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Waste energy harvest using thermoelectric (TE) materials will be a potential solution to the serious environmental pollution and energy shortage problems. Due to limitations of current manufacturing techniques in geometry complexity and high density, TE devices are not widely utilized in daily life to gather waste energy. 3D printing brings an opportunity to solve the fabrication limitations. In this paper, a hybrid process was developed to fabricate thermoelectric materials by integrating hot pressing with stereolithography. The mold and punch were designed and printed to fabricate thermoelectric devices used on hot water tubes via stereolithography. The Sb2Te3 powders filled the 3D printed mold in a layered manner, and each layer of powders was compacted under the pressing of punch at a certain temperature and compressive force. The polymer mold was removed after the sintering process to form the final TE components. A series of experiments were conducted to identify the optimal heating temperature and compressive force. The microstructures morphology and electrical conductivity of fabricated Sb2Te3 samples were evaluated. This research work conducted a scientific investigation into the fabrication of TE material with a hybrid process, including hot pressing and 3D printing, to solve the current manufacturing challenges, providing perspectives on developments of TE devices used in various energy harvest applications.

The complex structures and functional material systems of natural organisms effectively cope with crisis-ridden living environments such as high temperature, drought, toxicity, and predator. Behind these excellent survival strategies evolved over hundreds of millions of years is a series of effective mechanical, optical, hydraulic, and electromagnetic properties. Bionic design and manufacturing have always attracted extensive attention, but the progress has been limited by the inability of traditional manufacturing techniques to reproduce microscopically complex structures and the lack of functional materials. Therefore, there is an urgent need for a fabrication technique with a high degree of fabrication freedom and using composites derived from biological materials. Vat photopolymerization, an emerging additive manufacturing (aka 3D printing) technology, exhibits high manufacturing flexibility in the integrated manufacturing of multi-material systems and multi-scale structures. Here, biomaterial-inspired heterogeneous material systems based on polymer matrices and nanofillers, and the introduction of magnetic and electric fields on the basis of conventional 3D printing systems to spatially and programmably distribute nanofillers are summarized, which provides a new strategy for fabricating anisotropic structures. The application of this versatile 3D printing system in fabricating mechanically reinforced structures, polymer/metal structures, self-actuating, and superhydrophobic structures is also elaborated.

Heterogeneous material systems consisting of metallic structures and polymer matrixes are of significance for applications such as integrated circuits, microelectromechanical devices, antennas, sensors, actuators, and metamaterials. Scaly-foot snail which lives in the deep ocean exhibits high strength and temperature resistance due to unique shells made of metal and polymer. Recently, different multi-material structures have been fabricated with metal deposition using multiple manufacturing processes. However, using these complicated hybrid processes is challenging to construct complexthree-dimensional (3D) structures of heterogeneous material with enhanced properties, high resolution, and time efficiency. Here, we establish a novel manufacturing strategy to build bioinspired hierarchical structures with heterogeneous material systems using electrically assisted stereolithography. The photocurable printing solution that can act as an electrolyte for charge transfer was developed, and the curing characteristic of the printing solution was further investigated. A fundamental understanding of the formation mechanism of metallic structures on the polymer matrix was studied through physics-based multi-scale modeling and simulations. The correlation between metallic structures morphology, printing solution properties, and printing process parameters, and their effects in building bioinspired hierarchical structures with heterogeneous materials were identified. Demonstrative test cases were built to verify the printing performance of the proposed approach. This research work will deliver a scalable additive manufacturing (AM) process that can facilitate various interesting applications based on bioinspired heterogeneous material and structures.

Over millions of years of evolution, nature has created organisms with overwhelming performances due to their unique materials and structures, providing us with valuable inspirations for the development of next-generation biomedical devices. As a promising new technology, 3D printing enables the fabrication of multiscale, multi-material, and multi-functional three-dimensional (3D) biomimetic materials and structures with high precision and great flexibility. The manufacturing challenges of biomedical devices with advanced biomimetic materials and structures for various applications were overcome with the flourishing development of 3D printing technologies. In this paper, the state-of-the-art additive manufacturing of biomimetic materials and structures in the field of biomedical engineering were overviewed. Various kinds of biomedical applications, including implants, lab-on-chip, medicine, microvascular network, and artificial organs and tissues, were respectively discussed. The technical challenges and limitations of biomimetic additive manufacturing in biomedical applications were further investigated, and the potential solutions and intriguing future technological developments of biomimetic 3D printing of biomedical devices were highlighted.

Three-dimensional (3D)-printed scaffolds have attracted considerable attention in recent years as they provide a suitable environment for bone cell tissue regeneration and can be customized in shape. Among many other challenges, the material composition and geometric structure have major impacts on the performance of scaffolds. Hydroxyapatite and tricalcium phosphate (HA/TCP), as the major constituents of natural bone and teeth, possess attractive biological properties and are widely used in bone scaffold fabrication. Many fabrication methods have been investigated in attempts to achieve HA/TCP scaffolds with microporous structure enabling cell growth and nutrient transport. However, current 3D printing methods can only achieve the fabrication of HA/TCP scaffolds with certain range of microporous structure. To overcome this challenge, we developed a slurry-based microscale mask image projection stereolithography, allowing us to form a HA/TCP-based photocurable suspension with complex geometry including biomimetic features and hierarchical porosity. Here, the curing performance and physical properties of the HA/TCP suspension were investigated, and a circular movement process for the fabrication of highly viscous HA/TCP suspension was developed. Based on these investigations, the scaffold composition was optimized. We determined that a 30 wt% HA/TCP scaffold with biomimetic hierarchical structure exhibited superior mechanical properties and porosity. Cell proliferation was investigated in vitro, and the surgery was conducted in a nude mouse in vivo model of long bone with cranial neural crest cells and bone marrow mesenchymal stem cells. The results showed our 3D-printed HA/TCP scaffold with biomimetic hierarchical structure is biocompatible and has sufficient mechanical strength for surgery.

Flame-retardant and thermal management structures have attracted great attention due to the requirement of high-temperature exposure in industrial, aerospace, and thermal power fields, but the development of protective fire-retardant structures with complex shapes to fit arbitrary surfaces is still challenging. Herein, we reported a rotation-blade casting-assisted 3D printing process to fabricate nacre-inspired structures with exceptional mechanical and flame-retardant properties, and the related fundamental mechanisms are studied. 3-(Trimethoxysilyl)propyl methacrylate (TMSPMA) modified boron nitride nanoplatelets (BNs) were aligned by rotation-blade casting during the 3D printing process to build the “brick and mortar” architecture. The 3D printed structures are more lightweight, while having higher fracture toughness than the natural nacre, which is attributed to the crack deflection, aligned BN (a-BNs) bridging, and pull-outs reinforced structures by the covalent bonding between TMSPMA grafted a-BNs and polymer matrix. Thermal conductivity is enhanced by 25.5 times compared with pure polymer and 5.8 times of anisotropy due to the interconnection of a-BNs. 3D printed heat-exchange structures with vertically aligned BNs in complex shapes were demonstrated for efficient thermal control of high-power light-emitting diodes. 3D printed helmet and armor with a-BNs show exceptional mechanical and fire-retardant properties, demonstrating integrated mechanical and thermal protection.

Lightweight and cost-effective polymer matrix composites (PMCs) with extraordinary mechanical performance will be a key to the next generation of diverse industrial applications, such as aerospace, electric automobile, and biomedical devices. Limpet teeth made of mineral-polymer composites have been proved as nature’s strongest material due to the unique hierarchical architectures of mineral fiber alignment. Here, we present an approach to build limpet teeth inspired structural materials with precise control of geometric morphologies of microstructures by magnetic field-assisted 3D printing (MF-3DP). α-Iron (III) oxide-hydroxide nanoparticles (α-FeOOHs) are aligned by the magnetic field during 3D printing and aligned α-FeOOHs (aFeOOHs) bundles are further grown to aligned goethite-based bundles (aGBs) by rapid thermal treatment after printing. The mechanical reinforcement of aGBs in PMCs can be modulated by adjusting the geometric morphology and alignment of α-FeOOHs encapsulated inside the 3D printed PMCs. In order to identify the mechanical enhancement mechanism, physics-based modeling, simulation, and tests were conducted, and the results further guided the design of bioinspired goethite-based PMCs. The correlation of the geometric morphology of self-assembled α-FeOOHs, curing characteristics of α-FeOOHs/polymer composite, and process parameters were identified to establish the optimal design of goethite-based PMCs. The 3D printed PMCs with aGBs show promising mechanical reinforcement compared with PMCs without aGBs. This study opens intriguing perspectives for designing high strength 3D printed PMCs on the basis of bioinspired architectures with customized configurations.

Lightweight and cost-effective polymer matrix composites (PMCs) with extraordinary mechanical performance will be a key to the next generation of diverse industrial applications such as aerospace, electric automobile, and biomedical devices. Limpet teeth made of mineral-polymer composites have been proved as nature’s strongest material due to the unique hierarchical architectures of mineral fiber alignment. Here, we present an approach to build limpet teeth inspired structural materials with precise control of geometric morphologies of microstructures by magnetic field-assisted 3D printing (MF-3DP). α-Iron (III) oxide-hydroxide nanoparticles (α-FeOOHs) are aligned by the magnetic field during 3D printing and aligned α-FeOOHs bundles are further grown to aligned goethite-based bundles (aGBs) by rapid thermal treatment after printing. The mechanical reinforcement of goethite-based fillers in PMCs can be modulated by adjusting the geometric morphology and alignment of mineral particles encapsulated inside the 3D printed PMCs. In order to identify the mechanical enhancement mechanism, physics-based modeling, simulation, and tests were conducted and the results further guided the design of bioinspired goethite-based PMCs. The correlation of the geometric morphology of self-assembled α-FeOOHs, curing characteristics of α-FeOOHs/polymer composite, and process parameters were identified to establish the optimal design of goethite-based PMCs. The 3D-printed PMCs with aGBs show promising mechanical reinforcement. This study opens intriguing perspectives for designing high strength 3D printed PMCs on the basis of bioinspired architectures with customized configurations.

Collembola cuticle structures demonstrated attractive and robust liquid super repellence due to the special evolved hierarchical patterns. These multiscale patterns consist of nanoscale triangle-shaped granules that are connected with microscale hexagon-shaped base structures. However, it is a challenge to reproduce such complex cuticle structures by using traditional manufacturing techniques. Here, the cuticle inspired structures were replicated via fiber-optic assisted three-dimensional photopolymerization (F3DP). This newly developed 3D printing process provided the capability to manipulate the geometry of bionic structures based on the natural cuticle structures. To enhance the liquid-repellent function, the 3D printed artificial cuticle structures were treated by the vapor phase process. The results show that the artificial cuticle structures reveal interesting properties in terms of superhydrophobicity, and the surface wettability can be modulated by controlling the height of bioinspired granules. Based on 3D printed artificial cuticle structures, a new icing protection surface was investigated. Compared with conventional methods, the 3D printed superhydrophobic surface decorated with artificial cuticle structures demonstrated promising perspectives in deicing applications.

This article presents a detailed account of the processes involved in vat-photopolymerization-based fabrication of ceramics, namely bioceramics, structural ceramics, piezoelectric ceramics, optical ceramics, and polymer-derived ceramics. Information and methods of material preparation, curing characteristics, green-part fabrication, property identification, process design and planning, and quality control and optimization are introduced. The article also provides information on postprocessing techniques, namely debinding and sintering, as well as on the phenomenon of shrinkage and compensation.

Thermoelectric generators (TEGs) provide a unique solution for energy harvesting from waste heat, presenting a potential solution for green energy. However, traditional rigid and flexible TEGs cannot work on complex and dynamic surfaces. Here, we report a stretchable TEG (S-TEG) (over 50% stretchability of the entire device) that is geometrically suitable for various complex and dynamic surfaces of heat sources. The S-TEG consists of hot-pressed nanolayered p-(Sb2Te3) and n-(Bi2Te3)-type thermoelectric couple arrays and exploits the wavy serpentine interconnects to integrate all units. The internal resistance of a 10 × 10 array is 22 ohm, and the output power is ∼0.15 mW/cm2 at ΔT = 19 K on both developable and nondevelopable surfaces, which are much improved compared with those of existing S-TEGs. The energy harvesting of S-TEG from the dynamic surfaces of the human skin offers a potential energy solution for the wearable devices for health monitoring.

A new multiaxial fatigue life prediction model has been proposed in this paper. The concepts of nonlinear continuum damage mechanics and critical plane criteria were incorporated in the proposed model. The shear strain-based damage control parameter was chosen to account for multiaxial fatigue damage under constant amplitude loading. Fatigue tests were conducted on nickel-based superalloy GH4169 tubular specimens at the temperature of 400 °C under proportional and nonproportional loading. The proposed method was checked against the multiaxial fatigue test data of GH4169. Most of prediction results are within a factor of two scatter band of the test results.

Carbon Nanotube assemblies are currently of considerable interest for their properties of high-packing-density and aligned configurations, which offer a promising route to achieve higher strength and conductivity in macroscopic materials. While most researches have focused on their mechanical and electrical properties, recently, their light absorption and thermal conductivity have attracted more and more attention. This work introduces a novel freestanding optoacoustic transducer using continuous multilayered carbon nanotube yarns, gold nanoparticles and elastomeric polymer, which can be used for efficient conversion from laser energy to acoustic power and generating high acoustic pressure (~33.6 MPa) without focusing. The calculated energy conversion efficiency is as high as 2.74 × 10−2. Such excellent performances could be attributed to its freestanding structure and the enhanced optical absorption due to localized surface plasmon resonance (LSPR) caused by gold nanoparticles. The laser-generated ultrasound has been experimentally demonstrated to be capable of manipulating micro particles (50 µm) in a transparent channel.

Traditionally, the fabrication of temporary teeth restorations must go through multiple processes such as moulding, curing and post finishing, which requires extensive expertise of dentists. In addition, the handmade temporary restorations are usually unable to precisely fit the patient’s teeth due to limited formability of material. To address the problem, a three-dimensional (3D) printing technology named temperature controlled mask image projection based stereolithography (TCMIP-SL) is presented for dental materials in this paper, with aim to build customized temporary crown and bridge quickly for its use in dental offices. We first studied the photo-polymerization performance of commercial materials that are commonly used in dental industry. Then we discussed the temperature effect on curing performance and rheology of dental composite material. Based on the studies, we further developed our TCMIP-SL process by integrating a material coating system with controllable heating, so that the coated thin film of dental composite material can be selectively cured using high resolution patterned light beam. Several test cases are performed to demonstrate the TCMIP-SL process can 3D print high viscous temporary crown material with fast speed and high resolution.

Most additive manufacturing (AM) processes such as stereolithography (SLA) and selective laser sintering (SLS) use a layer-by-layer fabrication approach. They cannot be used to fabricate geometric features on the surfaces of a pre-existing three-dimensional (3D) object. In addition, existing AM processes are mainly based on a single size scale, e.g. macro-scale or micro-scale, and cannot be used to build a macro-scale object with micro-scale features on its surfaces. In this paper, we present a novel immersed surface accumulation process that can fabricate micro-scale features on macro-scale surfaces. In the process, a surface-based light guide tool is immersed inside liquid resin to fabricate high-resolution features on the surfaces of a pre-existing object. The system design and process settings to fabricate 3D features are presented. The relation between process control and related fabrication property is discussed. Two test cases are presented to demonstrate the effectiveness of the newly developed immersed surface accumulation process.

The mask image projection-based stereolithography (MIP-SL) is a low-cost and high-resolution additive manufacturing (AM) process. However, the slow speed of part separation and resin refilling is the primary bottleneck that limits the fabrication speed of the MIP-SL process. In addition, the stair-stepping effect due to the layer-based fabrication process limits the surface quality of built parts. To address the critical issues in the MIP-SL process related to resin refilling and layer-based fabrication, we present a mask video projection-based stereolithography (MVP-SL) process with continuous resin flow and light exposure. The newly developed AM process enables the continuous fabrication of three-dimensional (3D) objects with ultra-high fabrication speed. In the paper, the system design to achieve mask video projection and the process settings to achieve ultrafast fabrication speed are presented. The relationship between process parameters and the surface quality of the built parts is discussed. Test results illustrate that the MVP-SL process with a continuous resin flow can build three-dimensional objects within minutes, and the surface quality of the fabricated objects is significantly improved.

The revolution of additive manufacturing (AM) has led to many opportunities in fabricating complex and novel products. The increase of printable materials and the emergence of novel fabrication processes continuously expand the possibility of engineering systems in which product components are no longer limited to be single material, single scale, or single function. In fact, a paradigm shift is taking place in industry from geometry-centered usage to supporting functional demands. Consequently, engineers are expected to resolve a wide range of complex and difficult problems related to functional design. Although a higher degree of design freedom beyond geometry has been enabled by AM, there are only very few computational design approaches in this new AM-enabled domain to design objects with tailored properties and functions. The objectives of this review paper are to provide an overview of recent additive manufacturing developments and current computer-aided design methodologies that can be applied to multimaterial, multiscale, multiform, and multifunctional AM technologies. The difficulties encountered in the computational design approaches are summarized and the future development needs are emphasized. In the paper, some present applications and future trends related to additive manufacturing technologies are also discussed.

Electrically assisted 3D printing fabricates nacre-inspired graphene nanoplatelet alignment with self-sensing capability., Lightweight and strong structural materials attract much attention due to their strategic applications in sports, transportation, aerospace, and biomedical industries. Nacre exhibits high strength and toughness from the brick-and-mortar–like structure. Here, we present a route to build nacre-inspired hierarchical structures with complex three-dimensional (3D) shapes by electrically assisted 3D printing. Graphene nanoplatelets (GNs) are aligned by the electric field (433 V/cm) during 3D printing and act as bricks with the polymer matrix in between as mortar. The 3D-printed nacre with aligned GNs (2 weight %) shows lightweight property (1.06 g/cm3) while exhibiting comparable specific toughness and strength to the natural nacre. In addition, the 3D-printed lightweight smart armor with aligned GNs can sense its damage with a hesitated resistance change. This study highlights interesting possibilities for bioinspired structures, with integrated mechanical reinforcement and electrical self-sensing capabilities for biomedical applications, aerospace engineering, as well as military and sports armors.

The mask image projection based stereolithography (MIP-SL) is a low cost and high-resolution additive manufacturing (AM) process. However, the slow speed of part separation and resin refilling is the primary bottleneck that limits the fabrication speed of the MIP-SL process. In addition, the stair steeping effect due to the layer-based fabrication process limits the surface quality of built parts. To address the critical issues in the MIP-SL process related to resin refilling and layer-based fabrication, we present a mask video projection based stereolithography (MVP-SL) process with continuous resin flow and light exposure. The newly developed AM process enables the continuous fabrication of three-dimensional (3D) objects with ultra-high fabrication speed. In the paper. The system design to achieve mask video projection and the process settings to achieve ultrafast fabrication speed are presented. The relationship between process parameters and the surface quality of the fabricated parts is discussed. Test results illustrate the MVP-SL process with continuous resin flow can build three-dimensional objects within minutes and the surface quality of the fabricated objects can be significantly improved.

The focused ultrasound (FUS) systems are widely used due to their ability to generate a tight and intense focal region from a distributed source for the non-invasive and safely detection. However, these devices usually require elaborate arrays of a rather complex and cumbersome set of transducers. Holographic techniques are fundamental to applications such as volumetric displays and tweezers. The monolithic acoustic holograms can reconstruct diffraction-limited acoustic pressure fields and arbitrary ultrasound beams.

Biomimetic functional surfaces are attracting increasing attention for various technological applications, especially the superhydrophobic surfaces inspired by plant leaves. However, the replication of the complex hierarchical microstructures is limited by the traditional fabrication techniques. In this paper, superhydrophobic micro-scale artificial hairs with eggbeater heads inspired by Salvinia molesta leaf was fabricated by the Immersed surface accumulation three dimensional (3D) printing process. Multi-walled carbon nanotubes were added to the photocurable resins to enhance the surface roughness and mechanical strength of the microstructures. The 3D printed eggbeater surface reveals interesting properties in terms of superhydrophobilicity and petal effect. The results show that a hydrophilic material can macroscopically behave as hydrophobic if a surface has proper microstructured features. The controllable adhesive force (from 23 μN to 55 μN) can be easily tuned with different number of eggbeater arms for potential applications such as micro hand for droplet manipulation. Furthermore, a new energy-efficient oil/water separation solution based on our biomimetic structures was demonstrated. The results show that the 3D-printed eggbeater structure could have numerous applications, including water droplet manipulation, 3D cell culture, micro reactor, oil spill clean-up, and oil/water separation.

Offsetting of measured data, as a basic geometric operation, has already been widely used in many areas, like reverse engineering, rapid prototyping and NC machining. However, measured data always carry typical defects like caves and singular points. A fault-tolerant offset method is proposed to create the high quality offset surface of measured data with such defects. Firstly, we generated an expansion sphere model of measured data with the radius equivalent to the offset length. Secondly, using the computational geometry application of convex hull, we acquire the data of outermost enveloping surface of this expansion sphere model. Finally, we use local MLS projection fitting method to wipe out existing defects, and generate the high-quality triangular mesh surface of the offset model. The offset surface generated by this method is suitable for practical engineering application due to its high efficiency and accuracy.

This paper reported a method for designing monolithic acoustic holograms with consideration of both amplitude and phase modulation, which can be used for multifocal point beam forming by a single element ultrasonic transducer in the frequency of megahertz range. Mask-image-projection-based stereolithography 3D printing technology was induced to manufacture the complex hologram with a pixel resolution of 25 μm. A nine foci pattern and a letter U pattern acoustic field shown by a 2 MHz transducer and holograms were designed to validate the proposed designing method. Both the hydrophone testing and the simulation results indicated that the holograms designed by our method could form beam patterns accurately in the expected position.This paper reported a method for designing monolithic acoustic holograms with consideration of both amplitude and phase modulation, which can be used for multifocal point beam forming by a single element ultrasonic transducer in the frequency of megahertz range. Mask-image-projection-based stereolithography 3D printing technology was induced to manufacture the complex hologram with a pixel resolution of 25 μm. A nine foci pattern and a letter U pattern acoustic field shown by a 2 MHz transducer and holograms were designed to validate the proposed designing method. Both the hydrophone testing and the simulation results indicated that the holograms designed by our method could form beam patterns accurately in the expected position.

Nature has developed high-performance materials and structures over millions of years of evolution and provides valuable sources of inspiration for the design of next-generation structural materials, given the variety of excellent mechanical, hydrodynamic, optical, and electrical properties. Biomimicry, by learning from nature's concepts and design principles, is driving a paradigm shift in modern materials science and technology. However, the complicated structural architectures in nature far exceed the capability of traditional design and fabrication technologies, which hinders the progress of biomimetic study and its usage in engineering systems. Additive manufacturing (three-dimensional (3D) printing) has created new opportunities for manipulating and mimicking the intrinsically multiscale, multimaterial, and multifunctional structures in nature. Here, an overview of recent developments in 3D printing of biomimetic reinforced mechanics, shape changing, and hydrodynamic structures, as well as optical and electrical devices is provided. The inspirations are from various creatures such as nacre, lobster claw, pine cone, flowers, octopus, butterfly wing, fly eye, etc., and various 3D-printing technologies are discussed. Future opportunities for the development of biomimetic 3D-printing technology to fabricate next-generation functional materials and structures in mechanical, electrical, optical, and biomedical engineering are also outlined.