Laser writing of nitrogen-doped silicon carbide for biological modulation

Laser writing of nitrogen-doped silicon carbide for biological modulation
Schematic illustrations of the laser writing process and its applications. (A) CO2 laser writing a pattern on a PDMS substrate wherein the pattern can be (i) on the surface, (ii) a trench, or (iii) a cut-through leading to the formation of two distinct pieces. Architectures (i) to (iii) arise as a function of the laser power and writing speed. A graphite layer forms beneath the SiC because of the nature of the ablation process. (B) Laser-written electrodes are flexible electrodes that can integrate with a heart and stimulate it with electrical impulses leading to its pacing. (C) Laser-written circuits can be used for photoelectrochemical modulation of interconnected cellular ensembles. Credit: Science Advances, doi: 10.1126/sciadv.aaz2743

In materials science, conducting and semiconducting materials can be embedded in insulating polymeric substrates for useful biointerface applications. However, it is challenging to achieve the composite configuration directly using chemical processes. Laser-assisted synthesis is a fast and inexpensive technique used to prepare various materials but their applications in the construction of biophysical tools and biomedical materials remain to be explored. In a new report, Vishnu Nair and a research team in chemistry, molecular engineering, physics and atom probe tomography at the University of Chicago and Northwestern University, U.S., used laser writing to convert portions of polydimethylsiloxane (PDMS) into nitrogen-doped cubic silicon carbide (3C-SiC). They facilitated electrochemical and photoelectrochemical activity between the two surfaces by connecting the dense 3C-SiC surface layer to the PDMS matrix using a spongy graphite layer. They developed two-dimensional (2-D) silicon carbide patterns in PDMS and freestanding 3-D constructs. Nair et al. established the function of laser-produced composites by applying flexible electrodes for isolated heart pacing and photoelectrodes for local peroxide delivery to smooth muscle sheets. The work is now published on Science Advances.

Laser-assisted materials synthesis

Materials synthesis via laser-assisted processes are commonly used due to their ease of application, low cost and unique capacity to generate complex phases. Laser-produced composites can expand the design principles to develop materials and devices for biological sensing and activity. For example, scientists had previously used graphene/graphite-based conducting materials using laser writing to electrochemically sense metabolites in sweat. In the present work, the research team selected a material platform besides silicon to perform electronic, electrochemical, photochemical and photothermal control of multiscale biological components. Drawbacks of silicon (Si) include degradation under physiological conditions and limited electrochemical properties. Bioelectronics and biomaterials must facilitate operational flexibility more than structural precision. As a result, there is a demand in biointerface research to incorporate techniques of laser writing or nozzle-based printing to develop frugal and user-friendly materials and devices.

Laser writing of nitrogen-doped silicon carbide for biological modulation
SEM-EDS reveals the chemical composition of 3C-SiC-MnOx. a-d. Magnified view of crystals showing a coating of MnOx (x~2), created with electroless deposition. E-h. EDS map of various elements on crystals shown in (d). Credit: Science Advances, doi: 10.1126/sciadv.aaz2743

Silicon carbide

Nair et al. used silicon carbide (SiC) in this work due to its importance in the semiconductor industry. The cubic 3C polytype (3C-SiC) displayed high electron mobility, thermal conductivity, and saturation drift velocity, although its synthesis required stringent conditions. The team showed 2-D and 3-D laser patterning of 3C-SiC using PDMS (polydimethylsiloxane) as a precursor. They created a dense SiC layer using laser ablation under a nitrogen-rich atmosphere to produce composites with expected geometries. Together with an embedded graphite network, the SiC showed pseudocapacitive electrochemical behavior and photoelectrochemical activity. The scientists functionalized the SiC with manganese dioxide (MnO2 or MnOx) to improve its photoelectrochemical activity. Using these SiC-based devices, they directed the activity in isolated hearts and in cultured cells. The work showed how laser writing could efficiently produce flexible and multifunctional semiconductor/elastomer interactions for biointerface studies.

Laser writing of nitrogen-doped silicon carbide for biological modulation
Stimulation of primary human aortic smooth muscle cells with SiC biomimicry (A) The n-doping in 3C-SiC and subsequent band bending suggests the possibility of pure oxidation reactions after photostimulation. (B) Schematic of a photocurrent measurement setup (top) and a representative trace of a 3C-SiC photoresponse during a 10-ms light-emitting diode (LED) pulse of 375 nm revealing a photoanodic oxidation reaction. (C) Fluorescence kinetics measurements on 3C-SiC and 3C-SiC-MnO2 confirm oxidation of water to H2O2 and show the relative concentration of H2O2 produced by oxidation of water per square centimeter of material irradiated with light. (D) Schematic illustration of an exogenous H2O2 signaling pathway in smooth muscle cells. Exogenous peroxides cause increased activation of inositol triphosphate receptor (IP3R), inducing the release of calcium from internal stores such as endoplasmic reticulum (ER)/sarcoplasmic reticulum (SR) and uptake of exogenous calcium. (E) Representative traces of the differing calcium responses depending on the stimulation timing with respect to a contraction cycle. (F) Schematic of a device level implementation on a smooth muscle cell ensemble with a microscopy Z stack. Scale bar, 100 μm along the Z axis only. (G) 3D heatmaps showing a calcium wave propagating from the stimulation point in the cell ensemble. Scale bar, 50 μm. CB, conduction band; VB, valence band; GPCR, G protein–coupled receptor; RTK, receptor tyrosine kinase. Credit: Science Advances, doi: 10.1126/sciadv.aaz2743

Synthesis and structural characterization of silicon carbide

During the experiments, Nair et al. prepared a pure PDMS polymer slab and placed it on a commercial laser cutter platform to ablate the polymer into a pattern of interest. The process converted the material to a yellow solid with a thin, dark layer connection to the PDMS matrix. The team analyzed the structure using dark-field scanning transmission electron microscopy (HAADF-STEM), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). The results revealed an interface between a densely packed solid layer with well-faceted crystals and a spongy layered lattice network similar to graphite. The results confirmed the one-step synthesis of 3C-SiC connected to PDMS via a spongy graphite network, where a direct laser spot may have promoted the conversion of PDMS to SiC under high temperature, while lower temperature in the surrounding environment led to graphite formation. The resulting thermal gradient-based semiconductor-conductor junction is a requisite configuration for many electrochemical and photoelectrochemical devices.

2-D and 3-D printing and the pseudocapacitive nature of 3C-SiC electrodes

The team controlled the width and depth of the converted lines or trenches on a substrate after a single laser scan for controlled development of a semiconductor/elastomer composite. As proof of concept, they vectorized and printed a 2-D painting on PDMS and detected SiC in the details using Raman mapping. For 3-D printing, they used a layer-by-layer technique of SiC on the cut PDMS and a fresh layer of PDMS on top of it, to achieve interlayer SiC fusion. Using the printed 3C-SiC/graphite/PDMS composites, Nair et al explored the electrochemical properties of 3C-SiC. They accomplished this by preparing an electrode by electrically connecting the graphitic side of a scratched SiC/graphite patch to a copper wire using silver paste. Then they sealed the device and only exposed the densely packed SiC to the electrolyte. The recorded double-layer capacitance and reduced charge transfer resistance will be able to facilitate improved coupling between the composite surface and cells and tissues in biological modulation experiments.

Laser writing of nitrogen-doped silicon carbide for biological modulation
Structural analysis of laser-printed SiC using electron microscopy shows the formation of a graphite layer underneath. (A) HAADF-STEM image revealing a porous graphitic surface integrated with SiC. Higher-magnification images reveal layered graphite structures in the regions marked in blue and green. (B) Microtomed section of 3C-SiC with its diffraction pattern (C) taken on [011] indexing zone. (D) HAADF-STEM image showing cubic SiC lattice. (E) X-ray diffraction revealing a 3C polytype of SiC with stacking faults and graphite layer beneath. (F) An atom probe reconstruction of a 3C-SiC sample revealing nitrogen doping. Elemental mapping: red, C; blue, Si; and green, N. (G). Mass spectrum from APT indicating nitrogen present in SiC lattice and its bonding with carbon and silicon atoms. Scale bars, (A) 0.1 μm (left), 10 nm (middle), 10 nm (right); (B) 100 nm; (D) 1 nm; (F) 20 nm. A.U., arbitrary units. Credit: Science Advances, doi: 10.1126/sciadv.aaz2743

Developing flexible electrochemical electrodes for heart pacing and as photoelectrodes

The scientists then printed and tested the SiC-based flexible bioelectronic devices for tissue stimulation. After mounting a viable contracting rat heart, they placed a flexible SiC device against the left and right ventricles to deliver electrical stimulation to the heart. Upon stimulation, the heart rate simultaneously synchronized to the stimulation rate to disrupt the electrocardiography (ECG) signal indicating a clear overdrive pacing effect. When they ceased electrical stimulation, the heart returned to is slow atrioventricular node rhythm. The experiment showed how the SiC/graphite/PDMS composite was fully applicable for tissue and organ modulation. Nair et al. additionally studied the electrochemical activities of the SiC surface after optical excitation and the results indicated a photoanodic output of the printed 3C-SiC devices. They confirmed the observations via a chemical reaction to oxidize water to hydrogen peroxide and based on the results they proposed further investigations to understand the exact mechanism of the observed catalytic process. Since and other reactive oxygen species typically play an important role to modulate smooth muscle cells, the team studied the effects of H2O2 using 3C-SiC as a reservoir for muscle stimulation. Based on the outcomes they suggest remote therapeutic applications of the device to facilitate vasoconstriction in trauma surgeries or sphincter contraction after chronic spinal cord injury.

Laser writing of nitrogen-doped silicon carbide for biological modulation
Laser-printed SiC can form 2D and 3D structures. (A) Laser printing resolution determined by optical microscopy as a function of laser power and scan speed, represented as a contour plot. (B) Laser printing depth determined by optical microscopy as a function of laser power and scan speed, represented as a contour plot. (C) A painting that was vectorized and printed on PDMS. Scale bar, 1 cm. (D) Raman mapping of nitrogen defect luminescence on a printed pattern revealing SiC. Scale bar, 2 mm. (E) Nitrogen defect luminescence Raman spectrum of 3C-SiC. (F) Multilayer vector design for 3D printing. (G) Layer by layer printing method to obtain a stable 3D integrated structure. Photo credit: Jaeseok Yi, the University of Chicago. (i) Welding of subsequent layers, (ii) PDMS Piranha etching, and (iii) freestanding SiC-graphite structure. Credit: Science Advances, doi: 10.1126/sciadv.aaz2743

In this way, Vishnu Nair and colleagues demonstrated 2-D and 3-D laser writing of nitrogen-doped 3C-SiC on PDMS substrates. The resulting layer established a seamless hard-soft interface with PDMS. The flexible devices acted as stimulation electrodes for isolated hearts and as photoelectrodes for localized hydrogen peroxide production. The scientists aim to seamlessly integrate the semiconductor/elastomer composites in organ-on-a-chip or organoid-on-a-chip research, or in microfluidics systems for photoelectrochemical activity. Future studies will also precisely investigate the electrochemical mechanism underlying H2O2 production in the device.

More information: Vishnu Nair et al. Laser writing of nitrogen-doped silicon carbide for biological modulation, Science Advances (2020). DOI: 10.1126/sciadv.aaz2743

Duy Xuan Luong et al. Laser-Induced Graphene Composites as Multifunctional Surfaces, ACS Nano (2019). DOI: 10.1021/acsnano.8b09626

Yuanwen Jiang et al. Rational design of silicon structures for optically controlled multiscale biointerfaces, Nature Biomedical Engineering (2018). DOI: 10.1038/s41551-018-0230-1

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