug metabolism assays under hypoxia in vitro

Tissue oxygen levels are known to be critical to regulation of many cellular processes, including the hepatic metabolism of therapeutic drugs, but its impact is often ignored in in vitro assays. In this study, the material-induced oxygen scavenging property of off-stoichiometric thiol-enes (OSTE) was exploited to create physiologically relevant oxygen concentrations in microfluidic immobilized enzyme reactors (IMERs) incorporating human liver microsomes. This could facilitate rapid screening of, for instance, toxic drug metabolites possibly produced in hypoxic conditions typical for many liver injuries. The mechanism of OSTE-induced oxygen scavenging was examined in depth to enable precise adjustment of the on-chip oxygen concentration with the help of microfluidic flow. The oxygen scavenging rate of OSTE was shown to depend on the type and the amount of the thiol monomer used in the bulk composition, and the surface-to-volume ratio of the chip design, but not on the physical or mechanical properties of the bulk. Our data suggest that oxygen scavenging takes place at the polymer-liquid interface, likely via oxidative reactions of the excess thiol monomers released from the bulk with molecular oxygen. Based on the kinetic constants governing the oxygen scavenging rate in OSTE microchannels, a microfluidic device comprising monolithically integrated oxygen depletion and IMER units was designed and its performance validated with the help of oxygen-dependent metabolism of an antiretroviral drug, zidovudine, which yields a cytotoxic metabolite under hypoxic conditions.

A comparison of insertion methods for surgical placement of penetrating neural interfaces

Many implantable electrode arrays exist for the purpose of stimulating or recording electrical activity in brain, spinal, or peripheral nerve tissue, however most of these devices are constructed from materials that are mechanically rigid. A growing body of evidence suggests that the chronic presence of these rigid probes in the neural tissue causes a significant immune response and glial encapsulation of the probes, which in turn leads to gradual increase in distance between the electrodes and surrounding neurons. In recording electrodes, the consequence is the loss of signal quality and, therefore, the inability to collect electrophysiological recordings long term. In stimulation electrodes, higher current injection is required to achieve a comparable response which can lead to tissue and electrode damage. To minimize the impact of the immune response, flexible neural probes constructed with softer materials have been developed. These flexible probes, however, are often not strong enough to be inserted on their own into the tissue, and instead fail via mechanical buckling of the shank under the force of insertion. Several strategies have been developed to allow the insertion of flexible probes while minimizing tissue damage. It is critical to keep these strategies in mind during probe design in order to ensure successful surgical placement. In this review, existing insertion strategies will be presented and evaluated with respect to surgical difficulty, immune response, ability to reach the target tissue, and overall limitations of the technique. Overall, the majority of these insertion techniques have only been evaluated for the insertion of a single probe and do not quantify the accuracy of probe placement. More work needs to be performed to evaluate and optimize insertion methods for accurate placement of devices and for devices with multiple probes.

Near-infrared stimulation of the auditory nerve: A decade of progress toward an optical cochlear implant

OBJECTIVES

We provide an appraisal of recent research on stimulation of the auditory system with light. In particular, we discuss direct infrared stimulation and ongoing controversies regarding the feasibility of this modality. We also discuss advancements and barriers to the development of an optical cochlear implant.

METHODS

This is a review article that covers relevant animal studies.

RESULTS

The auditory system has been stimulated with infrared light, and in a much more spatially selective manner than with electrical stimulation. However, there are experiments from other labs that have not been able to reproduce these results. This has resulted in an ongoing controversy regarding the feasibility of infrared stimulation, and the reasons for these experimental differences still require explanation. The neural response characteristics also appear to be much different than with electrical stimulation. The electrical stimulation paradigms used for modern cochlear implants do not apply well to optical stimulation and new coding strategies are under development. Stimulation with infrared light brings the risk of heat accumulation in the tissue at high pulse repetition rates, so optimal pulse shapes and combined optical/electrical stimulation are being investigated to mitigate this. Optogenetics is another promising technique, which makes neurons more sensitive to light stimulation by inserting light sensitive ion channels via viral vectors. Challenges of optogenetics include the expression of light sensitive channels in sufficient density in the target neurons, and the risk of damaging neurons by the expression of a foreign protein.

CONCLUSION

Optical stimulation of the nervous system is a promising new field, and there has been progress toward the development of a cochlear implant that takes advantage of the benefits of optical stimulation. There are barriers, and controversies, but so far none that seem intractable.

LEVEL OF EVIDENCE

NA (animal studies and basic research).

A fully transparent, flexible PEDOT:PSS-ITO-Ag-ITO based microelectrode array for ECoG recording

Integrative neural interfaces combining neurophysiology and optogenetics with neural imaging provide numerous opportunities for neuroscientists to study the structure and function of neural circuits in the brain. Such a comprehensive interface demands miniature electrode arrays with high transparency, mechanical flexibility, electrical conductivity, and biocompatibility. Conventional transparent microelectrodes made of a single material, such as indium tin oxide (ITO), ultrathin metals, graphene and poly-(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS), hardly possess the desired combination of those properties. Herein, ultra-flexible, highly conductive and fully transparent microscale electrocorticogram (μECoG) electrode arrays made of a PEDOT:PSS-ITO-Ag-ITO assembly are constructed on thin parylene C films. The PEDOT:PSS-ITO-Ag-ITO assembly achieves a maximum ∼14% enhancement in light transmission over a broad spectrum (350-650 nm), a significant reduction in electrochemical impedance by 91.25%, and an increase in charge storage capacitance by 1229.78 μC cm^-2. Peeling, bending, and Young's modulus tests verify the enhanced mechanical flexibility and robustness of the multilayer assembly. The μECoG electrodes enable electrical recordings with high signal-to-noise ratios (SNRs) (∼35-36 dB) under different color photostimulations, suggesting that the electrodes are resilient to photon-induced artifacts. In vivo animal experiments confirm that our array can successfully record light-evoked ECoG oscillations from the primary visual cortex (V1) of an anesthetized rat.

Conformal Coating of Orthopedic Plates with X-ray Scintillators and pH Indicators for X-ray Excited Luminescence Chemical Imaging through Tissue

We describe a pH-indicating material that can be directly implanted or coated on orthopedic implant surfaces to provide high-spatial-resolution pH mapping through tissue by X-ray excited luminescence chemical imaging (XELCI). This is especially useful for detecting local pH changes during treatment of implant-associated infections. The material has two layers: an X-ray scintillator layer with Gd₂O₂S:Eu in epoxy, which emits 620 and 700 nm light when irradiated with X-rays, and a pH indicator dye layer, which absorbs some of the 620 nm light in a pH-dependent fashion. To acquire each pixel in the image, a focused X-ray beam irradiates a small region of scintillators and the ratio of 620 to 700 nm light is acquired through the tissue. Scanning the X-ray beam across the implant surface generates high-spatial-resolution chemical measurements. Two associated challenges are (1) to make robust sensors that can be implanted in tissue to measure local chemical concentrations specifically for metal orthopedic implants and (2) to conformally coat the implant surface with scintillators and pH indicator dyes in order to make measurements over a large area. Previously, we have physically pressed or glued a pH-sensitive hydrogel sensor onto the surface of an implant, but this is impractical for imaging over large irregular device areas such as an orthopedic plate with holes and edges. Herein, we describe a chemically sensitive and biocompatible XELCI sensor material that can conformally coat the implant surface. A two-part commercial-grade epoxy resin was mixed with Gd₂O₂S:Eu and adhered to the titanium surface. Sugar and salt particles were added to the surface of the epoxy as it cured to create a roughened surface and increase the surface area. On this roughened surface, a secondary layer of diacrylated polyethylene glycol (PEG) hydrogel, containing a pH sensitive dye, was polymerized. This combination of epoxy-PEG layers was found to adhere well to the metal implant unlike other previously tested polymer surfaces, which delaminated when exposed to water or humidity. The focused X-ray beam enabled 0.5 mm spatial resolution through 1 cm-thick tissue. The pH sensor-coated orthopedic plate was imaged with XELCI, through tissue, with different pH levels to acquire a calibration curve. The plates were also imaged through tissue, with a low pH region on one section due to growth of a Staphylococcus aureus biofilm. A pH sensor-coated stainless-steel rod with two distinct pH regions was inserted in a rabbit tibia specimen, and the pH was imaged through both bone and soft tissue. These studies demonstrate the use of pH sensor-coated orthopedic plates and rods for mapping the local pH through tissue during biofilm formation by XELCI.

Cell adhesion and proliferation on common 3D printing materials used in stereolithography of microfluidic devices

Three-dimensional (3D) printing has recently emerged as a cost-effective alternative for rapid prototyping of microfluidic devices. The feature resolution of stereolithography-based 3D printing is particularly well suited for manufacturing of continuous flow cell culture platforms. Poor cell adhesion or material-induced cell death may, however, limit the introduction of new materials to microfluidic cell culture. In this work, we characterized four commercially available materials commonly used in stereolithography-based 3D printing with respect to long-term (2 month) cell survival on native 3D printed surfaces. Cell proliferation rates, along with material-induced effects on apoptosis and cell survival, were examined in mouse embryonic fibroblasts. Additionally, the feasibility of Dental SG (material with the most favored properties) for culturing of human hepatocytes and human-induced pluripotent stem cells was evaluated. The strength of cell adhesion to Dental SG was further examined over a shear force gradient of 1-89 dyne per cm2 by using a custom-designed microfluidic shear force assay incorporating a 3D printed, tilted and tapered microchannel sealed with a polydimethylsiloxane lid. According to our results, autoclavation of the devices prior to cell seeding played the most important role in facilitating long-term cell survival on the native 3D printed surfaces with the shear force threshold in the range of 3-8 dyne per cm2.

Thiol-Ene Based Polymers as Versatile Materials for Microfluidic Devices for Life Sciences Applications

While there is a steady growth in the number of microfluidics applications, the search for an optimal material that delivers the diverse characteristics needed for the numerous tasks is still nowhere close to being settled. Often overlooked and still underrepresented, the thiol-ene family of polymer materials has an enormous potential for applications in organs-on-a-chip, droplet productions, microanalytics, and point of care testing. In this review, the main characteristics of the thiol-ene materials are given, and advantages and drawbacks with respect to their potential in microfluidic chip fabrication are critically assessed. Select applications, which exploit the versatility of the thiol-ene polymers, are presented and discussed. It is concluded that, in particular, the rapid prototyping possibility combined with the material's resulting mechanical strength, solvent resistance, and biocompatibility, as well as the inherently easy surface functionalization, are strong factors to make thiol-ene polymers strong contenders for promising future materials for many biological, clinical, and technical lab-on-a-chip applications.

Control of neural probe shank flexibility by fluidic pressure in embedded microchannel using PDMS/PI hybrid substrate

Implantable neural probes are widely used to record and stimulate neural activities. These probes should be stiff enough for insertion. However, it should also be flexible to minimize tissue damage after insertion. Therefore, having dynamic control of the neural probe shank flexibility will be useful. For the first time, we have successfully fabricated flexible neural probes with embedded microfluidic channels for dynamic control of neural probe stiffness by controlling fluidic pressure in the channels. The present hybrid neural probes consisted of polydimethylsiloxane (PDMS) and polyimide (PI) layers could provide the required stiffness for insertion and flexibility during operation. The PDMS channels were fabricated by reversal imprint using a silicon mold and bonded to a PI layer to form the embedded channels in the neural probe. The probe shape was patterned using an oxygen plasma generated by an inductively coupled plasma etching system. The critical buckling force of PDMS/PI neural probes could be tuned from 0.25-1.25 mN depending on the applied fluidic pressure in the microchannels and these probes were successfully inserted into a 0.6% agarose gel that mimicked the stiffness of the brain tissue. Polymer-based neural probes are typically more flexible than conventional metal wire-based probes, and they could potentially provide less tissue damage after implantation.

E-Beam Nanostructuring and Direct Click Biofunctionalization of Thiol-Ene Resist

Electron beam lithography (EBL) is of major importance for ultraminiaturized biohybrid system fabrication, as it allows combining biomolecular patterning and mechanical structure definition on the nanoscale. Existing methods are limited by multistep biomolecule immobilization procedures, harsh processing conditions that are harmful to sensitive biomolecules, or the structural properties of the resulting protein monolayers or hydrogel-based resists. This work introduces a thiol-ene EBL resist with chemically reactive thiol groups on its native surface that allow the direct and selective "click" immobilization of biomolecules under benign processing conditions. We constructed EBL structured features of size down to 20 nm, and direct functionalized the nanostructures with a sandwich of biotin and streptavidin. The facile combination of polymer nanostructuring with biomolecule immobilization enables mechanically robust biohybrid components of interest for nanoscale biomedical, electronic, photonic, and robotic applications.

Multichannel optrodes for photonic stimulation

An emerging method in the field of neural stimulation is the use of photons to activate neurons. The possible advantage of optical stimulation over electrical is attributable to its spatially selective activation of small neuron populations, which is promising in generating superior spatial resolution in neural interfaces. Two principal methods are explored for cochlear prostheses: direct stimulation of nerves with infrared light and optogenetics. This paper discusses basic requirements for developing a light delivery system (LDS) for the cochlea and provides examples for building such devices. The proposed device relies on small optical sources, which are assembled in an array to be inserted into the cochlea. The mechanical properties, the biocompatibility, and the efficacy of optrodes have been tested in animal models. The force required to insert optrodes into a model of the human scala tympani was comparable to insertion forces obtained for contemporary cochlear implant electrodes. Side-emitting diodes are powerful enough to evoke auditory responses in guinea pigs. Chronic implantation of the LDS did not elevate auditory brainstem responses over 26 weeks.

Amine functional magnetic nanoparticles via waterborne thiol-ene suspension photopolymerization for antibody immobilization

The modification of magnetic nanoparticles (MNPs) via different routes for biomolecule binding is an attractive area of research. Waterborne thiol-ene suspension photopolymerization (TESP) can be a useful method for preparing functional MNPs. In this study, for the very first time waterborne TESP was performed in the presence of MNPs. Neat MNPs were coated and in situ functionalized with amine groups by using thiol-ene chemistry. Engrailed-2 (EN2) protein, a potential biomarker for various cancers such as prostate cancer, bladder cancer, breast cancer and ovarian cancer, is known to be a strong binder to a specific DNA sequence (50-TAATTA-30) to regulate transcription. Anti-EN2 antibodies were immobilized onto these MNPs by physical adsorption and covalent bonding methods, respectively. The amount of the physically immobilized antibodies (0.54 mg/g) were found to be lower than the loading of the covalently bonded antibodies (1.775 mg/g). The biomarker level in the artificial solutions prepared was determined by enzyme-linked immunosorbent assay. Coated MNPs were characterized by FTIR, TGA, SEM and STEM. After TESP, the average diameter of the neat magnetite nanoparticles increased from ∼15 nm to ∼32 nm.

Histological evaluation of flexible neural implants; flexibility limit for reducing the tissue response?

OBJECTIVE

Flexible neural probes are hypothesized to reduce the chronic foreign body response (FBR) mainly by reducing the strain-stress caused by an interplay between the tethered probe and the brain's micromotion. However, a large discrepancy of Young's modulus still exists (3-6 orders of magnitude) between the flexible probes and the brain tissue. This raises the question of whether we need to bridge this gap; would increasing the probe flexibility proportionally reduce the FBR?

APPROACH

Using novel off-stoichiometry thiol-enes-epoxy (OSTE+) polymer probes developed in our previous work, we quantitatively evaluated the FBR to four types of probes with different softness: silicon (~150 GPa), polyimide (1.5 GPa), OSTE+_Hard (300 MPa), and OSTE+_Soft (6 MPa).

MAIN RESULTS

We observed a significant reduction in the fluorescence intensity of biomarkers for activated microglia/macrophages and blood-brain barrier (BBB) leakiness around the three soft polymer probes compared to the silicon probe, both at 4 weeks and 8 weeks post-implantation. However, we did not observe any consistent differences in the biomarkers among the polymer probes.

SIGNIFICANCE

The results suggest that the mechanical compliance of neural probes can mediate the degree of FBR, but its impact diminishes after a hypothetical threshold level. This infers that resolving the mechanical mismatch alone has a limited effect on improving the lifetime of neural implants.

Research trends in biomimetic medical materials for tissue engineering: 3D bioprinting, surface modification, nano/micro-technology and clinical aspects in tissue engineering of cartilage and bone

This review discusses about biomimetic medical materials for tissue engineering of bone and cartilage, after previous scientific commentary of the invitation-based, Korea-China joint symposium on biomimetic medical materials, which was held in Seoul, Korea, from October 22 to 26, 2015. The contents of this review were evolved from the presentations of that symposium. Four topics of biomimetic medical materials were discussed from different research groups here: 1) 3D bioprinting medical materials, 2) nano/micro-technology, 3) surface modification of biomaterials for their interactions with cells and 4) clinical aspects of biomaterials for cartilage focusing on cells, scaffolds and cytokines.