Local polymer replacement for neuron patterning and in situ neurite guidance

The Roles of Microprobe in Localized Electrodeposition: Electrolyte Localized Transport and Force-Displacement Sensitivity

Facing the rapid development of 6G communication, long-wave infrared metasurface and biomimetic microfluidics, the performance requirements for microsystems based on metal tiny structures are gradually increasing. As one of powerful methods for fabrication metal complex microstructures, localized electrochemical deposition microadditive manufacturing technology can fabricate copper metal micro overhanging structures without masks and supporting materials. In this study, the role of the microprobe cantilever (MC) in localized electrodeposition was studied. The MC can be used for precise deposition with electrolyte localized transport function and high accuracy force-displacement sensitivity. To prove this, the electrolyte flow was simulated when the MC was in bending or normal state. The simulation results can indicate the influence of turbulent flow on the electrolyte flow velocity and the pressure at the end of the pyramid. The results show that the internal flow velocity increased by 8.9% in the bending probe as compared with normal. Besides, this study analyzed the force-potential sensitivity characteristics of the MC. Using the deformation of the MC as an intermediate variable, the model of the probe tip displacement caused by the growth of the deposit and the voltage value displayed by the photodetector was mathematically established. In addition, the deposition of a single voxel was simulated by simulation process with the simulated height of 520 nm for one voxel, and the coincidence of simulation and experimental results was 93.1%. In conclusion, this method provides a new way for localized electrodeposition of complex microstructures.

Nonspecific Binding-Fundamental Concepts and Consequences for Biosensing Applications

Nature achieves differentiation of specific and nonspecific binding in molecular interactions through precise control of biomolecules in space and time. Artificial systems such as biosensors that rely on distinguishing specific molecular binding events in a sea of nonspecific interactions have struggled to overcome this issue. Despite the numerous technological advancements in biosensor technologies, nonspecific binding has remained a critical bottleneck due to the lack of a fundamental understanding of the phenomenon. To date, the identity, cause, and influence of nonspecific binding remain topics of debate within the scientific community. In this review, we discuss the evolution of the concept of nonspecific binding over the past five decades based upon the thermodynamic, intermolecular, and structural perspectives to provide classification frameworks for biomolecular interactions. Further, we introduce various theoretical models that predict the expected behavior of biosensors in physiologically relevant environments to calculate the theoretical detection limit and to optimize sensor performance. We conclude by discussing existing practical approaches to tackle the nonspecific binding challenge in vitro for biosensing platforms and how we can both address and harness nonspecific interactions for in vivo systems.

Collective dynamics of neuronal activities in various modular networks

Modularity is a key feature of structural and functional brain networks. However, the association between the structure and function of modular brain networks has not been revealed. We constructed three types of modular cortical networks in vitro and investigated their neuronal activities. The modular networks comprising 4, 3, or 2 modules were constructed using polydimethylsiloxane (PDMS) microstructures fabricated directly on a multi-electrode array (MEA) without transfer. The 4-module network had the strongest modular connectivity, followed by the 3-module and 2-module networks. To investigate how neuronal activities were affected by the modular network structure, spontaneous neuronal activities were recorded on different days in vitro and analyzed based on spike amplitudes, network bursts, and the propagation properties of individual spikes. Different characteristics were observed depending on the network topology and modular connectivity. Moreover, when an electrode was stimulated by biphasic voltage pulses, bursts were elicited for the 4-module network, whereas spikes were elicited for the 3-module and 2-module networks. Direct fabrication of the PDMS microstructures on the MEA without transfer allows microscale construction of modular networks and high-density functional recording; therefore, the technique utilizing the PDMS microstructures can be applied to the systematic study of the dynamics of modular neuronal networks in vitro.

Dissociation Constant of Integrin-RGD Binding in Live Cells from Automated Micropipette and Label-Free Optical Data

The binding of integrin proteins to peptide sequences such as arginine-glycine-aspartic acid (RGD) is a crucial step in the adhesion process of mammalian cells. While these bonds can be examined between purified proteins and their ligands, live-cell assays are better suited to gain biologically relevant information. Here we apply a computer-controlled micropipette (CCMP) to measure the dissociation constant (K_d) of integrin-RGD-binding. Surface coatings with varying RGD densities were prepared, and the detachment of single cells from these surfaces was measured by applying a local flow inducing hydrodynamic lifting force on the targeted cells in discrete steps. The average behavior of the populations was then fit according to the chemical law of mass action. To verify the resulting value of K_d^2d = (4503 ± 1673) 1/µm², a resonant waveguide grating based biosensor was used, characterizing and fitting the adhesion kinetics of the cell populations. Both methods yielded a K_d within the same range. Furthermore, an analysis of subpopulations was presented, confirming the ability of CCMP to characterize cell adhesion both on single cell and whole population levels. The introduced methodologies offer convenient and automated routes to quantify the adhesivity of living cells before their further processing.

Modeling and Experimental Study of the Localized Electrochemical Micro Additive Manufacturing Technology Based on the FluidFM

In this work, the localized electrochemical micro additive manufacturing technology based on the FluidFM (fluidic force microscope) has been introduced to fabricate micro three-dimensional overhang metal structures at sub-micron resolution. It breaks through the localized deposition previously achieved by micro-anode precision movement, and the micro-injection of the electrolyte is achieved in a stable electric field distribution. The structure of electrochemical facilities has been designed and optimized. More importantly, the local electrochemical deposition process has been analyzed with positive source diffusion, and the mathematical modeling has been revealed in the particle conversion process. A mathematical model is proposed for the species flux under the action of pulsed pressure in an innovatively localized liquid feeding process. Besides, the linear structure, bulk structure, complex structure, and large-area structure of the additive manufacturing are analyzed separately. The experimental diameter of the deposited cylinder structure is linearly fitted. The aspect ratio of the structure is greater than 20, the surface roughness value is between 0.1-0.2 μm at the surface of bulk structures, and the abilities are verified for deposition of overhang, hollow complex structures. Moreover, this work verifies the feasibility of 3D overhang array submicron structure additive manufacturing, with the application of pulsed pressure. Furthermore, this technology opens new avenues for the direct fabrication of nano circuit interconnection, tiny sensors, and micro antennas.

Growth and Spatial Control of Murine Neural Stem Cells on Reflectin Films

Stem cells have attracted significant attention due to their regenerative capabilities and their potential for the treatment of disease. Consequently, significant research effort has focused on the development of protein- and polypeptide-based materials as stem cell substrates and scaffolds. Here, we explore the ability of reflectin, a cephalopod structural protein, to support the growth of murine neural stem/progenitor cells (mNSPCs). We observe that the binding, growth, and differentiation of mNSPCs on reflectin films is comparable to that on more established protein-based materials. Moreover, we find that heparin selectively inhibits the adhesion of mNSPCs on reflectin, affording spatial control of cell growth and leading to a >30-fold change in cell density on patterned substrates. The described findings highlight the potential utility of reflectin as a stem cell culture material.

Asymmetric confinement for defining outgrowth directionality

Directional connectivity is required to develop accurate in vitro models of the nervous system. This research investigated the interaction of murine neuronal outgrowths with asymmetric microstructured geometries to provide insights into the mechanisms governing unidirectional outgrowth bias. The structures were designed using edge-guidance and critical turning angle principles to study different prohibitive to permissive edge-guidance ratios. The different structures enable outgrowth in the permissive direction, while reducing outgrowth in the prohibitive direction. Outgrowth bias was probabilistic in nature, requiring multiple structures for effective unidirectional bias in primary hippocampal cultures at 14 days in vitro. Arrowhead structures with acute posterior corners were optimal, enabling 100% unidirectional outgrowth bias by virtue of re-routing and delay effects.

A Versatile Protein and Cell Patterning Method Suitable for Long-Term Neural Cultures

Herein, we present an easy-to-use protein and cell patterning method relying solely on pipetting, rinsing steps and illumination with a desktop lamp, which does not require any expensive laboratory equipment, custom-built hardware or delicate chemistry. This method is based on the adhesion promoter poly(allylamine)-grafted perfluorophenyl azide, which allows UV-induced cross-linking with proteins and the antifouling molecule poly(vinylpyrrolidone). Versatility is demonstrated by creating patterns with two different proteins and a polysaccharide directly on plastic well plates and on glass slides, and by subsequently seeding primary neurons and C2C12 myoblasts on the patterns to form islands and mini-networks. Patterning characterization is done via immunohistochemistry, Congo red staining, ellipsometry, and infrared spectroscopy. Using a pragmatic setup, patterning contrasts down to 5 μm and statistically significant long-term stability superior to the gold standard poly(l-lysine)-grafted poly(ethylene glycol) could be obtained. This simple method can be used in any laboratory or even in classrooms and its outstanding stability is especially interesting for long-term cell experiments, e.g., for bottom-up neuroscience, where well-defined microislands and microcircuits of primary neurons are studied over weeks.

Three-dimensional direct laser writing of biomimetic neuron structures

Building biomimetic neuron structures that emulate the topological features of biological neural networks at multiple scales has been an active area in neuron cell culturing, neuron-chip interface and computer chip design. However, due to the fact that biological neural networks possess extraordinary connectivity and complexity from millimeter down to nanometer scale, with different dendritic branch angles, branch lengths, and branch diameters, previous methods to reproduce the topological features of biological neural networks are either limited to two dimensions or lack of fabrication resolution in building three-dimensional (3D) structures. Here we report on the generation of 3D biomimetic neuron structures at a micrometer scale, with high mechanical stability and controlled topologies by studying the effect of 3D direct laser writing (DLW) on the capillary force. This work provides an optical technology platform to replicate the topological features of biological neural networks and paves the avenue towards more applications of using 3D direct laser writing in engineered neural networks.

Unidirectional signal propagation in primary neurons micropatterned at a single-cell resolution

The structure and connectivity of cultured neuronal networks can be controlled by using micropatterned surfaces. Here, we demonstrate that the direction of signal propagation can be precisely controlled at a single-cell resolution by growing primary neurons on micropatterns. To achieve this, we first examined the process by which axons develop and how synapses form in micropatterned primary neurons using immunocytochemistry. By aligning asymmetric micropatterns with a marginal gap, it was possible to pattern primary neurons with a directed polarization axis at the single-cell level. We then examined how synapses develop on micropatterned hippocampal neurons. Three types of micropatterns with different numbers of short paths for dendrite growth were compared. A normal development in synapse density was observed when micropatterns with three or more short paths were used. Finally, we performed double patch clamp recordings on micropatterned neurons to confirm that these synapses are indeed functional, and that the neuronal signal is transmitted unidirectionally in the intended orientation. This work provides a practical guideline for patterning single neurons to design functional neuronal networks in vitro with the direction of signal propagation being controlled.

Controlled single-cell deposition and patterning by highly flexible hollow cantilevers

Single-cell patterning represents a key approach to decouple and better understand the role and mechanisms of individual cells of a given population. In particular, the bottom-up approach of engineering neuronal circuits with a controlled topology holds immense promises to perceive the relationships between connectivity and function. In order to accommodate these efforts, highly flexible SU-8 cantilevers with integrated microchannels have been fabricated for both additive and subtractive patterning. By directly squeezing out single cells onto adhesive surfaces, controlled deposition with a spatial accuracy of 5 μm could be achieved, while subtractive patterning has been realized by selective removal of targeted single cells. Complex cell patterns were created on substrates pre-patterned with cell-adhesive and repulsive areas, preserving the original pattern geometry for long-term studies. For example, a circular loop with a diameter of 530 μm has been realized using primary hippocampal neurons, which were fully connected to their respective neighbors along the loop. Using the same cantilevers, the versatility of the technique has also been demonstrated via in situ modification of already mature neuronal cultures by both detaching individual cells of the population and adding fresh ones, incorporating them into the culture.

Control over Neurite Directionality and Neurite Elongation on Anisotropic Micropillar Arrays

Control over neurite orientation in primary hippocampal neurons is achieved by using interrupted, anisotropic micropillar arrays as a cell culture platform. Both neurite orientation and neurite length are controlled by a function of interpillar distance.

Synthesis of Cell-Adhesive Anisotropic Multifunctional Particles by Stop Flow Lithography and Streptavidin-Biotin Interactions

Cell-adhesive particles are of significant interest in biotechnology, the bioengineering of complex tissues, and biomedical research. Their applications range from platforms to increase the efficiency of anchorage-dependent cell culture to building blocks to loading cells in heterogeneous structures to clonal-population growth monitoring to cell sorting. Although useful, currently available cell-adhesive particles can accommodate only homogeneous cell culture. Here, we report the design of anisotropic hydrogel microparticles with tunable cell-adhesive regions as first step toward micropatterned cell cultures on particles. We employed stop flow lithography (SFL), the coupling reaction between amine and N-hydroxysuccinimide (NHS) and streptavidin-biotin chemistry to adjust the localization of conjugated collagen and poly-L-lysine on the surface of microscale particles. Using the new particles, we demonstrate the attachment and formation of tight junctions between brain endothelial cells. We also demonstrate the geometric patterning of breast cancer cells on particles with heterogeneous collagen coatings. This new approach avoids the exposure of cells to potentially toxic photoinitiators and ultraviolet light and decouples in time the microparticle synthesis and the cell culture steps to take advantage of the most recent advances in cell patterning available for traditional culture substrates.

A Multifunctional Frontloading Approach for Repeated Recycling of a Pressure-Controlled AFM Micropipette

Fluid force microscopy combines the positional accuracy and force sensitivity of an atomic force microscope (AFM) with nanofluidics via a microchanneled cantilever. However, adequate loading and cleaning procedures for such AFM micropipettes are required for various application situations. Here, a new frontloading procedure is described for an AFM micropipette functioning as a force- and pressure-controlled microscale liquid dispenser. This frontloading procedure seems especially attractive when using target substances featuring high costs or low available amounts. Here, the AFM micropipette could be filled from the tip side with liquid from a previously applied droplet with a volume of only a few μL using a short low-pressure pulse. The liquid-loaded AFM micropipettes could be then applied for experiments in air or liquid environments. AFM micropipette frontloading was evaluated with the well-known organic fluorescent dye rhodamine 6G and the AlexaFluor647-labeled antibody goat anti-rat IgG as an example of a larger biological compound. After micropipette usage, specific cleaning procedures were tested. Furthermore, a storage method is described, at which the AFM micropipettes could be stored for a few hours up to several days without drying out or clogging of the microchannel. In summary, the rapid, versatile and cost-efficient frontloading and cleaning procedure for the repeated usage of a single AFM micropipette is beneficial for various application situations from specific surface modifications through to local manipulation of living cells, and provides a simplified and faster handling for already known experiments with fluid force microscopy.

Quantifying bacterial adhesion on antifouling polymer brushes via single-cell force spectroscopy

The adhesion forces between a single bacterial cell and different polymer brushes were measured directly with an atomic force microscope and correlated with their resistance to fouling.

Local surface modification via confined electrochemical deposition with FluidFM

Hollow AFM cantilevers enable local electroplating and grafting followed by the in situ imaging of the created surface patterns.