Replication of a Tissue Microenvironment by Thermal Scanning Probe Lithography

Combining thermal scanning probe lithography and dry etching for grayscale nanopattern amplification

Grayscale structured surfaces with nanometer-scale features are used in a growing number of applications in optics and fluidics. Thermal scanning probe lithography achieves a lateral resolution below 10 nm and a vertical resolution below 1 nm, but its maximum depth in polymers is limited. Here, we present an innovative combination of nanowriting in thermal resist and plasma dry etching with substrate cooling, which achieves up to 10-fold amplification of polymer nanopatterns into SiO2 without proportionally increasing surface roughness. Sinusoidal nanopatterns in SiO2 with 400 nm pitch and 150 nm depth are fabricated free of shape distortion after dry etching. To exemplify the possible applications of the proposed method, grayscale dielectric nanostructures are used for scalable manufacturing through nanoimprint lithography and for strain nanoengineering of 2D materials. Such a method for aspect ratio amplification and smooth grayscale nanopatterning has the potential to find application in the fabrication of photonic and nanoelectronic devices.

Advances in lithographic techniques for precision nanostructure fabrication in biomedical applications

Nano-fabrication techniques have demonstrated their vital importance in technological innovation. However, low-throughput, high-cost and intrinsic resolution limits pose significant restrictions, it is, therefore, paramount to continue improving existing methods as well as developing new techniques to overcome these challenges. This is particularly applicable within the area of biomedical research, which focuses on sensing, increasingly at the point-of-care, as a way to improve patient outcomes. Within this context, this review focuses on the latest advances in the main emerging patterning methods including the two-photon, stereo, electrohydrodynamic, near-field electrospinning-assisted, magneto, magnetorheological drawing, nanoimprint, capillary force, nanosphere, edge, nano transfer printing and block copolymer lithographic technologies for micro- and nanofabrication. Emerging methods enabling structural and chemical nano fabrication are categorised along with prospective chemical and physical patterning techniques. Established lithographic techniques are briefly outlined and the novel lithographic technologies are compared to these, summarising the specific advantages and shortfalls alongside the current lateral resolution limits and the amenability to mass production, evaluated in terms of process scalability and cost. Particular attention is drawn to the potential breakthrough application areas, predominantly within biomedical studies, laying the platform for the tangible paths towards the adoption of alternative developing lithographic technologies or their combination with the established patterning techniques, which depends on the needs of the end-user including, for instance, tolerance of inherent limits, fidelity and reproducibility.

Silica Replicas Derived from Mammalian Cells as an Innovative Approach to Physically Direct Cell Lineage Decisions of Mesenchymal Stem Cells

By means of a "live-cell" template strategy, silica replicas displaying the same morphology and topography of the mammalian cells used as templates are fabricated. The replicas are used as substrates to direct the differentiation of mesenchymal stem cells (MSCs) to predefined cell lineages. Upregulation of specific genes shows how the silica replica-based substrates have the ability to induce the molecular characteristics of the mature cell types from which they have been derived from. Thus, MSCs cultured in the presence of silica replicas of human osteoblasts (HObs) differentiate into HObs-like cells, as shown by the upregulation of specific osteogenic genes. Likewise, when MSCs are incubated with silica replicas derived from human chondrocytes, an enhanced expression of chondrogenic markers is observed. Importantly, the effects of the silica replicas are cell type-specific since the incubation of MSCs with HObs silica replicas does not result in upregulation of chondrogenic markers and vice versa. What is more, for both cases, the differentiation rate is enhanced when the silica replicas are used in combination with growth factors, suggesting a potential synergistic effect. These results demonstrate the potential of this innovative method as an efficient and cheap approach with the potential to eliminate, or at least reduce, the use of biochemically soluble compounds in stem cells research.

Thermal Scanning-Probe Lithography for Broad-Band On-Demand Plasmonic Nanostructures on Transparent Substrates

Thermal scanning-probe lithography (t-SPL) is a high-resolution nanolithography technique that enables the nanopatterning of thermosensitive materials by means of a heated silicon tip. It does not require alignment markers and gives the possibility to assess the morphology of the sample in a noninvasive way before, during, and after the patterning. In order to exploit t-SPL at its peak performances, the writing process requires applying an electric bias between the scanning hot tip and the sample, thereby restricting its application to conductive, optically opaque, substrates. In this work, we show a t-SPL-based method, enabling the noninvasive high-resolution nanolithography of photonic nanostructures onto optically transparent substrates across a broad-band visible and near-infrared spectral range. This was possible by intercalating an ultrathin transparent conductive oxide film between the dielectric substrate and the sacrificial patterning layer. This way, nanolithography performances comparable with those typically observed on conventional semiconductor substrates are achieved without significant changes of the optical response of the final sample. We validated this innovative nanolithography approach by engineering periodic arrays of plasmonic nanoantennas and showing the capability to tune their plasmonic response over a broad-band visible and near-infrared spectral range. The optical properties of the obtained systems make them promising candidates for the fabrication of hybrid plasmonic metasurfaces supported onto fragile low-dimensional materials, thus enabling a variety of applications in nanophotonics, sensing, and thermoplasmonics.

Biomimetic Surface Topography from the Rubus fruticosus Leaf as a Guidance of Angiogenesis in Tissue Engineering Applications

The promotion of angiogenesis is a fundamental step for efficient organ/tissue reconstitution and replacement. Thus, several strategies to promote vascularization of scaffolds were studied to satisfy this unsolved clinical need. The interface between cells and substrates is a determinant for the success of tissue engineering (TE) strategies. Substrate's topography is reported to play a key role in influencing endothelial cell behavior, namely, on its proliferation, metabolic activity, morphology, migration, and secretion of cytokines and chemokines. Therefore, surface topography of the biomaterial-based grafts is a crucial property that is considered in the development of a new TE approach. Herein, we hypothesize that the surface of Rubus fruticosus leaf plays a crucial role in driving angiogenesis since its architecture resembles the vascular structures at a biologically relevant size scale. For this, we produced biomimetic polycaprolactone (PCL) membranes (BpMs) replicating the surface topography of a R. fruticosus leaf by replica molding and nanoimprint lithography. Our results showed an enhanced performance in terms of proliferation of the human endothelial cell line on top of the BpM. Moreover, an asymmetric cellular spatial distribution among the surface of the BpM was observed. These cells seem to have higher density for longer time periods in the region that replicates the leaf veins. Finally, we assess the angiogenic capacity through a chick chorioallantoic membrane assay, revealing that BpMs are more prone to support angiogenesis than flat PCL membranes. We strongly believe that this strategy can bring new insights into developing TE strategies with an enhanced performance in terms of the vascular integration between the host and the scaffolds implanted.

Memory efficient constrained optimization of scanning-beam lithography

This article describes a memory efficient method for solving large-scale optimization problems that arise when planning scanning-beam lithography processes. These processes require the identification of an exposure pattern that minimizes the difference between a desired and predicted output image, subject to constraints. The number of free variables is equal to the number of pixels, which can be on the order of millions or billions in practical applications. The proposed method splits the problem domain into a number of smaller overlapping subdomains with constrained boundary conditions, which are then solved sequentially using a constrained gradient search method (L-BFGS-B). Computational time is reduced by exploiting natural sparsity in the problem and employing the fast Fourier transform for efficient gradient calculation. When it comes to the trade-off between memory usage and computational time we can make a different trade-off compared to previous methods, where the required memory is reduced by approximately the number of subdomains at the cost of more computations. In an example problem with 30 million variables, the proposed method reduces memory requirements by 67% but increases computation time by 27%. Variations of the proposed method are expected to find applications in the planning of processes such as scanning laser lithography, scanning electron beam lithography, and focused ion beam deposition, for example.

Anisotropic Particle Fabrication Using Thermal Scanning Probe Lithography

Size, shape, and chemical properties of nanoparticles are powerful tools to modulate the optical and physicochemical properties of a particle suspension. Despite having many methods to synthesize anisotropic nanoparticles, often there are challenges in terms of controlling the polydispersity, shape, size, or composition of anisotropic nanoparticles. This work has been inspired by the potential for developing a unique pathway to make different shaped monodispersed anisotropic nano- and microparticles with large flexibility in material choice. Compared to existing methods, this state-of-the-art nanolithographic method is fast, easy to prototype, and much simple in terms of its mechanical requirement. We show that this technique has been efficiently used to make a variety of anisotropic nano- and microparticles of different shapes, such as triangular prisms, ovals, disks, flowers, and stairs following the same pathway, at the same time showing the potential of being flexible with respect to the composition of the particles. The thermal scanning probe lithographic method in combination with dry reactive ion etching was used to make two-dimensional and three-dimensional templates for the fabrication of anisotropic nano- and microparticles. Deposition of different metal/metal oxides by the electron-beam evaporation method onto these templates allowed us to fabricate a range of nanomaterials according to the required functionality in potential applications. The particles were characterized by atomic force microscopy, He-ion microscopy, scanning electron microscopy, and dynamic light scattering to ensure that the developed method is reproducible, flexible, and robust in choosing the shapes for making monodispersed anisotropic nanoparticles with great control over shape and size.

The Bumpy Road to Stem Cell Therapies: Rational Design of Surface Topographies to Dictate Stem Cell Mechanotransduction and Fate

Cells sense and respond to a variety of physical cues from their surrounding microenvironment, and these are interpreted through mechanotransductive processes to inform their behavior. These mechanisms have particular relevance to stem cells, where control of stem cell proliferation, potency, and differentiation is key to their successful application in regenerative medicine. It is increasingly recognized that surface micro- and nanotopographies influence stem cell behavior and may represent a powerful tool with which to direct the morphology and fate of stem cells. Current progress toward this goal has been driven by combined advances in fabrication technologies and cell biology. Here, the capacity to generate precisely defined micro- and nanoscale topographies has facilitated the studies that provide knowledge of the mechanotransducive processes that govern the cellular response as well as knowledge of the specific features that can drive cells toward a defined differentiation outcome. However, the path forward is not fully defined, and the "bumpy road" that lays ahead must be crossed before the full potential of these approaches can be fully exploited. This review focuses on the challenges and opportunities in applying micro- and nanotopographies to dictate stem cell fate for regenerative medicine. Here, key techniques used to produce topographic features are reviewed, such as photolithography, block copolymer lithography, electron beam lithography, nanoimprint lithography, soft lithography, scanning probe lithography, colloidal lithography, electrospinning, and surface roughening, alongside their advantages and disadvantages. The biological impacts of surface topographies are then discussed, including the current understanding of the mechanotransductive mechanisms by which these cues are interpreted by the cells, as well as the specific effects of surface topographies on cell differentiation and fate. Finally, considerations in translating these technologies and their future prospects are evaluated.

Deconstructing, Replicating, and Engineering Tissue Microenvironment for Stem Cell Differentiation

One of the most crucial components of regenerative medicine is the controlled differentiation of embryonic or adult stem cells into the desired cell lineage. Although most of the reported protocols of stem cell differentiation involve the use of soluble growth factors, it is increasingly evident that stem cells also undergo differentiation when cultured in the appropriate microenvironment. When cultured in decellularized tissues, for instance, stem cells can recapitulate the morphogenesis and functional specialization of differentiated cell types with speed and efficiency that often surpass the traditional growth factor-driven protocols. This suggests that the tissue microenvironment (TME) provides stem cells with a holistic "instructive niche" that harbors signals for cellular reprogramming. The translation of this into medical applications requires the decoding of these signals, but this has been hampered by the complexity of TME. This problem is often addressed by a reductionist approach, in which cells are exposed to substrates decorated with simple, empirically designed geometries, textures, and chemical compositions ("bottom-up" approach). Although these studies are invaluable in revealing the basic principles of mechanotransduction mechanisms, their physiological relevance is often uncertain. This review examines the recent progress of an alternative, "top-down" approach, in which the TME is treated as a holistic biological entity. This approach is made possible by recent advances in systems biology and fabrication technologies that enable the isolation, characterization, and reconstitution of TME. It is hoped that these new techniques will elucidate the nature of niche signals so that they can be extracted, replicated, and controlled. This review summarizes these emerging techniques and how the data they generated are changing our view on TME. Impact statement This review summarizes the current state of art of the understanding of instructive niche in the field of tissue microenvironment. Not only did we survey the use of different biochemical preparations as stimuli of stem cell differentiation and summarize the recent effort in dissecting the biochemical composition of these preparations, through the application of extracellular matrix (ECM) arrays and proteomics, but we also introduce the use of open-source, high-content immunohistochemistry projects in contributing to the understanding of tissue-specific composition of ECM. We believe this review would be highly useful for our peer researching in the same field. "Mr. Tulkinghorn is always the same… so oddly out of place and yet so perfectly at home." -Charles Dickens, Bleak House.

Transferrin-targeted porous silicon nanoparticles reduce glioblastoma cell migration across tight extracellular space

Mortality of glioblastoma multiforme (GBM) has not improved over the last two decades despite medical breakthroughs in the treatment of other types of cancers. Nanoparticles hold tremendous promise to overcome the pharmacokinetic challenges and off-target adverse effects. However, an inhibitory effect of nanoparticles by themselves on metastasis has not been explored. In this study, we developed transferrin-conjugated porous silicon nanoparticles (Tf@pSiNP) and studied their effect on inhibiting GBM migration by means of a microfluidic-based migration chip. This platform, designed to mimic the tight extracellular migration tracts in brain parenchyma, allowed high-content time-resolved imaging of cell migration. Tf@pSiNP were colloidally stable, biocompatible, and their uptake into GBM cells was enhanced by receptor-mediated internalisation. The migration of Tf@pSiNP-exposed cells across the confined microchannels was suppressed, but unconfined migration was unaffected. The pSiNP-induced destabilisation of focal adhesions at the leading front may partially explain the migration inhibition. More corroborating evidence suggests that pSiNP uptake reduced the plasticity of GBM cells in reducing cell volume, an effect that proved crucial in facilitating migration across the tight confined tracts. We believe that the inhibitory effect of Tf@pSiNP on cell migration, together with the drug-delivery capability of pSiNP, could potentially offer a disruptive strategy to treat GBM.

Capturing instructive cues of tissue microenvironment by silica bioreplication

Cells in tissues are enveloped by an instructive niche made of the extracellular matrix. These instructive niches contain three general types of information: topographical, biochemical and mechanical. While the combined effects of these three factors are widely studied, the functions of each individual one has not been systematically characterised, because it is impossible to alter a single factor in a tissue microenvironment without simultaneously affecting the other two. Silica BioReplication (SBR) is a process that converts biological samples into silica, faithfully preserving the original topography at the nano-scale. We explored the use of this technique to generate inorganic replicas of intact mammalian tissues, including tendon, cartilage, skeletal muscle and spinal cord. Scanning electron and atomic force microscopy showed that the resulting replicas accurately preserved the three-dimensional ultrastructure of each tissue, while all biochemical components were eradicated by calcination. Such properties allowed the uncoupling the topographical information of a tissue microenvironment from its biochemical and mechanical components. Here, we showed that human mesenchymal stem cells (MSC) cultured on the replicas of different tissues displayed vastly different morphology and focal adhesions, suggesting that the topography of the tissue microenvironment captured by SBR could profoundly affect MSC biology. MSC cultured on tendon replica elongated and expressed tenocytes marker, while MSC on the spinal cord replica developed into spheroids that resembled neurospheres, in morphology and in the expression of neurosphere markers, and could be further differentiated into neuron-like cells. This study reveals the significance of topographical cues in a cell niche, as tissue-specific topography was sufficient in initiating and directing differentiation of MSC, despite the absence of any biochemical signals. SBR is a convenient and versatile method for capturing this topographical information, facilitating the functional characterisation of cell niches. STATEMENT OF SIGNIFICANCE: Various studies have shown that three major factors, topographical, biochemical and mechanical, in a tissue microenvironment (TME) are essential for cellular homeostasis and functions. Current experimental models are too simplistic to represent the complexity of the TME, hindering the detailed understanding of its functions. In particular, the importance each factor in a tissue microenvironment have not been individually characterised, because it is challenging to alter one of these factors without simultaneously affecting the other two. Silica bioreplication (SBR) is a process that converts biological samples into silica replicas with high structural fidelity. SBR is a convenient and versatile method for capturing this topographical information on to a biologically inert material, allowing the functional characterisation of the architecture of a TME.

Thermal scanning probe lithography-a review

Fundamental aspects and state-of-the-art results of thermal scanning probe lithography (t-SPL) are reviewed here. t-SPL is an emerging direct-write nanolithography method with many unique properties which enable original or improved nano-patterning in application fields ranging from quantum technologies to material science. In particular, ultrafast and highly localized thermal processing of surfaces can be achieved through the sharp heated tip in t-SPL to generate high-resolution patterns. We investigate t-SPL as a means of generating three types of material interaction: removal, conversion, and addition. Each of these categories is illustrated with process parameters and application examples, as well as their respective opportunities and challenges. Our intention is to provide a knowledge base of t-SPL capabilities and current limitations and to guide nanoengineers to the best-fitting approach of t-SPL for their challenges in nanofabrication or material science. Many potential applications of nanoscale modifications with thermal probes still wait to be explored, in particular when one can utilize the inherently ultrahigh heating and cooling rates.