Surface Area and Local Curvature: Why Roughness Improves the Bioactivity of Neural Implants

Improving Sensitivity and Longevity of In Vivo Glutamate Sensors with Electrodeposited NanoPt

In vivo glutamate sensing has provided valuable insight into the physiology and pathology of the brain. Electrochemical glutamate biosensors, constructed by cross-linking glutamate oxidase onto an electrode and oxidizing H2O2 as a proxy for glutamate, are the gold standard for in vivo glutamate measurements for many applications. While glutamate sensors have been employed ubiquitously for acute measurements, there are almost no reports of long-term, chronic glutamate sensing in vivo, despite demonstrations of glutamate sensors lasting for weeks in vitro. To address this, we utilized a platinum electrode with nanometer-scale roughness (nanoPt) to improve the glutamate sensors' sensitivity and longevity. NanoPt improved the GLU sensitivity by 67.4% and the sensors were stable in vitro for 3 weeks. In vivo, nanoPt glutamate sensors had a measurable signal above a control electrode on the same array for 7 days. We demonstrate the utility of the nanoPt sensors by studying the effect of traumatic brain injury on glutamate in the rat striatum with a flexible electrode array and report measurements of glutamate taken during the injury itself. We also show the flexibility of the nanoPt platform to be applied to other oxidase enzyme-based biosensors by measuring γ-aminobutyric acid in the porcine spinal cord. NanoPt is a simple, effective way to build high sensitivity, robust biosensors harnessing enzymes to detect neurotransmitters in vivo.

Improving the Biocompatibility and Functionality of Neural Interface Devices with Silica Nanoparticles

ConspectusNeural interface technologies enable bidirectional communication between the nervous system and external instrumentation. Advancements in neural interface devices not only open new frontiers for neuroscience research, but also hold great promise for clinical diagnosis, therapy, and rehabilitation for various neurological disorders. However, the performance of current neural electrode devices, often termed neural probes, is far from satisfactory. Glial scarring, neuronal degeneration, and electrode degradation eventually cause the devices to lose their connection with the brain. To improve the chronic performance of neural probes, efforts need to be made on two fronts: enhancing the physiochemical properties of the electrode materials and mitigating the undesired host tissue response.In this Account, we discuss our efforts in developing silica-nanoparticle-based (SiNP) coatings aimed at enhancing neural probe electrochemical properties and promoting device-tissue integration. Our work focuses on three approaches:(1) SiNPs' surface texturization to enhance biomimetic protein coatings for promoting neural integration. Through covalent immobilization, SiNP introduces biologically relevant nanotopography to neural probe surfaces, enhancing neuronal cell attachments and inhibiting microglia. The SiNP base coating further increases the binding density and stability of bioactive molecules such as L1CAM and facilitates the widespread dissemination of biomimetic coatings. (2) Doping SiNPs into conductive polymer electrode coatings improves the electrochemical properties and stability. As neural interface devices are moving to subcellular sizes to escape the immune response and high electrode site density to increase spatial resolution, the electrode sites need to be very small. The smaller electrode size comes at the cost of a high electrode impedance, elevated thermal noise, and insufficient charge injection capacity. Electrochemically deposited conductive polymer films reduce electrode impedance but do not endure prolonged electrical cycling. When incorporated into conductive polymer coatings as a dopant, the SiNP provides structural support for the polymer thin films, significantly increasing their stability and durability. Low interfacial impedance maintained by the conducting polymer/SiNP composite is critical for extended electrode longevity and effective charge injection in chronic neural stimulation applications. (3) Porous nanoparticles are used as drug carriers in conductive polymer coatings for local drug/neurochemical delivery. When triggered by external electrical stimuli, drug molecules and neurochemicals can be released in a controlled manner. Such precise focal manipulation of cellular and vascular behavior enables us to probe brain circuitry and develop therapeutic applications.We foresee tremendous opportunities for further advancing the functionality of SiNP coatings by incorporating new nanoscale components and integrating the coating with other design strategies. With an enriched nanoscale toolbox and optimized design strategies, we can create customizable multifunctional and multimodal neural interfaces that can operate at multiple spatial levels and seamlessly integrate with the host tissue for extended applications.

Single-cell fluid-based force spectroscopy reveals near lipid size nano-topography effects on neural cell adhesion

Nano-roughness has shown great potential in enhancing high-fidelity electrogenic cell interfaces, owing to its characteristic topography comparable to proteins and lipids, which influences a wide range of cellular mechanical responses. Gaining a comprehensive understanding of how cells respond to nano-roughness at the single-cell level is not only imperative for implanted devices but also essential for tissue regeneration and interaction with complex biomaterial surfaces. In this study, we quantify cell adhesion and biomechanics of single cells to nano-roughened surfaces by measuring neural cell adhesion and biomechanics via fluidic-based single-cell force spectroscopy (SCFS). For this, we introduce nanoscale topographical features on polyimide (PI) surfaces achieving roughness up to 25 nm without chemical modifications. Initial adhesion experiments show cell-specific response to nano-roughness for neuroblastoma cells (SH-SY5Y) compared to human astrocytes (NHA) around 15 and 20 nm surface roughness. In addition, our SCFS measurements revealed a remarkable 2.5-fold increase in adhesion forces (150-164 nN) for SH-SY5Y cells cultured on roughened PI (rPI) surfaces compared to smooth surfaces (60-107 nN). Our data also shows that cells can distinguish changes in nano-roughness as small 2 nm (close to the diameter of a single lipid) and show roughness dependence adhesion while favoring 15 nm. Notably, this enhanced adhesion is accompanied by increased cell elongation upon cell detachment without any significant differences in cell area spreading. The study provides valuable insights into the interplay between nano-topography and cellular responses and offers practical implications for designing biomaterial surfaces with enhanced cellular interactions.

Separating Geometric and Diffusive Contributions to the Surface Nucleation of Dislocations in Nanoparticles

While metal nanoparticles are widely used, their small size makes them mechanically unstable. Extensive prior research has demonstrated that nanoparticles with sizes in the range of 10-50 nm fail by the surface nucleation of dislocations, which is a thermally activated process. Two different contributions have been suggested to cause the weakening of smaller particles: first, geometric effects such as increased surface curvature reduce the barrier for dislocation nucleation; second, surface diffusion happens faster on smaller particles, thus accelerating the formation of surface kinks which nucleate dislocations. These two factors are difficult to disentangle. Here we use in situ compression testing inside a transmission electron microscope to measure the strength and deformation behavior of platinum particles in three groups: 12 nm bare particles, 16 nm bare particles, and 12 nm silica-coated particles. Thermodynamics calculations show that, if surface diffusion were the dominant factor, the last two groups would show equal strengthening. Our experimental results refute this, instead demonstrating a 100% increase in mean yield strength with increased particle size and no statistically significant increase in strength due to the addition of a coating. A separate analysis of stable plastic flow corroborates the findings, showing an order-of-magnitude increase in the rate of dislocation nucleation with a change in particle size and no change with coating. Taken together, these results demonstrate that surface diffusion plays a far smaller role in the failure of nanoparticles by dislocations as compared to geometric factors that reduce the energy barrier for dislocation nucleation.

Nanotopography-enhanced biomimetic coating maintains bioactivity after weeks of dry storage and improves chronic neural recording

We developed a nanoparticle base layer technology capable of maintaining the bioactivity of protein-based neural probe coating intended to improve neural recording quality. When covalently bound on thiolated nanoparticle (TNP) modified surfaces, neural adhesion molecule L1 maintained bioactivity throughout 8 weeks of dry storage at room temperature, while those bound to unmodified surfaces lost 66% bioactivity within 3 days. We tested the TNP + L1 coating in mouse brains on two different neural electrode arrays after two different dry storage durations (3 and 28 days). The results show that dry-stored coating is as good as the freshly prepared, and even after 28 days of storage, the number of single units per channel and signal-to-noise ratio of the TNP + L1 coated arrays were significantly higher by 32% and 40% respectively than uncoated controls over 16 weeks. This nanoparticle base layer approach enables the dissemination of biomolecule-functionalized neural probes to users worldwide and may also benefit a broad range of applications that rely on surface-bound biomolecules.

Metal-Phenolic Networks Assembled on TiO2 Nanospikes for Antimicrobial Peptide Deposition and Osteoconductivity Enhancement in Orthopedic Applications

The lack of antimicrobial and osteoconductive activities of titanium (Ti) for orthopedic implants has led to problems such as infection and structural looseness, which bring physical and psychological sufferings to patients as well as economic burden on the healthcare system. To endow Ti implants with anti-infective function and bioactivity, in this study, we successfully constructed TiO2 nanospike (TNS) structure on the surface of Ti followed by assembling metal-polyphenol networks (MPNs) and depositing antimicrobial peptides (AMPs). The TNSs' structure can disrupt the bacteria by physical puncture, and it was also proved to have excellent photothermal conversion performance upon near-infrared light irradiation. Furthermore, with the assistance of contact-active chemo bactericidal efficacy of AMPs, TNS-MPN-AMP nanocoating achieved physical/photothermal/chemo triple-synergistic therapy against pathogenic bacteria. The anti-infective efficiency of this multimodal treatment was obviously improved, with an antibacterial ratio of >99.99% in vitro and 95.03% in vivo. Moreover, the spike-like nanostructure of TNSs and the bioactive groups from MPNs and AMPs not only demonstrated desirable biocompatibility but also promoted the surface hydroxyapatite formation in simulated body fluid for further osseointegration enhancement. Altogether, this multifaceted TNS-MPN-AMP nanocoating endowed Ti implants with enhanced antibacterial activity, excellent cytocompatibility, and desirable osteoconductive ability.