Microvascular Composites Research Articles

Influence of microvascular structured void content on composites subjected to in-plane shear loads

Microvascular fiber reinforced composites can be used as multifunctional structures capable of self-healing, thermal regulation, and communication, among others. The inclusion of a small volume of microchannels generally has a minimal impact on the mechanical properties of the composites. In the current work, the in-plane shear properties of microvascular composites containing embedded stainless steel and wall-less microchannels produced through sacrificial material removal were characterized for the first time. Microvascular unidirectional composites were manufactured with precisely located microchannels both aligned with and transverse to the fiber direction, and their in-plane shear properties were tested. Microchannels aligned with the fiber direction were shown to cause a substantial (−7%) decrease in shear strength but had no effect on shear modulus or failure strain. Distortion of the laminate surface around the channels oriented transverse to the fiber axis was observed, resulting in an increase in void content (1–4%), a 9% loss in shear modulus, and a 27% loss in shear strength. The use of a caul plate made the loss in shear strength statistically insignificant, increased the shear modulus by 12–15%, and decreased the shear strain by −25% relative to the baseline composite without microchannels. Digital image correlation showed that the surface strains for transversely oriented samples were interrupted near the microchannels, but post-test x-ray computed tomography and optical microscopy do not show crack redirection. The newfound understanding of the shear response of microvascular composites provides important insights that will enable future engineering designs of multifunctional aerospace structures.

Modeling Thermal Regulation in Thin Vascular Systems: A Mathematical Analysis

Mimicking vascular systems in living beings, designers have realized microvascular composites to achieve thermal regulation and other functionalities, such as electromagnetic modulation, sensing, and healing. Such material systems avail circulating fluids through embedded vasculatures to accomplish the mentioned functionalities that benefit various aerospace, military, and civilian applications. Although heat transfer is a mature field, control of thermal characteristics in synthetic microvascular systems via circulating fluids is new, and a theoretical underpinning is lacking. What will benefit designers are predictive mathematical models and an in-depth qualitative understanding of vascular-based active cooling/heating. So, the central focus of this paper is to address the remarked knowledge gap. First, we present a reduced-order model with broad applicability, allowing the inlet temperature to differ from the ambient temperature. Second, we apply mathematical analysis tools to this reduced-order model and reveal many heat transfer properties of fluid-sequestered vascular systems. We derive point-wise properties (minimum, maximum, and comparison principles) and global properties (e.g., bounds on performance metrics such as the mean surface temperature and thermal efficiency). These newfound results deepen our understanding of active cooling/heating and propel the perfecting of thermal regulation systems.

Mechanical properties of additively printed, UV cured, continuous fiber unidirectional composites for multifunctional applications

This paper investigates the mechanical properties and microstructure of additively manufactured, continuous fiber reinforced composites made by the Continuous Fiber 3D Printing (CF3D®) process. We specifically investigate composites of GF-2, a high temperature thermosetting acrylic polymer, and T-1100, a high strength carbon fiber, with a 41.5% fiber volume fraction. The resulting composites have a longitudinal tensile modulus of 122 GPa and a strength of 1599 MPa, which is 89% and 55% of the theoretical value by rule of mixtures, respectively. This appears to be the highest strength additively manufactured composites to date, largely due to the high mechanical properties of the T-1100 fiber, low defects, and substantial fiber volume fraction. Comparable results are obtained for a cationic cure resin, CATPRO14. We optically measured the fiber–fiber contact surface area fraction of GF-2 and employing the unidirectional strength model of Karam to predict a strength efficiency of 0.6, which is in good agreement with mechanical estimate of 0.55. Comparing results with the efficiency factors of high fiber volume prepreg laminates, we determine the printing process yields composites with 88%–95% of the anticipated longitudinal modulus and 65%–70% of the anticipated longitudinal tensile strength of those processed by traditional methods. To extend the printing to multifunctional microvascular composites, we consider the mechanical implications of including sacrificial filaments of 100–300 μm within each tow. A modest decrease in longitudinal strength in proportion to the microchannel volume fraction is predicted, where the decrease is mainly attributed to the increased fiber-to-fiber interactions in more tightly packed tows.

Association of hemoglobin A1c time in range with risk for diabetes complications

IntroductionWe assessed the association between hemoglobin A1c time in range (A1c TIR), based on unique patient-level A1c target ranges, with risks of developing microvascular and macrovascular complications in older adults...

Multi-physics design of a new battery packaging for electric vehicles utilizing multifunctional composites

A multi-physics optimization framework is presented to design a new battery packaging for electric vehicles (EV). This battery packaging utilizes two types of multifunctional composites: structural battery composites (SBC) and microvascular composites (MVC). SBC has profound potential in harvesting electrical energy, and MVC shows promising capability in providing thermal regulation for battery packs. The proposed optimization scheme is aimed to maximize the driving range of EVs while making sure that the battery pack during operation would not overheat and also shows promising mechanical integrity. To reduce the computational burden of this multi-physics problem, a new reduced-order thermal/hydraulic model for transient analysis of MVC is proposed in this study as a part of the optimization framework. The thickness of constituents in the battery cell, carbon fiber volume fraction of the electrodes, number of microvascular composite panels for thermal regulations, and fiber directions in the battery cell plies are considered as design parameters. A parametric study is performed to evaluate the effect of each one of these design parameters on the driving range of an EV as well as overheating and structural integrity of battery packaging. The optimized battery packaging design obtained from the suggested optimization framework shows about a 23% increase in the driving range of Tesla model S.

Topology optimization of microvascular composites for active-cooling applications using a geometrical reduced-order model

In this study, we develop a design methodology with a basis in gradient-based topology optimization and a geometrical reduced-order thermal/hydraulic model for actively cooled microvascular composite panels. The proposed method is computationally very efficient owing to the suggested simplifications while preserving the required accuracy. The analytical sensitivity for the topology optimization scheme is derived. Several numerical examples are solved to demonstrate the applicability of the proposed method for active-cooling applications. Using topology optimization, the maximum temperature of the composite panel is reduced by up to 59% compared to a benchmark design. The optimization framework is compared to hybrid topology/shape (HyTopS) and shape optimization (SO) methods based on several measures such as maximum and average temperatures, temperature uniformity, network redundancy, and manufacturability. The solution obtained from the proposed TO scheme outperforms the other approaches in terms of the aforementioned measures.

Hybrid topology/shape optimization under uncertainty for actively-cooled nature-inspired microvascular composites

We present a computational framework for Hybrid Topology/Shape (HyTopS) optimization of actively-cooled microvascular composites under uncertainty. We developed a novel HyTopS optimization scheme for microvascular composites which can perform the topological change during the shape optimization process. This task has been done by introducing a new set of design parameters to add/remove microchannels and change the topology of the network. The profound advantage of the HyTopS method in contrast to a solely shape optimization approach is that the design space in HyTopS is not limited to the topology of the initial configuration. We integrate the HyTopS optimization method with the non-intrusive polynomial chaos expansion (PCE) approach to conduct a robust and reliable design. The non-intrusive nature of the proposed method allows for almost any source of uncertainty to be incorporated virtually in the design optimization process. The PCE representations of the response metrics enable the optimizer to efficiently and precisely approximate the statistical moments, failure probabilities, and their sensitivities with respect to the design variables. We solve several numerical examples to demonstrate the advantages of using the suggested optimization scheme over deterministic optimization method for microvascular composites. The results reveal that the optimized designs obtained from performing optimization process under uncertainty outperform the optimized configurations of the deterministic optimization approach in terms of reducing the sensitivity of the design performance to the various random variables.

How to design a blockage-tolerant cooling network?

Nature-inspired microvascular composites rely on microchannel networks to enable a variety of multifunctionality including self-healing and active cooling. However, the small size of the microchannels makes them highly susceptible to blockage from the presence of particulates in the fluid or damage in the vascular network. Blockage in the microchannels may disrupt the functionality of the microvascular composites. As an effort to alleviate the destructive effects of microchannel blockage, we present a Hybrid Topology/Shape (HyTopS) optimization approach to design actively-cooled microvascular networks for blockage tolerance. The proposed method shows a promising capability in producing a reliable design for blockage tolerance. In this hybrid scheme, the optimizer can modify the topology of the design during the shape optimization procedure. Being able to change the topology of the network enables the optimizer to provide network redundancy to effectively optimize the design for blockage tolerance. In this scheme, the optimizer takes into consideration the blockage scenarios in each iteration. As a result, the design will be optimized in such a way that it would perform satisfactorily when blockage of microchannels occurs. The main challenge in this method, similar to the other common approaches for blockage tolerance, is the extensive computational cost. To address this issue, we present a novel Importance Factor (IF) method and combined it with the HyTopS approach to reduce the computational cost significantly. For further reduction of computational cost, we use Interface-enriched Generalized Finite Element Method (IGFEM) wherein the design geometry is projected onto a fixed mesh. Thus, there is no need for remeshing during the optimization procedure. Moreover, a fully analytic sensitivity analysis of the shape optimization of the microvascular composites is also developed. Finally, we solve several numerical examples to investigate the pros and cons of using the proposed optimization approaches for blockage tolerance.

Gradient-based hybrid topology/shape optimization of bioinspired microvascular composites

Construction of bioinspired vasculature in synthetic materials enables multi-functional performance via mass transport through internal fluidic networks. However, exact reproduction of intricate, natural microvascular architectures is nearly impossible and thus there is a need to create practical, manufacturable designs guided by multi-physics principles. Here we present a Hybrid Topology/Shape (HyTopS) optimization scheme for microvascular materials using the Interface-enriched Generalized Finite Element Method (IGFEM). This new approach, which can simultaneously perform topological changes as well as shape optimization of microvascular materials, is demonstrated in the context of thermal regulation. In the current study, we present a new feature that enables the optimizer to augment network topology by creating/removing microchannels during the shape optimization process. This task has been accomplished by introducing a new set of design parameters, which act analogous to the penalization factor in the Solid Isotropic Material with Penalization (SIMP) method. The analytical sensitivity for the HyTopS optimization scheme has been derived and the sensitivity accuracy is verified against the finite difference method. We impose a set of geometrical constraints to account for manufacturing limitations and produce a design which is suitable for large-scale production without the need to perform post-processing on the obtained optimum. The method is validated by active-cooling experiments on vascularized carbon-fiber composites. Finally, we compare various application examples to demonstrate the advantages of the newly introduced HyTopS optimization scheme over solely shape optimization for microvascular materials.

Mechanical experiment evaluation of the microvascular self-healing capability of bitumen using hollow fibers containing oily rejuvenator

Smart microvascular self-healing bituminous composites are considered a promising approach to prolong pavement service life. The objective of this work was to investigate the microvascular self-healing capability of bituminous material by using hollow fibers with oily rejuvenator through a direct-tension mechanical experiment. The hollow fibers with oily rejuvenator were prepared using a wet-spinning approach by applying polyvinylidene fluoride material. The fixed-length hollow fibers with sealed ends were mixed in bitumen to form composite samples. Through a repetitive tensile method, the tension strength data were used to calculate the self-healing efficiency autonomously by considering crack closure and healing at various healing temperatures and times. The scanning-electron-microscope morphology showed that the polyvinylidene fluoride hollow fibers had a tight interfacial structure with bituminous material without debonding. X-ray computed tomography results indicated that the fibers were distributed homogeneously in bitumen. The fiber content and fiber orientation affected the self-healing capability of bituminous samples at 0 °C. To simplify the influence of the above two factors of the fibers, only one fiber was placed in bitumen samples parallel to the tension direction. With an increase in temperature from 0 to 30 °C, the self-healing capability of the bitumen samples increased dramatically. This phenomenon is attributed to the accelerated penetration speed of rejuvenator in the bitumen. An increase in time increased the self-healing capability of the bitumen samples. The rejuvenator may have sufficient time to leak from the hollow fibers and penetrate the bitumen. The results provide a guide to the microstructural design of the vascular fibers and the application of hollow fibers in self-healing asphalt pavement.

Active cooling of microvascular composites for battery packaging

Batteries in electric vehicles (EVs) require a packaging system that provides both thermal regulation and crash protection. A novel packaging scheme is presented that uses active cooling of microvascular carbon fiber reinforced composites to accomplish this multifunctional objective. Microvascular carbon fiber/epoxy composite panels were fabricated and their cooling performance assessed over a range of thermal loads and experimental conditions. Tests were performed for different values of coolant flow rate, channel spacing, panel thermal conductivity, and applied heat flux. More efficient cooling occurs when the coolant flow rate is increased, channel spacing is reduced, and thermal conductivity of the host composite is increased. Computational fluid dynamics (CFD) simulations were also performed and correlate well with the experimental data. CFD simulations of a typical EV battery pack confirm that microvascular composite panels can adequately cool battery cells generating 500 W m−2 heat flux below 40 °C.

A liquid metal-based structurally embedded vascular antenna: II. Multiobjective and parameterized design exploration

Research on the structurally embedded vascular antenna concept leverages past efforts on liquid metal (LM) reconfigurable electronics, microvascular composites, and structurally integrated and reconfigurable antennas. Such a concept has potential for reducing system weight or volume while simultaneously allowing in situ adjustment of resonant frequencies and/or changes in antenna directivity. This work considers a microvascular pattern embedded in a laminated composite and filled with LM. The conductive liquid provides radio frequency (RF) functionality while also allowing self-cooling. Models describing RF propagation and heat transfer, in addition to the structural effects of both the inclusion of channels and changes in temperature, were described in part 1 of this two-part work. In this part 2, the engineering models developed and demonstrated in part 1 toward the initial exploration of design trends are implemented into multiple optimization frameworks for more detailed design studies, one of which being novel and particularly applicable to this class of problem. The computational expense associated with the coupled multiphysical analysis of the structurally embedded LM transmitting antenna motivates the consideration of surrogate-based optimization methods. Both static and adaptive approaches are explored; it is shown that iteratively correcting the surrogate leads to more accurate optimized design predictions. The expected strong dependence of antenna performance on thermal environment motivates the consideration of a novel ‘parameterized’ optimization approach that simultaneously calculates whole families of optimal designs based on changes in design or operational variables generally beyond the control of the designer. The change in Pareto-optimal response with evolution in operating conditions is clearly demonstrated.

Gradient-based design of actively-cooled microvascular composite panels

Recent advances in manufacturing based on sacrificial fiber or template techniques have allowed complex networks of microchannels to be embedded in microvascular composites. In the thermal application of interest, a novel battery packaging scheme for electric vehicles is considered where each battery is surrounded by microvascular composite panels for temperature regulation and structural protection. We use simplified thermal and hydraulics models validated against more complex 3D FLUENT simulations and experiments to obtain the surface temperature distribution of the panel and the pressure drops across the microchannels. We further eliminate the cost and complexity associated with mesh generation by applying the interface-enriched generalized finite element method (IGFEM), which allows a non-conforming mesh to capture the discontinuous temperature gradient across the microchannels. The IGFEM thermal solver is then combined with a gradient-based shape optimization scheme to obtain optimal designs of a set of branched microchannel networks. The design parameters are the channel control points, which define the shape of the network. We use the p-mean as a differentiable objective function in place of the maximum temperature. To obtain accurate gradients with respect to the design parameters efficiently, we perform a sensitivity analysis based on a recently developed adjoint method for IGFEM. Starting from many distinct configurations, we obtain the optimal designs for a wide range of network topologies. We also investigate the effect of the coolant flow rate on the optimal design.

Survival of actively cooled microvascular polymer matrix composites under sustained thermomechanical loading

Exposure to high heat can cause polymer matrix composites (PMC) to fail under mechanical loads easily sustained at room temperature. However, heat is removed and temperature reduced in PMCs by active cooling through an internal vascular network. Here we compare structural survival of PMCs under thermomechanical loading with and without active cooling. Microchannels are incorporated into autoclave-cured carbon fiber/epoxy composites using sacrificial fibers. Time-to-failure, material temperature, and heat removal rates are measured during simultaneous heating on one face (5–75kW/m2) and compressive loading (100–250MPa). The effects of applied compressive load, heat flux, channel spacing, coolant flow rate, and channel distance from the heated surface are examined. Actively cooled composites containing 0.33% channel volume fraction survive without structural failure for longer than 30min under 200MPa compressive loading and 60kW/m2 heat flux. In dramatic comparison, non-cooled composites fail in less than a minute under the same loading conditions.

A gradient-based shape optimization scheme using an interface-enriched generalized FEM

Abstract A gradient-based shape optimization scheme using an Interface-enriched Generalized Finite Element Method (IGFEM) is presented wherein the design geometry is projected onto a fixed mesh and the IGFEM is used for analysis. This approach eliminates the mesh distortion present in conventional Lagrangian shape optimization methods, as well as the need for remeshing. An analytical sensitivity analysis using both the adjoint or direct approaches is presented to compute derivatives of the objective and constraint functions. Due to the fixed nature of the mesh, the so-called design velocity field only needs to be computed on the structure boundary/interface. A comparison between IGFEM- and conventional FEM-based shape optimization schemes is presented, showing an improved precision for the IGFEM approach. Finally, we solve various numerical examples to demonstrate the capability of the method including the computational design of particulate and microvascular composites.

Numerical simulations for microvascular shape memory polymer composites

Microvascular shape memory polymer (SMP) composites are a new class of active composites consisting of an embedded microvascular network in a SMP matrix. The microvascular network can be used to deliver thermal, chemical, electrical, and magnetic stimulation to the SMP matrix thus integrating the activation/deactivation mechanism and opening up a new functional space for active polymers. Here, we focus on thermomechanical coupling triggered through fluid transport within the polymer. A modified thermo-viscoelastic model is used to simulate the response of microvascular SMP composites. The model is developed within a finite deformation continuum mechanics framework and captures the free recovery response of the SMP composite. The present model includes two glass transition temperatures to describe the structural relaxation time and the stress relaxation time, respectively. The model results are calibrated with experimental data from the literature. Lined and unlined microvascular composites fabricated with a varying number of channels and thermal profiles were recorded by infrared camera. Using a range of thermal inputs, we calculate the corresponding free recovery response of microvascular SMP composites. We show that the response can be optimized by tailoring a number of controllable parameters including channel spacing, inlet temperature, fluid flow rate, and the heating/cooling protocol.

A NURBS-based interface-enriched generalized finite element scheme for the thermal analysis and design of microvascular composites

Motivated by recent advances in manufacturing techniques for high-temperature microvascular composites, a NURBS-based interface-enriched generalized finite element method (IGFEM) is developed to solve a simplified thermal model of microchannels embedded in the materials. This method is capable of handling curved and branched microchannels. Solutions more accurate than those achieved with the conventional finite element method can be obtained with coarse meshes that do not conform to the geometry of the microchannels. Near-optimal asymptotic convergence rate is also achieved with this method even for highly curved microchannels. The capability of the numerical scheme is demonstrated by solving problems with complex microchannel configurations.

Computational analysis of actively-cooled 3D woven microvascular composites using a stabilized interface-enriched generalized finite element method

The computational design of an actively-cooled 3D woven microvascular composite plate with sinusoidal and straight microchannels is presented. The design objectives include minimizing the maximum temperature of the composite, the microchannel volume fraction, and the pressure drop needed to circulate the coolant in the microchannels. We study the impact of a variety of parameters on the optimal design of a microvascular composite plate subjected to a uniform heat flux over its bottom surface. These parameters include the spacing, wavelength, and amplitude of the microchannels, the coolant type and flow rate, and the applied thermal loads. To facilitate the computational design process, a mesh-independent Interface-enriched Generalized Finite Element Method (IGFEM) is employed to evaluate the temperature field in the actively-cooled composite. The IGFEM solver also includes the streamline upwind Petrov-Galekin stabilization scheme to eliminate the spurious oscillations in the temperature field due to the convection-dominated heat transfer in the microchannels. This study reveals that the straight microchannels are often the optimal configuration. Design maps are presented to evaluate the required flow rate as a function of the applied thermal load and the plate dimensions.

Reducing cardiovascular risk in diabetes: insights from the ADVANCE study

BACKGROUND Blood pressure is an important determinant of the risks of macrovascular and microvascular complications of type 2 diabetes. That’s why guidelines recommend intensive lowering of blood pressure in those patients. The ADVANCE (Action in Diabetes and Vascular disease: preterAx and diamicroN-MR Controlled Evaluation) study has been recently introduced. It assessed the effects of such an approach on vascular disease using a fixed combination of the ACE-inhibitor perindopril, and the diuretic indapamide, in a population of patients with type 2 diabetes and a broad range of blood pressure values. METHODS After a 6-week active run-in period, 11,140 patients with type 2 diabetes were randomised to treatment with a fixed combination of perindopril and indapamide or matching placebo, in addition to current therapy. Primary end-points: major macrovascular and microvascular events, defined as death from cardiovascular disease, non-fatal stroke or non-fatal myocardial infarction, and new or worsening renal or diabetic eye disease. The macrovascular and microvascular composites were analysed jointly and separately. RESULTS To the end of the follow-up a meaningful reduction of the deaths has been obtained, both for cardiovascular causes and for macro and microvascular events, examined in together (in the total) (separately significance has not turned out). The best result has been obtained in the reduction of the renal events. DISCUSSION AND CONCLUSIONS The authors conclude that routine administration of a fixed combination of perindopril and indapamide to patients with type 2 diabetes and with a good controlled blood pressure, was well tolerated and reduced the risks of major vascular events, including death, but it remains the doubt if these results have been obtained because of the better pressure control or are specific of this preconstructed association.

Three‐Dimensional Microvascular Fiber‐Reinforced Composites

and materials or lack of scalability and vascular complexity of the fabrication approach. Here we show that the introduction of sacrificial fibers into woven preforms enables the seamless fabrication of 3D microvascular composites that are both strong and multifunctional. Underpinning the method is the efficient thermal depolymerization of catalyst-impregnated polylactide (PLA) fibers with simultaneous evaporative removal of the resulting lactide monomer. The hollow channels produced are high-fidelity inverse replicas of the original fiber’s diameter and trajectory. The method has yielded microvascular fiber-reinforced composites with channels over one meter in length that can be subsequently filled with a variety of liquids including aqueous solutions, organic solvents, and liquid metals. By circulating fluids with unique physical properties, we demonstrate the ability to create a new generation of biphasic pluripotent composite materials in which the solid phase provides strength and form while the liquid phase provides interchangeable functionality. Microvascular composite fabrication begins with the mechanized weaving of sacrifi cial fi bers into 3D woven glass