Soft Composites Research Articles

Nonlinear mechanical response of finite-length soft composites with random dislocations

The effects of random dislocations on the vibrational properties of finite-length RNA and DNA macro-structures have been investigated by means of a harmonic Hamiltonian and the Green’s function method. The RNA molecule has been modeled using a half ladder model, and three models (a fishbone model and two different strand models) have been employed to model the structure of DNA. The lengths of the finite and cyclic systems are gradually increased to more accurately approximate the structures of RNA and DNA. Springs whose behaviors are governed by Hooke’s constitutive law have been used to represent the bonds between the masses, with the stiffness of the vertical springs randomly changing along the length of each model. This results in a more realistic representation of the inherent randomness of the studied structures. To investigate the effect of random dislocations on the mechanical response of the studied systems, it has been assumed that a random mass-spring ensemble has been knocked out of place by an external force. It has been found that increasing the number of building blocks along the length of the models suppresses the influence of dislocations on the vibration spectra of RNA and DNA. It was also observed that at low frequencies, the influence of dislocations becomes more pronounced. Besides, taking into account the collective mass of the sugar-phosphate backbone results in the appearance of gaps in the vibration spectra. By introducing dislocations into the models, additional dislocation-induced vibrational states appear in the DOS curves. What stands out from the results is that the responses of the studied systems to these changes are strictly nonlinear. The employed methodology can be applied to investigate the mechanical response of damaged RNA and DNA.

Effect of isotropy and anisotropy: toward understanding the fracture behavior of magnetoactive elastomers

Magnetoactive elastomers (MAEs) exhibit magneto-mechanical coupling and typically contain micron-sized spherical ferromagnetic particles embedded in an elastomeric matrix. Anisotropic particle arrangement in MAEs, achieved by curing the material in a magnetic field, offers tailored and enhanced magneto-mechanical coupling compared to isotropic configurations. While tunable MAE properties such as the effective modulus and vibrational response have been relatively well studied, a thorough understanding of their fracture mechanisms, particularly in MAEs with microstructural anisotropy or composites with similar structures remains almost completely unexplored. Here, we characterize the fracture mechanisms of anisotropic and isotropic unmagnetized MAEs using experiments and simulations, focusing on the effects of spherical particles in anisotropic chain-like configurations. Specifically, we experimentally measure the fracture toughness of MAEs containing different volume fractions of particles, and with particles arranged both randomly and aligned at 0°, 45°, and 90° to the loading direction. Results show that anisotropic MAEs with particle chains aligned with the load direction can improve fracture toughness by up to almost 600%, whereas isotropic MAEs increase the fracture toughness by only 420%, compared to the pure elastomer. Scanning electron microscopy of the post-fractured surface reveals toughening mechanisms at micro-scale, such as chain waviness, particle agglomeration, and particle distribution. Simulations using a finite element method-based decoupled phase field-cohesive zone model on simplified geometries qualitatively support imaging interpretations. Overall, this work explains how internal geometry, including waviness in particle chains, chain alignment, and particle agglomerations affects fracture of MAEs and generally soft composites with chain-like geometries of spherical particles. These results have engineering applications in improving fracture properties of smart soft composites relevant to soft robotics, tunable vibration absorbers, and noise attenuators.

Flexible tactile sensing of magnetic hydrogel composites based on electrical impedance tomography

Flexible and stretchable tactile sensors show promising applications in robotics, prosthetics, and wearable electronics. This paper demonstrates an electrical impedance tomography (EIT) based highly flexible, touch-sensitive pressure sensor using magnetic hydrogels. EIT technology has been explored for continuous distributed strain sensing and tactile imaging. Magnetic hydrogel composites are synthesized by simultaneous polymerization and crosslinking of acrylamide with magnetic carbonyl iron particles (CIPs). The effects of CIPs percentage on morphology, sensitivity, and electrical/mechanical properties is investigated. EIT accurately discerns pressure quantity, shape, and spatial distribution. Lower CIPs volume fractions (3%) exhibit maximum sensitivity to strain fields. Mechanical testing show the hydrogels have excellent fatigue resistance, ideal for repeated compression loading. The results demonstrate the potential of EIT with magnetic hydrogel composites for tactile sensing and imaging, with applications in soft robotics, prosthetics, and wearable electronics. The tunable sensitivity and multifunctionality make these soft composites promising materials for flexible electronics.

An alternative stress boundary condition in small deformations and its application to soft elastic composites and structures

Linear elasticity theory has been used extensively in the study of the elastic behavior of various perforated structures and composite materials requiring the accompaniment of appropriate boundary conditions to derive qualitatively correct and quantitatively referential solutions. When incorporating conventional boundary conditions, however, linear elasticity theory fails to predict certain essential phenomena associated with perforate structures and composite materials even when they undergo small deformations. For example, a soft elastic porous medium is appreciably stiffened when inflated despite the fact that the internal air pressure is significantly lower than the modulus of the medium itself. In this paper, we propose an improved stress boundary condition by simply incorporating a small change in the normal to the boundary during deformation. We show via numerical examples that in the context of linear elasticity theory, the use of this improved boundary condition offers the possibility of predicting the influence of initial or residual stress in a perforated structure on the elastic response of the structure to external loadings (which can never be captured with the use of conventional boundary conditions). We perform also large-deformation-based finite element simulations to verify the accuracy of the closed-form results obtained from the improved boundary condition for a soft elastic perforated structure with initial internal pressure. We believe that the idea presented in this paper will extend the applicability of linear elasticity theory and yield more accurate referential analytic results for soft elastic structures and composites.

Modified with ferrocenyl-containing oligo- and polysiloxanes multi-walled carbon nanotubes for soft conductive silicone composites

Surface modification of multi-walled carbon nanotubes (MWCNT) was achieved by covalent grafting of 1,3,5,7-tetra(2-ferrocenylethyl)-1,3,5,7-tetramethylcyclotetrasiloxane (Fc4D4) and in situ forming polyFc4D4 in the presence of catalytic mixture AlCl3 and metallic Al. For composite preparation, MWCNT modified with ferrocenyl-containing oligo- and polysiloxanes were incorporated into the silicone compound and high-quality soft flexible silicone composites was obtained. In a stress-strain tests of the obtained composites, elongation at break value was an approx. two times higher that in the initial silicone matrix. The conducted modification of MWCNT surface significantly improve dispersion of modified MWCNT in silicone polymer matrix even at higher filler concentration using simple solution blending method of composite preparation. The composites demonstrated conductivity at the level of semiconductors.

Optimising 4D imaging of fast-oscillating structures using X-ray microtomography with retrospective gating

Imaging the internal architecture of fast-vibrating structures at micrometer scale and kilohertz frequencies poses great challenges for numerous applications, including the study of biological oscillators, mechanical testing of materials, and process engineering. Over the past decade, X-ray microtomography with retrospective gating has shown very promising advances in meeting these challenges. However, breakthroughs are still expected in acquisition and reconstruction procedures to keep improving the spatiotemporal resolution, and study the mechanics of fast-vibrating multiscale structures. Thereby, this works aims to improve this imaging technique by minimising streaking and motion blur artefacts through the optimisation of experimental parameters. For that purpose, we have coupled a numerical approach relying on tomography simulation with vibrating particles with known and ideal 3D geometry (micro-spheres or fibres) with experimental campaigns. These were carried out on soft composites, imaged in synchrotron X-ray beamlines while oscillating up to 400 Hz, thanks to a custom-developed vibromechanical device. This approach yields homogeneous angular sampling of projections and gives reliable predictions of image quality degradation due to motion blur. By overcoming several technical and scientific barriers limiting the feasibility and reproducibility of such investigations, we provide guidelines to enhance gated-CT 4D imaging for the analysis of heterogeneous, high-frequency oscillating materials.

Elastomer-based soft syntactic foam with broadly tunable mechanical properties and shapability

Fabrication of lightweight composites using thermally responsive expandable microspheres (EM) embedded in an elastomeric matrix to form syntactic foams offers significant opportunities to create functional and structural composites for diverse applications. The morphology of the EM-elastomer composite on the micro- and macro-scales is significantly influenced by the fabrication sequence and parameters (composition, expansion time/temperature, etc.). Herein, we report the morphological evolution of an EM-elastomer composite through the fabrication processes and elucidate the interaction of relevant fabrication parameters. Unlike the majority of published reports on hard EM composites, the in-situ expansion of the closed-cell foam structure within a soft matrix leads to larger void sizes, and the concomitant expansive force of the EM impacts the polymer chain mobility within the soft composite. At the same time, higher EM loading improves the mechanical properties; the evolved microstructure leads to a lightweight but stiff structure. For the first time the report details the thermal expansion of EM under applied pretension, leading to irreversible prestrain-dependent morphological changes toward anisotropic mechanical properties. Additionally, the composites are shaped under constrained expansion. The study provides a comprehensive guide and expands the pathways for the practical application of EM-embedded soft syntactic foams in aerospace, electronics, wearables, robotics, etc.

Poisson function and volume ratio of soft anisotropic materials under large deformations

Accurate transverse deformation measurements are required for the estimation of the Poisson function and volume ratio. In this study, pure silicone and soft composite specimens were subjected to uniaxial tension, and the digital image correlation method was used to measure longitudinal and in- and out-of-plane transverse stretches. To minimize the effects of measurement errors on parameter estimation, the measured transverse stretches were defined in terms of the longitudinal stretch using a new formulation based on Poisson's ratios and two stretch-dependent parameters. From this formulation, Poisson functions and volume ratio for soft materials under large deformations were obtained. The results showed that pure silicone can be considered isotropic and nearly incompressible under large deformations, as expected. In contrast, Poisson's ratio of silicone reinforced with extensible fabric can exceed classical bounds, including negative value (auxetic behavior). The incompressibility assumption can be employed for describing the stress-stretch curve of pure silicone, while volume ratios are required for soft composites. Data of human skin, aortic wall, and annulus fibrosus from the literature were selected and analyzed. Except for the aortic wall, which can be considered nearly incompressible, the studied soft tissues must be regarded as compressible. All tissues presented anisotropic behavior.

Multifunctional porous soft composites for bimodal wearable cardiac monitors

AbstractWearable heart monitors are crucial for early diagnosis and treatment of heart diseases in non‐clinical settings. However, their long‐term applications require skin‐interfaced materials that are ultrasoft, breathable, antibacterial, and possess robust, enduring on‐skin adherence—features that remain elusive. Here, we have developed multifunctional porous soft composites that meet all these criteria for skin‐interfaced bimodal cardiac monitoring. The composite consists of a bilayer structure featuring phase‐separated porous elastomer and slot‐die‐coated biogel. The porous elastomer ensures ultrasoftness, breathability, ease of handling, and mechanical integrity, while the biogel enables long‐term on‐skin adherence. Additionally, we incorporated ε‐polylysine in the biogel to offer antibacterial properties. Also, the conductive biogel embedded with silver nanowires was developed for use in electrocardiogram sensors to reduce contact impedance and ensure high‐fidelity recordings. Furthermore, we assembled a bimodal wearable cardiac monitoring system that demonstrates high‐fidelity recordings of both cardiac electrical (electrocardiogram) and mechanical (seismocardiogram) signals over a 14‐day testing period.

(Invited) 3D Printing-Assisted Casting of Conducting Polymers for Bioelectronics

3D structured conducting polymers, known for their high conductivity, low impedance, and tunable form factor, are promising candidates for various bioelectronics applications. However, achieving a balance between structural complexity, electrical conductance, and processing generality has been challenging in these materials. In this talk, I will discuss our efforts to address this trade-off through 3D printing-assisted casting. Light-based 3D printing is used to create hollow molds, providing structural complexity and design freedom, whereas the casting nature imparts material versatility. The casting molds are made of superabsorbent polymers and printed in a partially hydrated state. Their dehydration provides significant enhancement in feature resolution and excellent thermal and mechanical properties, making them versatile for casting. After casting, over-hydration of the molds facilitate their energy-efficient removal. Using poly(3,4-ethylenedioxythiophene) (PEDOT) as a model system, complex architectures such as octet and truncated octahedron can be achieved. Compositing silver flakes with PEDOT through a thermal injection process leads to prints with conductivity over 6000 S/cm and high environmental stability. This method can potentially be applied to other hard-to-print soft materials and composites.

Soft, Tough, Antifatigue Fracture Elastomer Composites with Low Thermal Resistance through Synergistic Crack Pinning and Interfacial Slippage.

Soft elastomer composites are promising functional materials for engineer interfaces, where the miniaturized electronic devices have triggered increasing demand for effective heat dissipation, high fracture energy, and antifatigue fracture. However, such a combination of these properties can be rarely met in the same elastomer composites simultaneously. Here a strategy is presented to fabricate a soft, extreme fracture tough (3316J m-2) and antifatigue fracture (1052.56J m⁻2) polydimethylsiloxane/aluminum elastomer composite. These outstanding properties are achieved by optimizing the dangling chains and spherical aluminum fillers, resulting in the combined effects of crack pinning and interfacial slippage. The dangling chains that lengthen the polymer chains between cross-linked points pin the cracks and the rigid fillers obstruct the cracks, enhancing the energy per unit area needed for fatigue failure. The dangling chains also promote polymer/filler interfacial slippage, enabling effective deflection and blunting of an advancing crack tip, thus enhancing mechanical energy dissipation. Moreover, the elastomer composite exhibits low thermal resistance (≈0.12K cm2 W-1), due to the formation of a thermally conductive network. These remarkable characteristics render this elastomer composite promising for application as a thermal interface material in electronic devices.

Insight into the configuration of hard-soft carbon composites for high performance sodium-ion batteries

Hard carbon materials are considered suitable anode materials for sodium-ion batteries due to their abundant defect sites and large interlayer distance, which are respectively beneficial to the adsorption/desorption and insertion/extraction behaviors of Na+ during the electrochemical process. However, the poor electrical conductivity resulting from the intrinsic short-range ordered structure in the carbon skeleton and limited active sites significantly hinders their electrochemical performance. Herein, hard and soft carbon composites with various configurations are developed using a layer-by-layer coating method with SiO2 nanospheres as templates. These composites, including the hard carbon coated soft carbon composite structure (SC@HC), soft carbon coated hard carbon composite structure (HC@SC), and uniformly mixed soft and hard carbon composites (HSC), are prepared through confined-carbonization under the outermost SiO2 layer. The resulting hollow nanospheres possess ultra-small diameters (approximately 50 nm), ultra-thin wall thicknesses (5-6 nm), and different hard/soft carbon layer arrangements. Evaluation of sodium storage properties showed that the combination of soft and hard carbon outperformed pure soft or hard carbon, with HC@SC configuration offering materials with larger interlayer spacing, higher defect concentration, and a more complete conductive network. This enhances sodium storage capacity (293 mAh g-1@1 A g-1) and rate performance (242 mAh g-1@5 A g-1). The findings of this study present a novel approach to designing soft and hard carbon composites for high-performance sodium-ion batteries.

Thermally Conductive Elastomer Composites with High Toughness, Softness, and Resilience Enabled by Regulating Interfacial Structure and Dynamics.

The emerging applications of thermally conductive elastomer composites in modern electronic devices for heat dissipation require them to maintain both high toughness and resilience under thermomechanical stresses. However, such a combination of thermal conductivity and desired mechanical characteristics is extremely challenging to achieve in elastomer composites. Here this long-standing mismatch is resolved via regulating interfacial structure and dynamics response. This regulation is realized both by tuning the molecular weight of the dangling chains in the polymer networks and by silane grafting of the fillers, thereby creating a broad dynamic-gradient interfacial region comprising of entanglements. These entanglements can provide the slipping topological constraint that allows for tension equalization between and along the chains, while also tightening into rigid knots to prevent chain disentanglement upon stretching. Combined with ultrahigh loading of aluminum-fillers (90 wt%), this design provides a low Young's modulus (350.0kPa), high fracture toughness (831.5 J m-2), excellent resilience (79%) and enhanced thermal conductivity (3.20 W m-1 k-1). This work presents a generalizable preparation strategy toward engineering soft, tough, and resilient high-filled elastomer composites, suitable for complex environments, such as automotive electronics, and wearable devices.

Soft Gel Filler Embedded Elastomer with Surfactant Improved Interface for Dielectric Elastomer Actuators.

Stretchable materials are the foundation of dielectric actuators (DEAs) for artificial muscle. However, the inadequate dielectric constant of stretchable materials has always greatly limited the performance of artificial muscle. Recently, soft fillers have been proposed to improve the dielectric property and preserve the stretchability for softness, aiming to avoid the stiffening effect of traditional rigid fillers. As composites, an amount of interfacial region is generated, which remarkably affects composites' performance from dielectrics to mechanics. Herein, we demonstrate that the size effect, interfacial binding, and compatibility have a great impact on soft filler doped composites. Particularly, according to the liquid characteristics of soft fillers, we explore an interfacial modification method using surfactants. Composite breakdown strength is thus enhanced 2.2-fold from that in the control group due to the reduction of mismatch between fillers and matrix. Moreover, surfactants alleviate the well-known stiffening effect in small fillers. The area strain of the composites reaches 10.3 ± 0.4% at a low electric field of 7 MV/m, and a soft micropump is successfully assembled. These findings demonstrate a unique and combined interfacial influence of soft filler doped elastomer, which promotes the advancements of the dielectric elastomer artificial muscle.

Higher-Order Structural Analysis of a Transparent and Flexible High Thermal Conductive Liquid Crystalline Elastomer Sheet and Its Composite.

Transparency, flexibility, and high thermal conductivity are trade-offs. Specifically, we have investigated a cross-linked acrylic liquid crystal elastomer (LCE) that exhibits both transparency and flexibility while maintaining a high level of thermal conductivity. The transparent monodomain LCE sheet was achieved through a process of stretching an initially opaque polydomain sheet to 80% elongation and subsequently subjecting it to photocuring. The thermal conductivity in the stretching direction (x) of the monodomain LCE sheet was found to be 1.8 times higher than that of the prestretched polydomain sheet, consistent with findings from previous studies. However, in the orthogonal direction (y) to the stretching (x) direction, the thermal conductivity exhibited an even higher value, being 1.7 times greater than in the x-direction, with a value of 3.0 W/(m·K). This unique observation prompted us to conduct further investigation through higher-order structural analysis of these LCE sheets using 2D wide-angle X-ray scattering (WAXS) analysis. In the transparent sheet, the LCE molecules were aligned in the sheet in the stretching x-direction (monodomain structure) for the out-of-plane direction. However, in the in-plane x-direction, the molecular plane spacing exhibited random orientation at a period of 0.45 nm. In contrast, within the y-direction of the inner layer, the molecular plane spacing exhibited a uniaxial horizontal orientation at the same period length as in the x-direction. The heat energy entering into the y-direction once spreads to the x-direction, but it was considered that the reason for the higher thermal conductivity to the y-direction would be forming covalent bonds that function as new heat transmission paths, in the direction intersecting to the x-direction during photocuring. Therefore, we concluded that the synergistic effect of the high level of the ordered inner structure and covalent bonding structure due to cross-linking in the y-direction contributes to its higher thermal conductivity compared to that in the x-direction, which exhibits a random in-plane structure. Additionally, we have fabricated an LCE composite sheet filled with 75 vol % of alumina particles using a polydomain-type LCE as the base material. The composite sheet exhibits remarkable thermal conductivity in the thickness direction, measuring at 9.8 W/(m·K), while maintaining a flexibility characterized by an elastic modulus of 70 MPa. This thermal conductivity surpasses that of a nonmesogenic acrylic composite sheet with identical alumina particle filling, which measured at 3.9 W/(m·K), more than twice as much. The presence of the mesogen skeleton has been demonstrated to enhance heat transfer, even within soft composites, by facilitating the formation of an ordered structure.

Functional soft robotic composites based on organic photovoltaic and dielectric elastomer actuator.

Improving the energy efficiency of robots remains a crucial challenge in soft robotics, with energy harvesting emerging as a promising approach to address it. This study presents a functional soft robotic composite called OPV-DEA, which integrates flexible organic photovoltaic (OPV) and dielectric elastomer actuator (DEA). The composite can simultaneously generate electrostatic bending actuation and harvest energy from external lights. Owing to its simplicity and inherent flexibility, the OPV-DEA is poised to function as a fundamental building block for soft robots. This study aimed to validate this concept by initially establishing the fabrication process of OPV-DEA. Subsequently, experimental samples are fabricated and characterized. The results show that the samples exhibit a voltage-controllable bending actuation of up to 15.6° and harvested power output of 1.35 mW under an incident power irradiance of 11.7 mW/cm2. These performances remain consistent even after 1000 actuation cycles. Finally, to demonstrate the feasibility of soft robotic applications, an untethered swimming robot equipped with two OPV-DEAs is fabricated and tested. The robot demonstrates swimming at a speed of 21.7mm/s. The power consumption of the robot is dominated by a high-voltage DC-DC converter, with a value approximately 1.5 W. As a result, the on-board OPVs cannot supply the necessary energy during locomotion simultaneously. Instead, they contribute to the overall system by charging a battery used for the controller on board. Nevertheless, these findings suggest that the OPV-DEA could pave the way for the development of an unprecedented range of functional soft robots.

Understanding the Role of Loss Modulus of Viscoelastic Substrates in the Evaporation Dynamics of Sessile Drops.

Viscoelastic properties of soft substrates play a crucial role in the evaporation dynamics of sessile drops. Recent studies have revealed that the modification of the viscoelastic properties of substrates changes the dynamics of the three-phase contact line, consequently affecting the evaporation behavior of sessile drops. Notably, these modifications occur without any noticeable changes to the substrate's wetting characteristics or surface topography. However, the individual role of storage (G') and loss (G″) moduli of substrates on drop evaporation dynamics remains unexplored. In this study, we investigate the evaporation dynamics of water drops on two groups of poly(dimethylsiloxane)-based viscoelastic substrates possessing either identical G' with varying G″ or identical G″ with varying G'. Our study reveals that on a substrate with constant shear modulus (G'), a reduction of an order of magnitude in loss modulus shifts the evaporation process from the constant contact radius mode to the constant contact angle mode. We hypothesize that this observed shift in behavior stems from the varying viscoelastic dissipation influenced by the plateau modulus and characteristic relaxation time of polymer gels. Our hypothesis is further supported from the observation that the evaporation process persists on the substrate with constant loss modulus (G″). Our study advances the current understanding of drop evaporation on soft substrates that may find potential applications involving soft composites, biological entities, tissue engineering, and wearable electronics.

Measurement of tensile and shear strength of interfacial bonding in fiber reinforced soft composites combining experiments and simulation

Interfacial bonding strength plays a key role in the mechanical performance of fiber reinforced polymers. However, measurement of interfacial bonding strength in fiber reinforced soft composites (FRSCs) becomes a challenging issue, because the large deformation capability of soft matrix invalidates most of current testing methods. This paper demonstrates an advanced method combining experiments and simulation, which provides a convenient and reliable means to measure both tensile strength and shear strength of interfacial bonding for FRSCs. The specimen is made by embedding a fiber bundle in a soft plate for transverse tensile test. Various stress states along the fiber–matrix interface can be realized by controlling the inclination angle of fiber bundle to the stretching direction. Tensile strength and shear strength of interfacial bonding in FRSCs can be estimated based on the analysis of critical stress states. The comparison with the widely adopted fiber pull-out test indicates that the nominal shear strength obtained in pull-out test is severely underestimated.

Rheological and electrical percolation of reinforced soft polyvinyl chloride composites with different morphological carbon fillers

The study analyzed the electrical properties, mechanical performance, and rheological behavior of three kinds of high plasticizer-content polyvinyl chloride (PVC) composites, herein referred as SCBC, EGC and MCNTC, respectively filled with porous superconductive carbon black (SCB), flake-like expanded graphite (EG), and tubular multi-walled carbon nanotubes (MCNT). The impact of the conductive fillers' morphology on the electrical percolation and viscoelastic percolation performance of the composites were investigated to understand the conductive mechanisms. It revealed that the incorporation of SCB and MCNT can induce both viscoelastic and electrical percolation in the composites. The thresholds for viscoelastic percolation for SCBC and MCNTC are between 2.35-3.11 vol% and 2.72–3.59 vol%, respectively, which are lower than their electrical percolation thresholds of 3.85 vol% for SCBC and 3.65 vol% for MCNTC. The nature of PVC chains, which can transmit force but not electrons, results in a higher electrical percolation threshold than the rheological percolation threshold. Despite MCNT's poorer dispersion, its high aspect ratio and one-dimensional tubular shape enabled it to achieve electrical percolation at a lower loading level. Due to the poor dispersion, EG did not cause either viscoelastic or electrical percolation in the composites within the 0–9 vol% range.

Bottom-up and top-down VAT photopolymerization bioprinting for rapid fabrication of multi-material microtissues

Over the years, three-dimensional (3D) bioprinting has attracted attention for being a highly automated manufacturing system that allows for the precise design of living constructs where cells and biomaterials are displaced in predefined positions to recreate cell–matrix and cell–cell interactions similar to native tissues. Such technologies rarely offer multi-material features. In this paper, we present a new approach for bioprinting of multi-material tissue constructs using VAT photopolymerization at high resolution and fidelity. We developed a versatile dual-mode bioprinter that can easily be modulated to print in both top-down and bottom-up approaches. The custom-built platform was then used to fabricate microtissues and hydrogel microfluidic models. Combining bottom-up and top-down biofabrication tools can offer an optimal solution for hard–soft multi-material composites and for bioprinting tissue–tissue interface models. We demonstrated the possibility for hard–soft multi-material bioprinting by generating musculoskeletal tissue with integrated microvasculature. Combining multiple material bioprinting and microfluidic chips shows advantages in two aspects: precise regulation of microenvironment and accurate emulation of multi-tissue interfaces.