Ordered Silicon Pillar Arrays Prepared by Electrochemical Micromachining: Substrates for High-Efficiency Cell Transfection

The Roles of Micro- and Nanoscale Materials in Cell-Engineering Systems

Customizable manufacturing of ex vivo cell engineering is driven by the need for innovations in the biomedical field and holds substantial potential for addressing current therapeutic challenges; but it is still only in its infancy. Micro- and nanoscale-engineered materials are increasingly used to control core cell-level functions in cellular engineering. By reprogramming or redirecting targeted cells for extremely precise functions, these advanced materials offer new possibilities. This influences the modularity of cell reprogramming and reengineering, making these materials part of versatile and emerging technologies. Here, the roles of micro- and nanoscale materials in cell engineering are highlighted, demonstrating how they can be adaptively controlled to regulate cellular reprogramming and core cell-level functions, including differentiation, proliferation, adhesion, user-defined gene expression, and epigenetic changes. The current reprogramming routes used to achieve pluripotency from somatic cells and the significant potential of induced pluripotent stem cell technology for translational biomedical research are covered. Recent advances in nonviral intracellular delivery modalities for cell reprogramming and their constraints are evaluated. This paper focuses on emerging physical and combinatorial approaches of intracellular delivery for cell engineering, revealing the capabilities and limitations of these routes. It is showcased how these programmable materials are continually being explored as customizable tools for inducing biophysical stimulation. Harnessing the power of micro- and nanoscale-engineered materials will be a step change in the design of cell engineering, producing a suite of powerful tools for addressing potential future challenges in therapeutic cell engineering.

Voltage- and Metal-assisted Chemical Etching of Micro and Nano Structures in Silicon: A Comprehensive Review

Sculpting silicon at the micro and nano scales has been game-changing to mold bulk silicon properties and expand, in turn, applications of silicon beyond electronics, namely, in photonics, sensing, medicine, and mechanics, to cite a few. Voltage- and metal-assisted chemical etching (ECE and MaCE, respectively) of silicon in acidic electrolytes have emerged over other micro and nanostructuring technologies thanks to their unique etching features. ECE and MaCE have enabled the fabrication of novel structures and devices not achievable otherwise, complementing those feasible with the deep reactive ion etching (DRIE) technology, the gold standard in silicon machining. Here, a comprehensive review of ECE and MaCE for silicon micro and nano machining is provided. The chemistry and physics ruling the dissolution of silicon are dissected and similarities and differences between ECE and MaCE are discussed showing that they are the two sides of the same coin. The processes governing the anisotropic etching of designed silicon micro and nanostructures are analyzed, and the modulation of etching profile over depth is discussed. The preparation of micro- and nanostructures with tailored optical, mechanical, and thermo(electrical) properties is then addressed, and their applications in photonics, (bio)sensing, (nano)medicine, and micromechanical systems are surveyed. Eventually, ECE and MaCE are benchmarked against DRIE, and future perspectives are highlighted.

Fabricating mesoscale polymer ribbons with tunable mechanical properties via evaporative deposition and dewetting

Synthetic replication of the precise mesoscale control found in natural systems poses substantial experimental challenges due to the need for manipulation across multiple length scales (from nano- to millimeter). We address this challenge by using a 'flow coating' method to fabricate polymer ribbons with precisely tunable dimensions and mechanical properties. Overcoming barriers that previously limited the achievable range of properties with this method, we eliminate the need for substrate patterning and post-processing etching to facilitate the production of high aspect ratio, filament-like ribbons across a range of polymers-from glassy polystyrene to elastomeric poly(butadiene), as well as poly(butadiene-block-styrene). Our method uniquely enables the preservation of chemical fidelity, composition, and dimensions of these ribbons, leveraging polymers with elastic moduli from GPa to tens of MPa to achieve multi-scale features. We demonstrate the role of the elastocapillary length (γ/E) in determining morphological outcomes, revealing the increase in curvature with lower elastic modulus. This finding underscores the intricate relationship among surface tension, elastic modulus, and resultant structural form, enabling control over the morphology of mesoscale ribbons. The soft (MPa) polybutadiene-based ribbons exemplify our method's utility, offering structures with significant extensibility, resilience, and ease of handling, thus expanding the potential for future applications. This work advances our understanding of the fundamental principles governing mesoscale structure formation and unlocks new possibilities for designing soft materials with tailored properties, mirroring the complexity and functionality observed in nature.

Catalytic nickel silicide as an alternative to noble metals in metal-assisted chemical etching of silicon

Metal-assisted chemical etching (MACE) has received much attention from researchers because it can be used to fabricate plasma-free anisotropic etching profiles for semiconductors. However, the etching mechanism of MACE is based on the catalytic reaction of noble metals, which restricts its use in complementary metal oxide semiconductor (CMOS) processes. To obtain process compatibility, we developed catalytic Ni after alloying it with Si as a substitute for noble metals in the MACE of Si substrates. Nickel silicide is a material commonly used as a contact electrode in CMOS processes. When NiSi was used as the catalyst, the anisotropic etching of Si with a smooth surface was successfully demonstrated. Silicidation increased the standard reduction potential of the Ni alloy and enhanced the electrochemical stability in the MACE of Si. In contrast, when pure Ni was used as the catalyst, a rough-etched surface was fabricated because of the low standard reduction potential. Based on the experimental results, the factors affecting the MACE of Si were systematically analyzed to optimize the catalytic NiSi properties. The implementation of the NiSi alloy potentially eliminates the use of noble metals in MACE and allows the technology to be adopted in contemporary CMOS processes.

The influence of dysfunctional actin on polystyrene-nanotube-mediated mRNA nanoinjection into mammalian cells

The advancement of nanofabrication technologies has transformed the landscape of engineered nano-bio interfaces, especially with vertically aligned nanoneedles (NNs). This enables scientists to venture into new territories, widening NN applications into increasingly more complex cellular manipulation and interrogation. Specifically, for intracellular delivery application, NNs have been shown to mediate the delivery of various bioactive cargos into a wide range of cells-a physical method termed "nanoinjection". Silicon (Si) nanostructures demonstrated great potential in nanoinjection, whereas the use of polymeric NNs for nanoinjection has rarely been explored. Furthermore, the underlying mechanism of interaction at the cell-NN interface is subtle and multifaceted, and not fully understood-underpinned by the design versatility of the NN biointerface. Recent studies have suggested that actin dynamic plays a pivotal role influencing the delivery efficacy. In this study, we fabricated a new class of NNs-a programmable polymeric nanotubes (NTs)-from polystyrene (PS) cell cultureware, designed to facilitate mRNA delivery into mouse embryonic fibroblast GPE86 cells. The PSNT delivery platform was able to mediate mRNA delivery with high delivery efficiency (∼83%). We also investigated the role of actin cytoskeleton in PSNTs mediated intracellular delivery by introducing two actin inhibitors-cytochalasin D (Cyto D) and jasplakinolide (Jas)-to cause dysfunctional cytoskeleton, via inhibiting actin polymerization and depolymerization, respectively (before and after the establishment of cell-PSNT interface). By inhibiting actin dynamics 12 h before cell-PSNT interfacing (pre-interface treatment), the mRNA delivery efficiencies were significantly reduced to ∼3% for Cyto D-treated samples and ∼1% for Jas-treated sample, as compared to their post-interface (2 h after cell-PSNT interfacing) counterpart (∼46% and ∼68%, respectively). The added flexibility of PSNTs have shown to help withstand mechanical breakage stemming from cytoskeletal forces in contrast to the SiNTs. Such findings will step-change our capacity to use programmable polymeric NTs in fundamental cellular processes related to intracellular delivery.

Optimal Stopping Rules for Preventing Overloading of Multicomponent Systems

When random-strength components work as an interconnected parallel system, then its carrying capacity is random as well. In a case where such a multicomponent system is a subject of the stepwise-growing workload, some of its components fail and their loads are taken over by the ones that are intact. When the loading process is continued, the additional loads trigger consecutive failures that degrade the system, eventually leading to a complete failure. If the goal of the system is to carry as much load as possible, then the loading process should be continued, but no longer than until the loading capacity of the whole system is reached. On the other hand, with every additional load step, a failure of the system becomes more probable, as the carrying capacity is random and known solely through its probability distribution. In such cases, the decision on when to cease the loading process is not obvious. We introduce and analyse a minimal model of failure spreading in an array of progressively loaded pillars controlled by a decision-maker who stops the process when a required load is attained. We show how to construct an optimal stopping rule. Under some additional assumptions regarding the adopted loss function, it is argued that the optimal stopping rule is of the threshold type and it significantly depends on the shape of the load-step probability distribution.

Role of actin cytoskeleton in cargo delivery mediated by vertically aligned silicon nanotubes

Nanofabrication technologies have been recently applied to the development of engineered nano–bio interfaces for manipulating complex cellular processes. In particular, vertically configurated nanostructures such as nanoneedles (NNs) have been adopted for a variety of biological applications such as mechanotransduction, biosensing, and intracellular delivery. Despite their success in delivering a diverse range of biomolecules into cells, the mechanisms for NN-mediated cargo transport remain to be elucidated. Recent studies have suggested that cytoskeletal elements are involved in generating a tight and functional cell–NN interface that can influence cargo delivery. In this study, by inhibiting actin dynamics using two drugs—cytochalasin D (Cyto D) and jasplakinolide (Jas), we demonstrate that the actin cytoskeleton plays an important role in mRNA delivery mediated by silicon nanotubes (SiNTs). Specifically, actin inhibition 12 h before SiNT-cellular interfacing (pre-interface treatment) significantly dampens mRNA delivery (with efficiencies dropping to 17.2% for Cyto D and 33.1% for Jas) into mouse fibroblast GPE86 cells, compared to that of untreated controls (86.9%). However, actin inhibition initiated 2 h after the establishment of GPE86 cell–SiNT interface (post-interface treatment), has negligible impact on mRNA transfection, maintaining > 80% efficiency for both Cyto D and Jas treatment groups. The results contribute to understanding potential mechanisms involved in NN-mediated intracellular delivery, providing insights into strategic design of cell–nano interfacing under temporal control for improved effectiveness.

Delivering Mechanical Stimulation to Cells: State of the Art in Materials and Devices Design

Biochemical signals, such as growth factors, cytokines, and transcription factors are known to play a crucial role in regulating a variety of cellular activities as well as maintaining the normal function of different tissues and organs. If the biochemical signals are assumed to be one side of the coin, the other side comprises biophysical cues. There is growing evidence showing that biophysical signals, and in particular mechanical cues, also play an important role in different stages of human life ranging from morphogenesis during embryonic development to maturation and maintenance of tissue and organ function throughout life. In order to investigate how mechanical signals influence cell and tissue function, tremendous efforts have been devoted to fabricating various materials and devices for delivering mechanical stimuli to cells and tissues. Here, an overview of the current state of the art in the design and development of such materials and devices is provided, with a focus on their design principles, and challenges and perspectives for future research directions are highlighted.

Shining light in blind alleys: deciphering bacterial attachment in silicon microstructures

With new advances in infectious disease, antifouling surfaces, and environmental microbiology research comes the need to understand and control the accumulation and attachment of bacterial cells on a surface. Thus, we employ intrinsic phase-shift reflectometric interference spectroscopic measurements of silicon diffraction gratings to non-destructively observe the interactions between bacterial cells and abiotic, microstructured surfaces in a label-free and real-time manner. We conclude that the combination of specific material characteristics (i.e., substrate surface charge and topology) and characteristics of the bacterial cells (i.e., motility, cell charge, biofilm formation, and physiology) drive bacteria to adhere to a particular surface, often leading to a biofilm formation. Such knowledge can be exploited to predict antibiotic efficacy and biofilm formation, and enhance surface-based biosensor development, as well as the design of anti-biofouling strategies.

Topography-Mediated Enhancement of Nonviral Gene Delivery in Stem Cells

Gene delivery holds great promise for bioengineering, biomedical applications, biosensors, diagnoses, and gene therapy. In particular, the influence of topography on gene delivery is considered to be an attractive approach due to low toxicity and localized delivery properties. Even though many gene vectors and transfection systems have been developed to enhance transfection potential and combining it with other forms of stimulations could even further enhance it. Topography is an interesting surface property that has been shown to stimulate differentiation, migration, cell morphology, and cell mechanics. Therefore, it is envisioned that topography might also be able to stimulate transfection. In this study, we tested the hypothesis “topography is able to regulate transfection efficiency”, for which we used nano- and microwave-like topographical substrates with wavelengths ranging from 500 nm to 25 µm and assessed the transfectability of human bone marrow-derived mesenchymal stem cells (hBM-MSCs) and myoblasts. For transfection, Lipofectamine 2000 and a gene encoding plasmid for red-fluorescent protein (m-Cherry) were used and topography-induced cell morphology and transfection efficiency was analyzed. As a result, topography directs cell spreading, elongation, and proliferation as well as the transfection efficiency, which were investigated but were found not to be correlated and dependent on the cell type. A 55% percent improvement of transfection efficiency was identified for hBM-MSCs grown on 2 µm wrinkles (24.3%) as compared to hBM-MSCs cultured on flat controls (15.7%). For myoblast cells, the highest gene-expression efficiency (46.1%) was observed on the 10 µm topography, which enhanced the transfection efficiency by 64% as compared to the flat control (28.1%). From a qualitative assessment, it was observed that the uptake capacity of cationic complexes of TAMRA-labeled oligodeoxynucleotides (ODNs) was not topography-dependent but that the intracellular release was faster, as indicated by the positively stained nuclei on 2 μm for hBM-MSCs and 10 μm for myoblasts. The presented results indicate that topography enhances the gene-delivery capacity and that the responses are dependent on cell type. This study demonstrates the important role of topography on cell stimulation for gene delivery as well as understanding the uptake capacity of lipoplexes and may be useful for developing advanced nonviral gene delivery strategies.

Cellular nanointerface of vertical nanostructure arrays and its applications

Vertically standing nanostructures with various morphologies have been developed with the emergence of the micro-/nanofabrication technology. When cells are cultured on them, various bio-nano interfaces between cells and vertical nanostructures would impact the cellular activities, depending on the shape, density, and height of nanostructures. Many cellular pathway activation processes involving a series of intracellular molecules (proteins, RNA, DNA, enzymes, etc.) would be triggered by the cell morphological changes induced by nanostructures, affecting the cell proliferation, apoptosis, differentiation, immune activation, cell adhesion, cell migration, and other behaviors. In addition, the highly localized cellular nanointerface enhances coupled stimulation on cells. Therefore, understanding the mechanism of the cellular nanointerface can not only provide innovative tools for regulating specific cell functions but also offers new aspects to understand the fundamental cellular activities that could facilitate the precise monitoring and treatment of diseases in the future. This review mainly describes the fabrication technology of vertical nanostructures, analyzing the formation of cellular nanointerfaces and the effects of cellular nanointerfaces on cells' fates and functions. At last, the applications of cellular nanointerfaces based on various nanostructures are summarized.

Survivability of Suddenly Loaded Arrays of Micropillars

When a multicomponent system is suddenly loaded, its capability of bearing the load depends not only on the strength of components but also on how a load released by a failed component is distributed among the remaining intact ones. Specifically, we consider an array of pillars which are located on a flat substrate and subjected to an impulsive and compressive load. Immediately after the loading, the pillars whose strengths are below the load magnitude crash. Then, loads released by these crashed pillars are transferred to and assimilated by the intact ones according to a load-sharing rule which reflects the mechanical properties of the pillars and the substrate. A sequence of bursts involving crashes and load transfers either destroys all the pillars or drives the array to a stable configuration when a smaller number of pillars sustain the applied load. By employing a fibre bundle model framework, we numerically study how the array integrity depends on sudden loading amplitudes, randomly distributed pillar strength thresholds and varying ranges of load transfer. Based on the simulation, we estimate the survivability of arrays of pillars defined as the probability of sustaining the applied load despite numerous damaged pillars. It is found that the resulting survival functions are accurately fitted by the family of complementary cumulative skew-normal distributions.

Tutorial: using nanoneedles for intracellular delivery

Intracellular delivery of advanced therapeutics, including biologicals and supramolecular agents, is complex because of the natural biological barriers that have evolved to protect the cell. Efficient delivery of therapeutic nucleic acids, proteins, peptides and nanoparticles is crucial for clinical adoption of emerging technologies that can benefit disease treatment through gene and cell therapy. Nanoneedles are arrays of vertical high-aspect-ratio nanostructures that can precisely manipulate complex processes at the cell interface, enabling effective intracellular delivery. This emerging technology has already enabled the development of efficient and non-destructive routes for direct access to intracellular environments and delivery of cell-impermeant payloads. However, successful implementation of this technology requires knowledge of several scientific fields, making it complex to access and adopt by researchers who are not directly involved in developing nanoneedle platforms. This presents an obstacle to the widespread adoption of nanoneedle technologies for drug delivery. This tutorial aims to equip researchers with the knowledge required to develop a nanoinjection workflow. It discusses the selection of nanoneedle devices, approaches for cargo loading and strategies for interfacing to biological systems and summarises an array of bioassays that can be used to evaluate the efficacy of intracellular delivery.

Nanopatterning with Photonic Nanojets: Review and Perspectives in Biomedical Research

Nanostructured surfaces and devices offer astounding possibilities for biomedical research, including cellular and molecular biology, diagnostics, and therapeutics. However, the wide implementation of these systems is currently limited by the lack of cost-effective and easy-to-use nanopatterning tools. A promising solution is to use optical methods based on photonic nanojets, namely, needle-like beams featuring a nanometric width. In this review, we survey the physics, engineering strategies, and recent implementations of photonic nanojets for high-throughput generation of arbitrary nanopatterns, along with applications in optics, electronics, mechanics, and biosensing. An outlook of the potential impact of nanopatterning technologies based on photonic nanojets in several relevant biomedical areas is also provided.

Optically transparent vertical silicon nanowire arrays for live-cell imaging

Programmable nano-bio interfaces driven by tuneable vertically configured nanostructures have recently emerged as a powerful tool for cellular manipulations and interrogations. Such interfaces have strong potential for ground-breaking advances, particularly in cellular nanobiotechnology and mechanobiology. However, the opaque nature of many nanostructured surfaces makes non-destructive, live-cell characterization of cellular behavior on vertically aligned nanostructures challenging to observe. Here, a new nanofabrication route is proposed that enables harvesting of vertically aligned silicon (Si) nanowires and their subsequent transfer onto an optically transparent substrate, with high efficiency and without artefacts. We demonstrate the potential of this route for efficient live-cell phase contrast imaging and subsequent characterization of cells growing on vertically aligned Si nanowires. This approach provides the first opportunity to understand dynamic cellular responses to a cell-nanowire interface, and thus has the potential to inform the design of future nanoscale cellular manipulation technologies.

Low-Load Metal-Assisted Catalytic Etching Produces Scalable Porosity in Si Powders

The recently discovered low-load metal-assisted catalytic etching (LL-MACE) creates nanostructured Si with controllable and variable characteristics that distinguish this technique from the conventional high-load variant. LL-MACE employs 150 times less metal catalyst and produces porous Si instead of Si nanowires. In this work, we demonstrate that some of the features of LL-MACE cannot be explained by the present understanding of MACE. With mechanistic insight derived from extensive experimentation, it is demonstrated that (1) the method allows the use of not only Ag, Pd, Pt, and Au as metal catalysts but also Cu and (2) judicious combinations of process parameters such as the type of metal, Si doping levels, and etching temperatures facilitate control over yield (0.065-88%), pore size (3-100 nm), specific surface area (20-310 m²·g^-1), and specific pore volume (0.05-1.05 cm³·g^-1). The porous structure of the product depends on the space-charge layer, which is controlled by the Si doping and the chemical identity of the deposited metal. The porous structure was also dependent on the dynamic structure of the deposited metal. A distinctive comet-like structure of metal nanoparticles was observed after etching with Cu, Ag, Pd, and, in some cases, Pt; this structure consisted of 10-50 nm main particles surrounded by smaller (<5 nm) nanoparticles. With good scalability and precise control of structural properties, LL-MACE facilitates Si applications in photovoltaics, energy storage, biomedicine, and water purification.

Emerging Roles of 1D Vertical Nanostructures in Orchestrating Immune Cell Functions

Engineered nano-bio cellular interfaces driven by 1D vertical nanostructures (1D-VNS) are set to prompt radical progress in modulating cellular processes at the nanoscale. Here, tuneable cell-VNS interfacial interactions are probed and assessed, highlighting the use of 1D-VNS in immunomodulation, and intracellular delivery into immune cells-both crucial in fundamental and translational biomedical research. With programmable topography and adaptable surface functionalization, 1D-VNS provide unique biophysical and biochemical cues to orchestrate innate and adaptive immunity, both ex vivo and in vivo. The intimate nanoscale cell-VNS interface leads to membrane penetration and cellular deformation, facilitating efficient intracellular delivery of diverse bioactive cargoes into hard-to-transfect immune cells. The unsettled interfacial mechanisms reported to be involved in VNS-mediated intracellular delivery are discussed. By identifying up-to-date progress and fundamental challenges of current 1D-VNS technology in immune-cell manipulation, it is hoped that this report gives timely insights for further advances in developing 1D-VNS as a safe, universal, and highly scalable platform for cell engineering and enrichment in advanced cancer immunotherapy such as chimeric antigen receptor-T therapy.

Efficient Transmission Electron Microscopy Characterization of Cell-Nanostructure Interfacial Interactions

Engineered nano-bio interfaces driven by tunable vertically configured nanostructures have recently emerged as a powerful tool for cellular manipulations and interrogations. Yet the interplay between substrate topography and cellular behavior is highly complex and not fully understood. A new experimental design is proposed that enables generation of ultrathin sections (lamellae) of cell-nanostructure imprints with minimal artifacts. We demonstrate the potential of such lamellae for efficient transmission electron microscopy (TEM) characterization of interfacial interactions between adherent cells and vertically aligned Si nanostructures. This approach will advance understanding of cellular responses to extracellular biophysical and biochemical cues-which is likely to facilitate the design of improved cellular manipulation technologies.

Surface-Mediated Intracellular Delivery by Physical Membrane Disruption

Effective and nondestructive intracellular delivery of exogenous molecules and other functional materials into living cells is of importance for diverse biological fundamental research and therapeutic applications, such as gene editing and cell-based therapies. However, for most exogenous molecules, the cell plasma membrane is effectively impermeable and thus remains the greatest barrier to intracellular delivery. In recent years, methods based on surface-mediated physical membrane disruption have attracted considerable attention. These methods exploit the physical properties of the surface to transiently increase the membrane permeability of cells come in contact thereto, thereby facilitating the efficient intracellular delivery of molecules regardless of molecule or target cell type. In this Review, we focus on recent progress, particularly over the past decade, on these surface-mediated membrane disruption-based delivery systems. According to the membrane disruption mechanism, three categories can be recognized: (i) mechanical penetration, (ii) electroporation, and (iii) photothermal poration. Each of these is discussed in turn and a brief perspective on future developments in this promising area is presented.

Silicon-Nanotube-Mediated Intracellular Delivery Enables Ex Vivo Gene Editing

Engineered nano-bio cellular interfaces driven by vertical nanostructured materials are set to spur transformative progress in modulating cellular processes and interrogations. In particular, the intracellular delivery-a core concept in fundamental and translational biomedical research-holds great promise for developing novel cell therapies based on gene modification. This study demonstrates the development of a mechanotransfection platform comprising vertically aligned silicon nanotube (VA-SiNT) arrays for ex vivo gene editing. The internal hollow structure of SiNTs allows effective loading of various biomolecule cargoes; and SiNTs mediate delivery of those cargoes into GPE86 mouse embryonic fibroblasts without compromising their viability. Focused ion beam scanning electron microscopy (FIB-SEM) and confocal microscopy results demonstrate localized membrane invaginations and accumulation of caveolin-1 at the cell-NT interface, suggesting the presence of endocytic pits. Small-molecule inhibition of endocytosis suggests that active endocytic process plays a role in the intracellular delivery of cargo from SiNTs. SiNT-mediated siRNA intracellular delivery shows the capacity to reduce expression levels of F-actin binding protein (Triobp) and alter the cellular morphology of GPE86. Finally, the successful delivery of Cas9 ribonucleoprotein (RNP) to specifically target mouse Hprt gene is achieved. This NT-enhanced molecular delivery platform has strong potential to support gene editing technologies.

High-Aspect-Ratio Nanostructured Surfaces as Biological Metamaterials

Materials patterned with high-aspect-ratio nanostructures have features on similar length scales to cellular components. These surfaces are an extreme topography on the cellular level and have become useful tools for perturbing and sensing the cellular environment. Motivation comes from the ability of high-aspect-ratio nanostructures to deliver cargoes into cells and tissues, access the intracellular environment, and control cell behavior. These structures directly perturb cells' ability to sense and respond to external forces, influencing cell fate, and enabling new mechanistic studies. Through careful design of their nanoscale structure, these systems act as biological metamaterials, eliciting unusual biological responses. While predominantly used to interface eukaryotic cells, there is growing interest in nonanimal and prokaryotic cell interfacing. Both experimental and theoretical studies have attempted to develop a mechanistic understanding for the observed behaviors, predominantly focusing on the cell-nanostructure interface. This review considers how high-aspect-ratio nanostructured surfaces are used to both stimulate and sense biological systems.

Cellular Deformations Induced by Conical Silicon Nanowire Arrays Facilitate Gene Delivery

Engineered cell-nanostructured interfaces generated by vertically aligned silicon nanowire (SiNW) arrays have become a promising platform for orchestrating cell behavior, function, and fate. However, the underlying mechanism in SiNW-mediated intracellular access and delivery is still poorly understood. This study demonstrates the development of a gene delivery platform based on conical SiNW arrays for mechanical cell transfection, assisted by centrifugal force, for both adherent and nonadherent cells in vitro. Cells form focal adhesions on SiNWs within 6 h, and maintain high viability and motility. Such a functional and dynamic cell-SiNW interface features conformational changes in the plasma membrane and in some cases the nucleus, promoting both direct penetration and endocytosis; this synergistically facilitates SiNW-mediated delivery of nucleic acids into immortalized cell lines, and into difficult-to-transfect primary immune T cells without pre-activation. Moreover, transfected cells retrieved from SiNWs retain the capacity to proliferate-crucial to future biomedical applications. The results indicate that SiNW-mediated intracellular delivery holds great promise for developing increasingly sophisticated investigative and therapeutic tools.

High-Aspect-Ratio Semiconducting Polymer Pillars for 3D Cell Cultures

Hybrid interfaces between living cells and nano/microstructured scaffolds have huge application potential in biotechnology, spanning from regenerative medicine and stem cell therapies to localized drug delivery and from biosensing and tissue engineering to neural computing. However, 3D architectures based on semiconducting polymers, endowed with responsivity to visible light, have never been considered. Here, we apply for the first time a push-coating technique to realize high aspect ratio polymeric pillars, based on polythiophene, showing optimal biocompatibility and allowing for the realization of soft, 3D cell cultures of both primary neurons and cell line models. HEK-293 cells cultured on top of polymer pillars display a remarkable change in the cell morphology and a sizable enhancement of the membrane capacitance due to the cell membrane thinning in correspondence to the pillars' top surface, without negatively affecting cell proliferation. Electrophysiology properties and synapse number of primary neurons are also very well preserved. In perspective, high aspect ratio semiconducting polymer pillars may find interesting applications as soft, photoactive elements for cell activity sensing and modulation.

High Anodic-Voltage Focusing of Charge Carriers in Silicon Enables the Etching of Regularly-Arranged Submicrometer Pores at High Density and High Aspect-Ratio

The anodic dissolution of silicon in acidic electrolytes is a well-known technology enabling the silicon machining to be accurately controlled down to the micrometer scale in low-doped n-type silicon electrodes. Attempts to scale down this technology to the submicrometer scale has shown to be challenging, though it premises to enable the fabrication of meso and nano structures/systems that would greatly impact the fields of biosensors and nanomedicine. In this work, we report on the electrochemical etching at high anodic voltages (up to 40 V) of two-dimensional regular arrays of millions pores per square centimeter (up to 30 × 10⁶ cm^-2) with sub-micrometric diameter (down to ~860 nm), high depth (up to ~40 μm), and high aspect-ratio (up to ~45) using low-doped n-type silicon electrodes (resistivity 3-8 Ω cm). The use of high anodic voltages, which are over one order of magnitude higher than that commonly used in electrochemical etching of silicon, tremendously improves hole focusing at the pore tips during the etching and enables, in turn, the control of electrochemical etching of submicrometer-sized pores when spatial period reduces below 2 μm. A theoretical model allows experimental results to be interpreted in terms of an electric-field-enhanced focusing of holes at the tip apex of the pores at high anodic voltages, with respect to the pore base, which leads to a smaller curvature radius of the tip apex and enables, in turn, the etching of pore tips to be preferentially sustained over time and space.

Cell Membrane Disruption by Vertical Micro-/Nanopillars: Role of Membrane Bending and Traction Forces

Gaining access to the cell interior is fundamental for many applications, such as electrical recording and drug and biomolecular delivery. A very promising technique consists of culturing cells on micro-/nanopillars. The tight adhesion and high local deformation of cells in contact with nanostructures can promote the permeabilization of lipids at the plasma membrane, providing access to the internal compartment. However, there is still much experimental controversy regarding when and how the intracellular environment is targeted and the role of the geometry and interactions with surfaces. Consequently, we investigated, by coarse-grained molecular dynamics simulations of the cell membrane, the mechanical properties of the lipid bilayer under high strain and bending conditions. We found out that a high curvature of the lipid bilayer dramatically lowers the traction force necessary to achieve membrane rupture. Afterward, we experimentally studied the permeabilization rate of the cell membrane by pillars with comparable aspect ratios but different sharpness values at the edges. The experimental data support the simulation results: even pillars with diameters in the micron range may cause local membrane disruption when their edges are sufficiently sharp. Therefore, the permeabilization likelihood is connected to the local geometric features of the pillars rather than diameter or aspect ratio. The present study can also provide significant contributions to the design of three-dimensional biointerfaces for tissue engineering and cellular growth.

Inductively actuated micro needles for on-demand intracellular delivery

Methods that provide controlled influx of molecules into cells are of critical importance for uncovering cellular mechanisms, drug development and synthetic biology. However, reliable intracellular delivery without adversely affecting the cells is a major challenge. We developed a platform for on-demand intracellular delivery applications, with which cell membrane penetration is achieved by inductive heating of micro needles. The micro needles of around 1 μm in diameter and 5 μm in length are made of gold using a silicon-based micro fabrication process that provides flexibility with respect to the needles' dimensions, pitch, shell thickness and the covered area. Experiments with HCT 116 colon cancer cells showed a high biocompatibility of the gold needle platform. Transmission electron microscopy of the cell-needle interface revealed folding of the cell membrane around the needle without penetration, preventing any delivery, which was confirmed using the EthD-1 fluorescent dye. The application of an alternating magnetic field, however, resulted in the delivery of EthD-1 by localized heating of the micro needles. Fluorescence quantification showed that intracellular delivery, with as high as 75% efficiency, is achieved for specific treatment times between 1 and 5 minutes. Overexposure of the cells to the heated micro needles, i.e. longer magnetic field application, leads to an increase in cell death, which can be exploited for cleaning the platform. This method allows to perform intracellular deliver by remotely activating the micro needles via a magnetic field, and it is controlled by the application time, making it a versatile and easy to use method. The wireless actuation could also be an attractive feature for in-vivo delivery and implantable devices.

Interfacing Cells with Vertical Nanoscale Devices: Applications and Characterization

Measurements of the intracellular state of mammalian cells often require probes or molecules to breach the tightly regulated cell membrane. Mammalian cells have been shown to grow well on vertical nanoscale structures in vitro, going out of their way to reach and tightly wrap the structures. A great deal of research has taken advantage of this interaction to bring probes close to the interface or deliver molecules with increased efficiency or ease. In turn, techniques have been developed to characterize this interface. Here, we endeavor to survey this research with an emphasis on the interface as driven by cellular mechanisms.

Nanomedicine and advanced technologies for burns: Preventing infection and facilitating wound healing

According to the latest report from the World Health Organization, an estimated 265,000 deaths still occur every year as a direct result of burn injuries. A widespread range of these deaths induced by burn wound happens in low- and middle-income countries, where survivors face a lifetime of morbidity. Most of the deaths occur due to infections when a high percentage of the external regions of the body area is affected. Microbial nutrient availability, skin barrier disruption, and vascular supply destruction in burn injuries as well as systemic immunosuppression are important parameters that cause burns to be susceptible to infections. Topical antimicrobials and dressings are generally employed to inhibit burn infections followed by a burn wound therapy, because systemic antibiotics have problems in reaching the infected site, coupled with increasing microbial drug resistance. Nanotechnology has provided a range of molecular designed nanostructures (NS) that can be used in both therapeutic and diagnostic applications in burns. These NSs can be divided into organic and non-organic (such as polymeric nanoparticles (NPs) and silver NPs, respectively), and many have been designed to display multifunctional activity. The present review covers the physiology of skin, burn classification, burn wound pathogenesis, animal models of burn wound infection, and various topical therapeutic approaches designed to combat infection and stimulate healing. These include biological based approaches (e.g. immune-based antimicrobial molecules, therapeutic microorganisms, antimicrobial agents, etc.), antimicrobial photo- and ultrasound-therapy, as well as nanotechnology-based wound healing approaches as a revolutionizing area. Thus, we focus on organic and non-organic NSs designed to deliver growth factors to burned skin, and scaffolds, dressings, etc. for exogenous stem cells to aid skin regeneration. Eventually, recent breakthroughs and technologies with substantial potentials in tissue regeneration and skin wound therapy (that are as the basis of burn wound therapies) are briefly taken into consideration including 3D-printing, cell-imprinted substrates, nano-architectured surfaces, and novel gene-editing tools such as CRISPR-Cas.

Mapping cell behavior across a wide range of vertical silicon nanocolumn densities

Over the past decade, vertical nanostructures have provided novel approaches for biomedical applications such as intracellular delivery/detection, specific cell capture, membrane potential measurement, and cellular activity regulation. Although the feasibility of the vertical nanostructures as a new biological tool has been thoroughly demonstrated, a better understanding of cell behavior on vertical nanostructures, in particular the effects of geometry, is essential for advanced applications. To investigate the cell behavior according to the variation of the spacing between vertical nanostructures, we have interfaced fibroblasts (NIH3T3) with density-controlled vertical silicon nanocolumn arrays (vSNAs). Over a wide range of vSNA densities, we observe three distinct cell settling regimes and investigate both overall cell behavior (adhesions, morphology, and mobility) and detailed biomacromolecule variance (F-actin and focal adhesion) across these regimes. We expect that these systematic observations could serve as a guide for improved nanostructure array design for the desired cell manipulation.