Microscale Resolution Research Articles

Residual stress effects of CMAS infiltration in high temperature jet engine ceramic coatings captured non-destructively with confocal Raman-based 3D rendering

Calcium-magnesium-aluminosilicates (CMAS) are ingested by jet engines and infiltrate into high temperature ceramic coatings during operation. This infiltration increases coating stiffness and promotes coating phase destabilization, encouraging micro-crack formation. Thermomechanical effects from CMAS infiltration were mapped over time with confocal Raman spectroscopy through residual stresses within EB-PVD 7YSZ coatings with microscale resolution. The results show an interplay between physical and chemical components influencing the residual stress. Physical mechanisms influence residual stress more after 1h of infiltration, inducing tensile loading up to 100 MPa on tetragonal ZrO2 Raman bands. Chemical mechanisms impart greater influence after 10h, inducing compressive loading up to 100 MPa. A monoclinic phase volume fraction of about 35% was observed as a transitional point for chemical mechanisms overtaking physical mechanisms in influencing residual stress. These results non-destructively elucidate changes within a coating’s residual stress during CMAS infiltration, aiding coating degradation monitoring during maintenance and towards implementing CMAS-mitigation strategies.

Additive manufacturing of micro-architected metals via hydrogel infusion.

Metal additive manufacturing (AM) enables the production of high value and high performance components1 with applications from aerospace2 to biomedical3 fields. Layer-by-layer fabrication circumvents the geometric limitations of traditional metalworking techniques, allowing topologically optimized parts to be made rapidly and efficiently4,5. Existing AM techniques rely on thermally initiated melting or sintering for part shaping, a costly and material-limited process6-8. We report an AM technique that produces metals and alloys with microscale resolution via vat photopolymerization (VP). Three-dimensional-architected hydrogels are infused with metal precursors, then calcined and reduced to convert the hydrogel scaffolds into miniaturized metal replicas. This approach represents a paradigm shift in VP; the material is selected only after the structure is fabricated. Unlike existing VP strategies, which incorporate target materials or precursors into the photoresin during printing9-11, our method does not require reoptimization of resins and curing parameters for different materials, enabling quick iteration, compositional tuning and the ability to fabricate multimaterials. We demonstrate AM of metals with critical dimensions of approximately 40 µm that are challenging to fabricate by using conventional processes. Such hydrogel-derived metals have highly twinned microstructures and unusually high hardness, providing a pathway to create advanced metallic micromaterials.

Biomechanical assessment of chronic liver injury using quantitative micro-elastography.

Hepatocellular carcinoma is one of the most lethal cancers worldwide, causing almost 700,000 deaths annually. It mainly arises from cirrhosis, which, in turn, results from chronic injury to liver cells and corresponding fibrotic changes. Although it is known that chronic liver injury increases the elasticity of liver tissue, the role of increased elasticity of the microenvironment as a possible hepatocarcinogen is yet to be investigated. One reason for this is the paucity of imaging techniques capable of mapping the micro-scale elasticity variation in liver and correlating that with cancerous mechanisms on the cellular scale. The clinical techniques of ultrasound elastography and magnetic resonance elastography typically do not provide micro-scale resolution, while atomic force microscopy can only assess the elasticity of a limited number of cells. We propose quantitative micro-elastography (QME) for mapping the micro-scale elasticity of liver tissue into images known as micro-elastograms, and therefore, as a technique capable of correlating the micro-environment elasticity of tissue with cellular scale cancerous mechanisms in liver. We performed QME on 13 freshly excised healthy and diseased mouse livers and present micro-elastograms, together with co-registered histology, in four representative cases. Our results indicate a significant increase in the mean (×6.3) and standard deviation (×6.0) of elasticity caused by chronic liver injury and demonstrate that the onset and progression of pathological features such as fibrosis, hepatocyte damage, and immune cell infiltration correlate with localized variations in micro-elastograms.

Understanding Key Factors during Dod Inkjet Printing Towards Precise Fabrication of Micro Energy Systems

There is an ever-increasing need for micro energy systems that can power miniature, unmanned, or remotely-located devices. Drop-on-Demand (DOD) inkjet fabrication is uniquely positioned for this demand due to its low-cost processing with microscale resolution, high throughput, reproducibility, and ease in shape design. This study seeks to capitalize on the advantages of DOD inkjet printing for application to manufacturing micro energy conversion/storage systems, by defining the fundamental jet kinematics and key factors underpinning the inkjet printing process.The ink used in this study was a dilute colloidal ink suspension composed of a common solid oxide fuel cell (SOFC) cathode material [La0.6Sr0.4Fe0.8Co0.2O3 (LSFC)] and organic solvent α-terpineol. The results were evaluated systematically in terms os final deposition quality, resolution, and microstructure. Favorable fluid kinematics were identified that attained uniform, well-shaped, circular 0-D dots about 0.1 µm thick and 60 µm in diameter. To increases the microfeature thickness and x/y plane dimensions , multiple and/or sequential ink passes were employed. For printing 1-D lines, the spacing between droplets was imperative to fabricating quality micro features. Using appropriate conditions, 1-D lines with x/y dimensions < 100 µm and controllable z axis dimensions at 0.1 µm per printing pass with dense, open or networked microstructures were demonstrated. 2-D planes at the dimensions as small as 100 µm by 100 µm were also achieved with smooth surface and continuous intra-planar ceramic coverage. In the course of post-processing the inkjetted cathodes, it was observed that the submicron cathode films printed at optimal conditions, after being sintered at controlled conditions, were uniform, smooth and free of cracks or delamination.This research demonstrated through fundamental understanding and control of the inkjet fabrication process, cathode features can be printed with controlled dimension and quality towards producing a functioning micro SOFC operable at low temperatures. The knowledge gained from this research will be appliable to other micro energy conversion and storage systems.

Freeform 3D Ice Printing (3D-ICE) at the Micro Scale.

Water is one of the most important elements for life on earth. Water's rapid phase‐change ability along with its environmental and biological compatibility also makes it a unique structural material for 3D printing of ice structures reproducibly and accurately. This work introduces the freeform 3D ice printing (3D‐ICE) process for high‐speed and reproducible fabrication of ice structures with micro‐scale resolution. Drop‐on‐demand deposition of water onto a −35 °C platform rapidly transforms water into ice. The dimension and geometry of the structures are critically controlled by droplet ejection frequency modulation and stage motions. The freeform approach obviates layer‐by‐layer construction and support structures, even for overhang geometries. Complex and overhang geometries, branched hierarchical structures with smooth transitions, circular cross‐sections, smooth surfaces, and micro‐scale features (as small as 50 µm) are demonstrated. As a sample application, the ice templates are used as sacrificial geometries to produce resin parts with well‐defined internal features. This approach could bring exciting opportunities for microfluidics, biomedical devices, soft electronics, and art.

Dynamic volumetric imaging and cilia beat mapping in the mouse male reproductive tract with optical coherence tomography.

Spermatozoa transport within the male reproductive tract is a highly dynamic and biologically important reproductive event. However, due to the lack of live volumetric imaging technologies and quantitative measurements, there is little information on the dynamic aspect and regulation of this process. Here, we presented ex vivo dynamic volumetric imaging of the mouse testis, efferent duct, epididymis, and vas deferens at a micro-scale spatial resolution with optical coherence tomography (OCT). Micro computed tomography imaging is presented as a reference for the proposed OCT imaging. Application of functional OCT analysis allowed for 3D mapping of the cilia beat frequency in the efferent duct, which volumetrically visualized the spatial distribution of the ciliated cells and corresponding ciliary activities. Potentially these analyses could be expanded to in vivo settings through intravital approach. In summary, this study demonstrated that OCT has a great potential to investigate the microstructure and dynamics, such as cilia beating, muscle contractions, and sperm transport, within the male reproductive tract.

Direct Laser Writing of Graphitic Carbon from Liquid Precursors

One-step synthesis and micropatterning of different types of carbon nanomaterials, such as amorphous carbon or three-dimensional graphene which have versatile electrochemical, thermal, and mechanical properties, are advantageous for the fabrication of microelectronics, sensors, and wearable devices. Here, we report a direct laser writing method to simultaneously synthesize and pattern graphitic carbon from liquid organic precursors. We have tested a wide range of liquid organic precursors and identified the chemical characteristics that are beneficial for successful laser-induced solvothermal deposition. The laser-deposited carbon exhibits a paracrystalline-to-polycrystalline structure and has electrical resistivities on the order of 10–3 to 10–4 Ω m which is tunable through variations in the laser power. Such properties of the laser-deposited carbon, coupled with the ability to direct-write custom patterns with microscale resolution, make these carbon materials exciting candidates for use in applications such as energy storage and sensing.

Electrochemical 3D printing of silver and nickel microstructures with FluidFM

Electrochemical 3D printing of conductors with microscale resolution holds a great promise for a wide range of applications, but the choice of suitable metals for these technologies remains limited. Most efforts so far have been focused on deposition of copper, however, other metals are also of interest, especially when tuning of mechanical, electrical, or optical properties is required for a given application. Here we address the issue of a limited materials choice in electrochemical 3D printing by extending the materials library to silver and nickel. Free-standing microscale structures are fabricated in a single step via locally confined electrochemical 3D printing using FluidFM - a microchanneled cantilever nanopipette capable to deliver electrolyte through sub-microscale opening. The 3D printed structures are constructed in a layer-by-layer fashion, which allows complex geometrical shapes such as double rings, helices and pyramids. We report the process performance in terms of printing speed (in the range 7 - 40 nm s-1 for silver and 27 - 42 nm s-1 for nickel) and reveal dense inner structure and chemical purity of the printed features.

Longitudinal shear wave elasticity measurements of millimeter-sized biomaterials using a single-element transducer platform.

Temporal variations of the extracellular matrix (ECM) stiffness profoundly impact cellular behaviors, possibly more significantly than the influence of static stiffness. Three-dimensional (3D) cell cultures with tunable matrix stiffness have been utilized to characterize the mechanobiological interactions of elasticity-mediated cellular behaviors. Conventional studies usually perform static interrogations of elasticity at micro-scale resolution. While such studies are essential for investigations of cellular mechanotransduction, few tools are available for depicting the temporal dynamics of the stiffness of the cellular environment, especially for optically turbid millimeter-sized biomaterials. We present a single-element transducer shear wave (SW) elasticity imaging system that is applied to a millimeter-sized, ECM-based cell-laden hydrogel. The single-element ultrasound transducer is used both to generate SWs and to detect their arrival times after being reflected from the side boundaries of the sample. The sample’s shear wave speed (SWS) is calculated by applying a time-of-flight algorithm to the reflected SWs. We use this noninvasive and technically straightforward approach to demonstrate that exposing 3D cancer cell cultures to X-ray irradiation induces a temporal change in the SWS. The proposed platform is appropriate for investigating in vitro how a group of cells remodels their surrounding matrix and how changes to their mechanical properties could affect the embedded cells in optically turbid millimeter-sized biomaterials.

Deep-Brain Three-Photon Imaging Enabled by Aggregation-Induced Emission Luminogens with Near-Infrared-III Excitation.

Understanding the morphology and hemodynamics of cerebral vasculature at large penetration depths and microscale resolution is fundamentally important to decipher brain diseases. Among the various imaging technologies, three-photon (3P) microscopy is of significance by virtue of its deep-penetrating capability and submicron resolution, which especially benefits in vivo vascular imaging. Aggregation-induced emission luminogens (AIEgens) have been recognized to be extraordinarily powerful as 3P probes. However, systematic studies on the structure-performance relationship of 3P AIEgens have been seldom reported. Herein, a series of AIEgens has been designed and synthesized. By intentionally introducing benzene rings onto electron donors (D) and acceptors (A), the molecular distortion, conjugation strength, and the D-A relationship can be facilely manipulated. Upon encapsulation with DSPE-PEG2000, the optimized AIEgens are successfully applied for 3P microscopy with emission in the far-red/near-infrared-I (NIR-I, 700-950 nm) region under the near-infrared-III (NIR-III, 1600-1870 nm) excitation. Impressively, using mice with an opened skull, vasculature within 1700 μm and a microvessel with a diameter of 2.2 μm in deep mouse brain were clearly visualized. In addition, the hemodynamics of blood vessels were well-characterized. Thus, this work not only proposes a molecular design strategy of 3P AIEgens but also promotes the performance of 3P imaging in cerebral vasculature.

Tissue Preparation Techniques for Contrast-Enhanced Micro Computed Tomography Imaging of Large Mammalian Cardiac Models with Chronic Disease.

Structural remodeling is a common consequence of chronic pathological stresses imposed on the heart. Understanding the architectural and compositional properties of diseased tissue is critical to determine their interactions with arrhythmic behavior. Microscale tissue remodeling, below the clinical resolution, is emerging as an important source of lethal arrhythmia, with high prevalence in young adults. Challenges remain in obtaining high imaging contrast at sufficient microscale resolution for preclinical models, such as large mammalian whole hearts. Moreover, tissue composition-selective contrast enhancement for three-dimensional high-resolution imaging is still lacking. Non-destructive imaging using micro-computed tomography shows promise for high-resolution imaging. The objective was to alleviate sufferance from X-ray over attenuation in large biological samples. Hearts were extracted from healthy pigs (N = 2), and sheep (N = 2) with either induced chronic myocardial infarction and fibrotic scar formation or induced chronic atrial fibrillation. Excised hearts were perfused with: a saline solution supplemented with a calcium ion quenching agent and a vasodilator, ethanol in serial dehydration, and hexamethyldisilizane under vacuum. The latter reinforced the heart structure during air-drying for 1 week. Collagen-dominant tissue was selectively bound by an X-ray contrast-enhancing agent, phosphomolybdic acid. Tissue conformation was stable in air, permitting long-duration microcomputed tomography acquisitions to obtain high-resolution (isotropic 20.7 µm) images. Optimal contrast agent loading by diffusion showed selective contrast enhancement of the epithelial layer and sub-endocardial Purkinje fibers in healthy pig ventricles. Atrial fibrillation (AF) hearts showed enhanced contrast accumulation in the posterior walls and appendages of the atria, attributed to greater collagen content. Myocardial infarction hearts showed increased contrast selectively in regions of cardiac fibrosis, which enabled the identification of interweaving surviving myocardial muscle fibers. Contrast-enhanced air-dried tissue preparations enabled microscale imaging of the intact large mammalian heart and selective contrast enhancement of underlying disease constituents.

Tissue Preparation Techniques for Contrast-Enhanced Micro Computed Tomography Imaging of Large Mammalian Cardiac Models with Chronic Disease

Structural remodeling is a common consequence of chronic pathological stresses imposed on the heart. Understanding the architectural and compositional properties of diseased tissue is critical to determine their interactions with arrhythmic behavior. Microscale tissue remodeling, below the clinical resolution, is emerging as an important source of lethal arrhythmia, with high prevalence in young adults. Challenges remain in obtaining high imaging contrast at sufficient microscale resolution for preclinical models, such as large mammalian whole hearts. Moreover, tissue composition-selective contrast enhancement for three-dimensional high-resolution imaging is still lacking. Non-destructive imaging using micro-computed tomography shows promise for high-resolution imaging. The objective was to alleviate sufferance from X-ray over attenuation in large biological samples. Hearts were extracted from healthy pigs (N = 2), and sheep (N = 2) with either induced chronic myocardial infarction and fibrotic scar formation or induced chronic atrial fibrillation. Excised hearts were perfused with: a saline solution supplemented with a calcium ion quenching agent and a vasodilator, ethanol in serial dehydration, and hexamethyldisilizane under vacuum. The latter reinforced the heart structure during air-drying for 1 week. Collagen-dominant tissue was selectively bound by an X-ray contrast-enhancing agent, phosphomolybdic acid. Tissue conformation was stable in air, permitting long-duration microcomputed tomography acquisitions to obtain high-resolution (isotropic 20.7 µm) images. Optimal contrast agent loading by diffusion showed selective contrast enhancement of the epithelial layer and sub-endocardial Purkinje fibers in healthy pig ventricles. Atrial fibrillation (AF) hearts showed enhanced contrast accumulation in the posterior walls and appendages of the atria, attributed to greater collagen content. Myocardial infarction hearts showed increased contrast selectively in regions of cardiac fibrosis, which enabled the identification of interweaving surviving myocardial muscle fibers. Contrast-enhanced air-dried tissue preparations enabled microscale imaging of the intact large mammalian heart and selective contrast enhancement of underlying disease constituents.

Messy or Ordered? Multiscale Mechanics Dictates Shape‐Morphing of 2D Networks Hierarchically Assembled of Responsive Microfibers

AbstractShape‐morphing active networks of mesoscale filaments are a common hierarchical feature in biology for applying forces, transporting materials, and inducing motility with microscale resolution. Synthetic morphing systems of similar dimensions and capabilities hold potential for a range of technological applications, from micro‐muscles to shape‐morphing optical devices. Here, the fabrication of highly‐ordered 2D networks hierarchically constructed of thermoresponsive mesoscale polymeric fibers, which can exhibit morphing with microscale resolution, is presented. It is demonstrated both experimentally and computationally that the morphing of such networks strongly depends on the physical attributes of the single fiber, in particular on two intrinsic length scales—the fiber diameter and mesh size, which stems from network's density. It is shown that depending on these parameters, such fiber‐networks exhibit one of two thermally driven morphing behaviors: i) the fibers stay straight, and the network preserves its ordered morphology, exhibiting a bulk‐like behavior; or ii) the fibers buckle and the network becomes messy and highly disordered. Notably, in both cases, the networks display memory and regain their original ordered morphology upon shrinking. This hierarchically induced phase transition, demonstrated here on a range of networks, offers a new way of controlling the shape‐morphing of synthetic materials with mesoscale resolutions.

Fabrication of Biomaterials and Biostructures Based On Microfluidic Manipulation.

Biofabrication technologies are of importance for the construction of organ models and functional tissue replacements. Microfluidic manipulation, a promising biofabrication technique with micro-scale resolution, can not only help to realize the fabrication of specific microsized structures but also build biomimetic microenvironments for biofabricated tissues. Therefore, microfluidic manipulation has attracted attention from researchers in the manipulationof particles and cells, biochemical analysis, tissue engineering, disease diagnostics, and drug discovery. Herein, biofabrication based on microfluidic manipulation technology is reviewed. The application of microfluidic manipulation technology in the manufacturing of biomaterials and biostructures with different dimensions and the control of the microenvironment is summarized. Finally, current challenges are discussed and a prospect of microfluidic manipulation technology is given. The authors hope this review can provide an overview of microfluidic manipulation technologies used in biofabrication and thus steer the current efforts in this field.

Aerosol Jet® Printing on stereolithography resin substrates for in-vitro dual bioreactor sensing

Aerosol Jet® Printing (AJ®P) is a relatively novel additive manufacturing technique, of the type direct writing, principally applied for printed electronics. Particularly, AJ®P has the ability to print various functional inks (conductive, dielectrics, biological solutions) at micro-scale resolution and on free-from substrates. This work investigates AJ®P to fabricate conductive patterns on stereolithography (SLA) samples towards the development of an innovative bioelectrical device for in-vitro dual bioreactor sensing. Silver nanoparticle (AgNPs) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) inks were both tested. Preliminary results show a low metal-polymer AgNPs-SLA interaction, revealing the presence of pores and cracks along with the printed pattern after thermal sintering, with no electrical resistance (R) detected. In contrast, the PEDOT:PSS ink reveals a good covering of the SLA samples, with R decreasing with the number of printed layers (from ~700 KΩ to ~60 Ω for 15 and 40 layers, respectively, on lines 1 cm long). However, using the PEDOT:PSS ink, the total printing time was ~45 minutes, hence not ideal for an industry-oriented vision. Therefore, a study on the optimization of the AgNPs-SLA sample interaction was carried out. SLA samples were coated with Parylene-C (ParC), and further treated with a low pressure plasma. The final results show excellent conductivity (R~2.40 Ω), with a printing time of ~5.5 minutes (7 layers), demonstrating the possibility to obtain a functional circuit. Eventually, the circuit was encapsulated with polydimethylsiloxane (PDMS) in order to avoid a release of Ag+ ions, potentially toxic in the medium culture.

The diversity and specificity of functional connectivity across spatial and temporal scales

Macroscopic neuroimaging modalities in humans have revealed the organization of brain-wide activity into distributed functional networks that re-organize according to behavioral demands. However, the inherent coarse-graining of macroscopic measurements conceals the diversity and specificity in responses and connectivity of many individual neurons contained in each local region. New invasive approaches in animals enable recording and manipulating neural activity at meso- and microscale resolution, with cell-type specificity and temporal precision down to milliseconds. Determining how brain-wide activity patterns emerge from interactions across spatial and temporal scales will allow us to identify the key circuit mechanisms contributing to global brain states and how the dynamic activity of these states enables adaptive behavior.

Monolithic digital patterning of polyimide by laser-induced pyrolytic jetting

Based on its outstanding mechanical, thermal, and chemical properties, a Polyimide (PI) is useful in a wide range of applications. Its usage in biomedicine is drawing great attention owing to the recent confirmation of the biocompatibility of various PIs. However, the conventional patterning of a PI, based on photolithographic methods, which is expensive and time-consuming, hampers the rapid advancement of research-oriented fields that require frequent design changes. To resolve this problem, we introduce the method of the monolithic digital patterning of PI up to the quasi-three-dimensional (3D) structures at the microscale resolution by laser-induced jetting of highly porous Laser-induced graphene (LIG) from the PI matrix. Pyrolytic jetting of the LIG is dependent not only on the laser-induced temperature but also on its temporal and spatial gradients. However, the surfaces of the remaining PI can be exceptionally smooth at the optimum laser condition, comparable to the pristine surface at the microscopic level, as confirmed by Raman spectroscopy and Atomic force microscopy (AFM) measurements. On-demand microfluidic channels and multilevel imprinting molds are created in a single step using the proposed method as a proof-of-concept, substantiating its potential application in relevant research.

Micro-scale resolution of carbon turnover in soil - Insights from laser ablation isotope ratio mass spectrometry on water-glass embedded aggregates

Soil aggregates may stabilize carbon at mineral surfaces and in the interior, but resolving such micro-scale carbon (C) turnover at a scale of soil aggregates <2 mm is challenged by C contaminations during sample preparation such as from resin embedding. Here we introduce a novel C-free embedding method using silica gel for water glass formation, and applied it to soil aggregates from a C3/C4 vegetation change soil chronosequence (4, 10 and 19 years of Miscanthus cropping on former C3 soil) for subsequent δ13C and C turnover analyses using laser ablation isotope ratio mass spectrometry (LA-IRMS). We hypothesized that C-free embedding allows for the first time the comparison of C turnover at soil aggregates both in their center and at their outer surface. We found that using water glass embedding enabled δ13C analyses via LA-IRMS in all parts of the sample, free of any contaminations. There was a large micro-scale heterogeneity in δ 13C signals within aggregates, which increased with cropping duration (total range −5.5 to –41.5‰). Noteworthy, after 19 years Miscanthus materials were still found preferentially at soil aggregate surfaces and hardly in interior parts, documenting slow aggregate turnover but also success of the embedding technique for future micro-scale C dynamic analyses in environmental samples.

Scanning Electrochemical Cell Microscope Study of Individual H2 Gas Bubble Nucleation on Platinum: Effect of Surfactants

Adsorption of gas bubbles on the electrode surface is one of important issues responsible for the reduced efficiency of water electrolysis. Understanding the bubble phenomena can benefit the management of gas bubbles. In this study, we carried out a single-entity electrochemical study of individual gas bubbles on the Pt electrode using scanning electrochemical cell microscopy. Different from conventional ensemble methods, the bubble nucleation behaviour and the effect of surfactants (perfluorooctanoic acid (PFOA), hexadecyl trimethyl ammonium bromide (CTAB) and siloxane defoamer) on the electrode surface were investigated at microscale resolution. Statistical analysis of individual nucleation event showed the stochastic characteristic and the addition of surfactant could significantly improve the stability of gas bubbles especially those formed in small nanopipets. A correlation between the critical supersaturation (or Faradic current) for bubble nucleation and surface tension with was observed, which was consistent with classic nucleation theory. However, chronoamperometry study showed that the water electrolysis efficiency after surfactant addition was not significantly improved. This study provided unique insights into understanding of nucleation event at single-entity.

Stress distribution at the AlN/SiC heterointerface probed by Raman spectroscopy

We grow AlN/4H-SiC and AlN/6H-SiC heterostructures by physical vapor deposition and characterize the heterointerface with micro-scale resolution. We investigate the spatial stress and strain distribution in these heterostructures using confocal Raman spectroscopy. We measure the spectral shifts of various vibrational Raman modes across the heterointerface and along the entire depth of the 4H- and 6H-SiC layers. Using the earlier experimental prediction for the phonon-deformation potential constants, we determine the stress tensor components in SiC as a function of the distance from the AlN/SiC heterointerface. Despite the fact that the lattice parameter of SiC is smaller than that of AlN, the SiC layers are compressively strained at the heterointerface. This counterintuitive behavior is explained by different coefficients of thermal expansion of SiC and AlN when the heterostructures are cooled from growth to room temperature. The compressive stress values are maximum at the heterointerface, approaching 1 GPa, and relax to the equilibrium value on the scale of several tens of micrometers from the heterointerface.