Modulating patterned adhesion and repulsion of HEK 293 cells on microengineered parylene-C/SiO(2) substrates

Geometric micro-shapes facilitate trackless connections between human astrocytes

Objective.Cell patterning approaches commonly employed to direct the cytoplasmic outgrowth from cell bodies have been via chemical cues or biomaterial tracks. However, complex network designs using these approaches create problems where multiple tracks lead to manifold obstructions in design. A less common but alternative cell patterning modality is to geometrically design the nodes to project the cytoplasmic processes into a specific direction, thus, removing the need for tracks. Janget alperformed an in-depth study of how rodent neuron primaries could be directed accurately using geometric micro-shapes. In parallel and in contrast, to the work of Janget alwe investigate, for the first time, the effect that micro-shape geometry has on the cytoplasmic process outgrowth of human cells of astrocyte origin using the biomaterial parylene-C.Approach.We investigated eight different types of parylene-C micro-shape on SiO₂substrates consisting of the: circle, square, pentagon, hexagon, equilateral triangle and three isosceles triangles with top vertex angles of 14.2°, 28.8°, and 97.6°, respectively. We quantified how each micro-shape influenced the: cell patterning, the directionality of the cytoplasmic process outgrowth and the functionality for human astrocyte.Main results.Human astrocytes became equally well patterned on all different micro-shapes. Human astrocytes could discriminate the underlying micro-shape geometry and preferentially extended processes from the vertices of equilateral triangles and isosceles triangles where the vertex angle equal to 28.8° in a repeatable manner whilst remaining functional.Significance.We demonstrate how human astrocytes are extremely effective at directing their cytoplasmic process outgrowth from the vertices of geometric micro-shapes, in particular the top vertex of triangular shapes. The significance of this work is that it demonstrates that geometric micro-shapes offer an alternative patterning modality to direct cytoplasmic process outgrowth for human astrocytes, which can serve to simplify complex network design, thus, removing the need for tracks.

The Effect of Basic Microshapes on hNT Astrocytes Cytoplasmic Process Outgrowth

Astrocytes are a non-homogeneous cell type, highly mobile which constantly extend and retract their cytoplasmic processes in what would seem random in direction. In this paper, we investigate how simple geometric microshapes can be used to control the outgrowth of human astrocytes cytoplasmic processes. We investigate the effect of how five regular microshapes: the circle, triangle, square, pentagon and hexagon control astrocyte cytoplasmic process outgrowth. For all the different microshape types, we observe that it is the corners of the shapes that that cause the astrocyte to produce spontaneous outgrowth except for the circle where the outgrowth occurs at a random radial position. This work suggests that the geometry of cell adhesive regions effects the outgrowth of hNT astrocytes.

Evaluation of parylene derivatives for use as biomaterials for human astrocyte cell patterning

Cell patterning is becoming increasingly popular in neuroscience because it allows for the control in the location and connectivity of cells. A recently developed cell patterning technology uses patterns of an organic polymer, parylene-C, on a background of SiO₂. When cells are cultured on the parylene-C/SiO₂ substrate they conform to the underlying parylene-C geometry. Parylene-C is, however, just one member of a family of parylene polymers that have varying chemical and physical properties. In this work, we investigate whether two commercially available mainstream parylene derivatives, parylene-D, parylene-N and a more recent parylene derivative, parylene-HT to determine if they enable higher fidelity hNT astrocyte cell patterning compared to parylene-C. We demonstrate that all parylene derivatives are compatible with the existing laser fabrication method. We then demonstrate that parylene-HT, parylene-D and parylene-N are suitable for use as an hNT astrocyte cell attractive substrate and result in an equal quality of patterning compared to parylene-C. This work supports the use of alternative parylene derivatives for applications where their different physical and chemical properties are more suitable.

Selective PEGylation of Parylene-C/SiO2 Substrates for Improved Astrocyte Cell Patterning

Controlling the spatial distribution of glia and neurons in in vitro culture offers the opportunity to study how cellular interactions contribute to large scale network behaviour. A recently developed approach to cell-patterning uses differential adsorption of animal-serum protein on parylene-C and SiO₂ surfaces to enable patterning of neurons and glia. Serum, however, is typically poorly defined and generates reproducibility challenges. Alternative activation methods are highly desirable to enable patterning without relying on animal serum. We take advantage of the innate contrasting surface chemistries of parylene-C and SiO₂ to enable selective bonding of polyethylene glycol SiO₂ surfaces, i.e. PEGylation, rendering them almost completely repulsive to cell adhesion. As the reagents used in the PEGylation protocol are chemically defined, the reproducibility and batch-to-batch variability complications associated with the used of animal serum are avoided. We report that PEGylated parylene-C/SiO₂ substrates achieve a contrast in astrocyte density of 65:1 whereas the standard serum-immersion protocol results in a contrast of 5.6:1. Furthermore, single-cell isolation was significantly improved on PEGylated substrates when astrocytes were grown on close-proximity parylene-C nodes, whereas isolation was limited on serum-activated substrates due tolerance for cell adhesion on serum-adsorbed SiO₂ surfaces.

Patterning of functional human astrocytes onto parylene-C/SiO2 substrates for the study of Ca2+ dynamics in astrocytic networks

OBJECTIVE

Recent literature suggests that astrocytes form organized functional networks and communicate through transient changes in cytosolic Ca²⁺. Traditional techniques to investigate network activity, such as pharmacological blocking or genetic knockout, are difficult to restrict to individual cells. The objective of this work is to develop cell-patterning techniques to physically manipulate astrocytic interactions to enable the study of Ca²⁺ in astrocytic networks.

APPROACH

We investigate how an in vitro cell-patterning platform that utilizes geometric patterns of parylene-C on SiO₂ can be used to physically isolate single astrocytes and small astrocytic networks.

MAIN RESULTS

We report that single astrocytes are effectively isolated on 75  ×  75 µm square parylene nodes, whereas multi-cellular astrocytic networks are isolated on larger nodes, with the mean number of astrocytes per cluster increasing as a function of node size. Additionally, we report that astrocytes in small multi-cellular clusters exhibit spatio-temporal clustering of Ca²⁺ transients. Finally, we report that the frequency and regularity of Ca²⁺ transients was positively correlated with astrocyte connectivity.

SIGNIFICANCE

The significance of this work is to demonstrate how patterning hNT astrocytes replicates spatio-temporal clustering of Ca²⁺ signalling that is observed in vivo but not in dissociated in vitro cultures. We therefore highlight the importance of the structure of astrocytic networks in determining ensemble Ca²⁺ behaviour.

Human astrocytic grid networks patterned in parylene-C inlayed SiO2 trenches

Recent literature suggests that glia, and in particular astrocytes, should be studied as organised networks which communicate through gap junctions. Astrocytes, however, adhere to most surfaces and are highly mobile cells. In order to study, such organised networks effectively in vitro it is necessary to influence them to pattern to certain substrates whilst being repelled from others and to immobilise the astrocytes sufficiently such that they do not continue to migrate further whilst under study. In this article, we demonstrate for the first time how it is possible to facilitate the study of organised patterned human astrocytic networks using hNT astrocytes in a SiO2 trench grid network that is inlayed with the biocompatible material, parylene-C. We demonstrate how the immobilisation of astrocytes lies in the depth of the SiO2 trench, determining an optimum trench depth and that the optimum patterning of astrocytes is a consequence of the parylene-C inlay and the grid node spacing. We demonstrate high fidelity of the astrocytic networks and demonstrate that functionality of the hNT astrocytes through ATP evoked calcium signalling is also dependent on the grid node spacing. Finally, we demonstrate that the location of the nuclei on the grid nodes is also a function of the grid node spacing. The significance of this work, is to describe a suitable platform to facilitate the study of hNT astrocytes from the single cell level to the network level to improve knowledge and understanding of how communication links to spatial organisation at these higher order scales and trigger in vitro research further in this area with clinical applications in the area of epilepsy, stroke and focal cerebral ischemia.

Investigating parylene-HT as a substrate for human cell patterning

We demonstrate, for the first time, how parylene-HT on SiO2 substrates can be used as a human cell patterning platform. We demonstrate this platform with hNT astrocytes, derived from the human NTera2.D1 cell line. We show how hNT astrocytes are attracted to Parylene-HT and repelled by the SiO2 and are shown to adopt a similar morphology as that attained on standard tissue culture polystyrene. Furthermore, parylene-HT was capable of patterning the astrocytes achieving a ratio of 8:1 for cells on parylene compared to SiO2. Thus, as parylene-HT has similar physical properties to parylene-C with the addition of UV and thermal resistance, parylene-HT represents a desirable alternative substrate for human cell patterning.

Towards a 'siliconeural computer': technological successes and challenges

Electronic signals govern the function of both nervous systems and computers, albeit in different ways. As such, hybridizing both systems to create an iono-electric brain-computer interface is a realistic goal; and one that promises exciting advances in both heterotic computing and neuroprosthetics capable of circumventing devastating neuropathology. 'Neural networks' were, in the 1980s, viewed naively as a potential panacea for all computational problems that did not fit well with conventional computing. The field bifurcated during the 1990s into a highly successful and much more realistic machine learning community and an equally pragmatic, biologically oriented 'neuromorphic computing' community. Algorithms found in nature that use the non-synchronous, spiking nature of neuronal signals have been found to be (i) implementable efficiently in silicon and (ii) computationally useful. As a result, interest has grown in techniques that could create mixed 'siliconeural' computers. Here, we discuss potential approaches and focus on one particular platform using parylene-patterned silicon dioxide.

Serum protein layers on parylene-C and silicon oxide: effect on cell adhesion

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We studied how cell adhesion is affected by serum protein adsorbed on parylene-C.

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Serum proteins form distinct layers when adsorbed onto parylene-C or silicon oxide.

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Biosensing technique elucidates contrasting protein layer densities and thicknesses.

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Fibronectin supports cell adhesion on both surfaces.

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Albumin outcompetes fibronectin on parylene-C and vice versa on silicon oxide.

Among the range of materials used in bioengineering, parylene-C has been used in combination with silicon oxide and in presence of the serum proteins, in cell patterning. However, the structural properties of adsorbed serum proteins on these substrates still remain elusive. In this study, we use an optical biosensing technique to decipher the properties of fibronectin (Fn) and serum albumin adsorbed on parylene-C and silicon oxide substrates. Our results show the formation of layers with distinct structural and adhesive properties. Thin, dense layers are formed on parylene-C, whereas thicker, more diffuse layers are formed on silicon oxide. These results suggest that Fn acquires a compact structure on parylene-C and a more extended structure on silicon oxide. Nonetheless, parylene-C and silicon oxide substrates coated with Fn host cell populations that exhibit focal adhesion complexes and good cell attachment. Albumin adopts a deformed structure on parylene-C and a globular structure on silicon oxide, and does not support significant cell attachment on either surface. Interestingly, the co-incubation of Fn and albumin at the ratio found in serum, results in the preferential adsorption of albumin on parylene-C and Fn on silicon oxide. This finding is supported by the exclusive formation of focal adhesion complexes in differentiated mouse embryonic stem cells (CGR8), cultured on Fn/albumin coated silicon oxide, but not on parylene-C. The detailed information provided in this study on the distinct properties of layers of serum proteins on substrates such as parylene-C and silicon oxide is highly significant in developing methods for cell patterning.

Cell patterning on photolithographically defined parylene-C: SiO2 substrates

Cell patterning platforms support broad research goals, such as construction of predefined in vitro neuronal networks and the exploration of certain central aspects of cellular physiology. To easily combine cell patterning with Multi-Electrode Arrays (MEAs) and silicon-based ‘lab on a chip’ technologies, a microfabrication-compatible protocol is required. We describe a method that utilizes deposition of the polymer parylene-C on SiO₂ wafers. Photolithography enables accurate and reliable patterning of parylene-C at micron-level resolution. Subsequent activation by immersion in fetal bovine serum (or another specific activation solution) results in a substrate in which cultured cells adhere to, or are repulsed by, parylene or SiO₂ regions respectively. This technique has allowed patterning of a broad range of cell types (including primary murine hippocampal cells, HEK 293 cell line, human neuron-like teratocarcinoma cell line, primary murine cerebellar granule cells, and primary human glioma-derived stem-like cells). Interestingly, however, the platform is not universal; reflecting the importance of cell-specific adhesion molecules. This cell patterning process is cost effective, reliable, and importantly can be incorporated into standard microfabrication (chip manufacturing) protocols, paving the way for integration of microelectronic technology.

Patterning human neuronal networks on photolithographically engineered silicon dioxide substrates functionalized with glial analogues

Interfacing neurons with silicon semiconductors is a challenge being tackled through various bioengineering approaches. Such constructs inform our understanding of neuronal coding and learning and ultimately guide us toward creating intelligent neuroprostheses. A fundamental prerequisite is to dictate the spatial organization of neuronal cells. We sought to pattern neurons using photolithographically defined arrays of polymer parylene-C, activated with fetal calf serum. We used a purified human neuronal cell line [Lund human mesencephalic (LUHMES)] to establish whether neurons remain viable when isolated on-chip or whether they require a supporting cell substrate. When cultured in isolation, LUHMES neurons failed to pattern and did not show any morphological signs of differentiation. We therefore sought a cell type with which to prepattern parylene regions, hypothesizing that this cellular template would enable secondary neuronal adhesion and network formation. From a range of cell lines tested, human embryonal kidney (HEK) 293 cells patterned with highest accuracy. LUHMES neurons adhered to pre-established HEK 293 cell clusters and this coculture environment promoted morphological differentiation of neurons. Neurites extended between islands of adherent cell somata, creating an orthogonally arranged neuronal network. HEK 293 cells appear to fulfill a role analogous to glia, dictating cell adhesion, and generating an environment conducive to neuronal survival. We next replaced HEK 293 cells with slower growing glioma-derived precursors. These primary human cells patterned accurately on parylene and provided a similarly effective scaffold for neuronal adhesion. These findings advance the use of this microfabrication-compatible platform for neuronal patterning. © 2013 The Authors. Journal ofBiomedicalMaterials Research Part APublished byWiley Periodicals, Inc.Wiley Periodicals, Inc. J Biomed Mater Res Part A: 102A: 1350–1360, 2014.