Freeform Structures Research Articles

Advanced Design and 3D Printing Strategies With Alginate‐Nanoclay Nanocomposites: From Microstructure to Bioprinting

AbstractNanocomposites made from alginate and nanoclay are extensively applied for diverse biomedical applications. However, the lack of a clear understanding of the interactions between alginate and nanoclay makes it difficult to rationally design the nanocomposites for different material extrusion‐based 3D bioprinting strategies. Here, a combined analytical model is proposed to accurately predict the interaction mechanisms between alginate and nanoclay through small‐angle neutron scattering. These mechanisms are summarized into a phase diagram that can guide the design of alginate‐nanoclay nanocomposites for different bioprinting applications. The rheological properties of various nanocomposites are measured to validate the proposed interaction mechanisms at the macroscale. Accordingly, three representative extrusion‐based bioprinting strategies are linked with the nanocomposite design and applied to freeform fabricate complex structures. A roadmap is summarized to bridge the gap between biomaterial design and bioprinting processes, enabling the rapid and rational selection of biomaterial formula based on available 3D printing methods, and vice versa.

Grid Optimization of Free-Form Spatial Structures Considering the Mechanical Properties

In recent years, the application of free-form surface spatial grid structures in large public buildings has become increasingly common. The layouts of grids are important factors that affect both the mechanical performance and aesthetic appeal of such structures. To achieve a triangular grid with good mechanical performance and uniformity on free-form surfaces, this study proposes a new method called the “strain energy gradient optimization method”. The grid topology is optimized to maximize the overall stiffness, by analyzing the sensitivity of nodal coordinates to the overall strain energy. The results indicate that the overall strain energy of the optimized grid has decreased, indicating an improvement in the structural stiffness. Specifically, compared to the initial grid, the optimized grid has a 30% decrease in strain energy and a 43.3% decrease in maximum nodal displacement. To optimize the smoothness of the grid, the study further applies the Laplacian grid smoothing method. Compared to the mechanically adjusted grid, the structural mechanical performance does not significantly change after smoothing, while the geometric indicators are noticeably improved, with smoother lines and regular shapes. On the other hand, compared to the initial grid, the smoothed grid has a 21.4% decrease in strain energy and a 28.3% decrease in maximum nodal displacement.

Bamboo spatial structure, developing an integrated computational workflow and a tailored semi-automated fabrication apparatus

This paper studies the development of a semi-automated process for designing and fabricating node-based lattice spatial structures using bamboo as an ecological material. The study addresses the challenges of integrating bamboo—a naturally variable material—into spatial structures and was conducted through a hybrid workshop at the Digital Craft House, University of Art, Tehran, during COVID-19. Traditional CAD tools are often limited when applied to non-industrialized, organic materials like bamboo. This research introduces a tailored computational workflow and a semi-automated fabrication apparatus explicitly designed to create complex, freeform bamboo structures. Combining online and in-person sessions allowed participants to engage with both the theoretical and practical aspects of the design-to-fabrication process. This approach enabled the continuous development and refinement of custom tools and methods during COVID. The workshop showcased how synthesizing advanced computational approaches and natural materials could trigger new architectural solutions and autonomy by developing a single-layer, freeform spatial structure, promoting ecological and advanced architectural practices.

Direct ink writing of customized polymeric structures embedded in a nano-silicate based supporting matrix

The increase in use of intricate structures for engineering applications has encouraged researchers to find new fabrication techniques. This work presents an innovative approach of embedded 3D printing, which allows fabrication of freeform structures by directly 3D printing inside a supporting matrix. The polymer ink solution was prepared by blending polylactic acid (PLA) and polypropylene carbonate (PPC) in 1,4-dioxane. Laponite RD was used to prepare the aqueous nano-silicate suspension, which exhibited yield stress and thixotropic properties due to its house-of-cards arrangement. Solidification of the 3D printed polymer ink occurred due to the hydrogen bonding between the oxygen atoms of dioxane molecules and the protons of water molecules. Particle image velocimetry study showed that the increasing nano-silicate concentration caused a reduction in yield region as a result of increasing yield stress. Dimensionless parameters (Oldroyd number and ratio of yield stress to hydrostatic pressure) for nano-silicate concentrations with values less than 1 were found to facilitate the nozzle movement without crevice formation, while the values greater than 1 were found to be unsuitable for printing. Different intricate structures such as tubular structures with varying geometry that mimic the native trachea, and a bifurcated tube were 3D printed as a proof-of-concept study. The proposed method introduced a novel approach by utilizing the solvent-water interaction capability to fabricate objects with overhanging geometry. This approach allows 3D printing of a wide variety of polymers by leveraging the miscibility between its corresponding solvent and supporting matrix.

Sustainable Support Material for Overhang Printing in 3D Concrete Printing Technology

The advantage of 3DCP technologies is the ability to fabricate free-form structures. However, printing openings in concrete structures are limited by the presence of overhanging sections. While various 3D printing and additive manufacturing technologies have established methods for handling overhangs with temporary supports, many existing techniques for 3D concrete printing still rely on wooden planks and corbelling, which restrict the design flexibility and slope angles. The objective of this study is to develop a removable and sustainable support material with high printability performance. This support material serves as temporary support for overhang sections in 3D-printed structures and can be removed once the primary concrete has hardened sufficiently. This study observed that increasing the recycled glass content in the mixture raises both the dynamic and static yield stresses, with only mixtures containing up to 60% recycled glass remaining pumpable. Optimization of the mixture design aimed to balance high flowability and buildability, and the results indicated that a mixture with 60% recycled glass content is optimal. The effectiveness of the optimized support material was validated through the successful printing of a structure featuring a free-form opening and overhang section.

An Efficient Progressive Grid Generation Method Considering Internal Quality and Boundary Grid Adjustment

Due to its elegant appearance and high structural efficiency, free-form grid structures are increasingly adopted in architectural design. However, it is a challenging task for engineers to generate a uniform and well-shaped grid on a free-form surface while considering the processing of both interior and boundary of grid, especially for composite surfaces. To generate well-shaped grids with uniform rods, regular cells and smooth visual effects over free-form surfaces, this paper develops an innovative triangular grid generation method capable of automatically managing grid boundary with extending initial surface. In the method, nodes of grid structure are considered to be zero mass particles and are progressively added to the extended surface from geometric center to the boundary of surface. A boundary-processing algorithm is then established to evenly distribute nodes on initial boundary curve, while ensuring that grid cells at the boundary do not exhibit elongation. A mechanical simulation system based on spring-mass model is improved to optimize spatial grid structure, rods of grid are regarded as linear springs, a surface attraction force and an anchoring force are performed on grid nodes. Moreover, the proposed method allows architects change grid direction and rods length easily, which can greatly improve the efficiency of design. Several case studies show that the method can effectively avoid distortion of grids and generate well-shaped grids that can meet aesthetic requirements.

Rapid, Micron-Resolution 3D Printing of Nd:YAG Ceramic with Optical Gain.

Polycrystalline yttrium aluminum garnet (YAG) ceramic doped with neodymium (Nd), referred to as Nd:YAG, is widely used in solid-state lasers. However, conventional powder metallurgy methods suffer from expenses, time consumption, and limitations in customizing structures. This study introduces a novel approach for creating Nd:YAG ceramics with 3D free-form structures from micron (∼70µm) to centimeter scales. Firstly, sol-gel synthesis is employed to form photocurable colloidal solutions. Subsequently, by utilizing a home-built micro-continuous liquid interface printing process, precursors are printed into 3D poly(acrylic acid) hydrogels containing yttrium, aluminum, and neodymium hydroxides, with a resolution of 5.8µm pixel-1 at a speed of 10µm s-1. After the hydrogels undergo thermal dehydration, debinding, and sintering, polycrystalline Nd:YAG ceramics featuring distinguishable grains are successfully produced. By optimizing the concentrations of the sintering aids (tetraethyl orthosilicate) and neodymium trichloride (NdCl3), the resultant samples exhibit satisfactory photoluminescence, emitting light concentrated at 1064nm when stimulated by a 532nm laser. Additionally, Nd:YAG ceramics with various 3D geometries (e.g., cone, spiral, and angled pillar) are printed and characterized, which demonstrates the potential for applications, such as laser and amplifier fibers, couplers, and splitters in optical circuits, as well as gain metamaterials or metasurfaces.

Out-of-plane cross-angle bending stiffness of cross-laminated timber

The increasing use of cross-laminated timber (CLT) in construction worldwide can be attributed to its combination of sustainability, structural efficiency, biaxial strength and stiffness, short construction times, and architectural flexibility. However, when applied in free-form structures, CLT panels may exhibit irregular element geometries, resulting in deviations from their major or minor strength axes. The required out-of-plane cross-angle bending stiffness is not addressed by commonly used design methods. This study addresses this gap by examining the cross-angle bending stiffness of CLT panel segments using experimental, analytical, and numerical methods. Three different lay-ups (3-ply, 4-ply, and 5-ply) and five cross-angles (ranging from 0° to 90°) were tested under 3-point bending, demonstrating the nonlinear relation between cross-angle and out-of-plane bending stiffness. The lowest stiffness was observed for intermediate cross-angles and not when loaded along the minor strength direction. A proposed analytical model that combines the shear analogy with Hankinson's equation yielded adequate approximations. The model was verified by numerical simulations and then applied to predict the out-of-plane cross-angle bending stiffness of a wide range of CLT layups. This research provides valuable insights for working with CLT in complex geometrical designs, enhancing preliminary design.

Laser Direct Writing for Micromachining of Freeform Surface on n-Type GaAs via Photoelectrochemical Etching

The fabrication of microstructures featuring freeform surfaces on semiconductor substrates confronts substantial obstacles due to their inherent material difficulties, including considerable hardness and brittleness, as well as geometric complexity. In this investigation, we leverage laser direct writing (LDW) combined with photoelectrochemical etching to achieve precise material removal from semiconductor surfaces. By conducting a series of experiments on a home-made LDW apparatus under varying conditions, we established a correlation between the etching depth and both the power intensity and motion speed of the laser spot. The analysis revealed that the etching depth exhibited linearly with the power intensity of the laser spot and inversely with the motion speed. Additionally, the half widths of the grooves maintained consistently within the range of 1–2 μm. By leveraging this methodology, we successfully fabricated a freeform micro-optical structure on an n-GaAs wafer by precisely adjusting the power intensity of the laser spot during scanning, thus demonstrating the feasibility for achieving nanoscale machining precision. This research underscores the promise of this technique in significantly advancing the fabrication processes of semiconductor devices.

Direct 3D printing of freeform anisotropic bioactive structure based on shear-oriented ink system

Various anisotropic tissue structures exist in organisms, including muscle tissue, skin tissue, and nerve tissue. Replicating anisotropic tissue structuresin vitrohas posed a significant challenge. Three-dimensional (3D) printing technology is often used to fabricate biomimetic structures due to its advantages in manufacturing principle. However, direct 3D printing of freeform anisotropic bioactive structures has not been reported. To tackle this challenge, we developed a ternary F/G/P ink system that integrates the printability of Pluronic F127 (F), the robust bioactivity and photocrosslinking properties of gelatin methacryloyl (G), and the shear-induced alignment functionality of high-molecular-weight polyethylene glycol (P). And through this strategic ternary system combination, freeform anisotropic tissue structures can be 3D printed directly. Moreover, these anisotropic structures exhibit excellent bioactivity, and promote orientational growth of different cells. This advancement holds promise for the repair and replacement of anisotropic tissues within the human body.

Self-shaping free-form spatial structure with large-deformable bunched CFRP rods

Based on the bending-active concept, this paper presents a self-shaping system with bunched carbon fiber-reinforced polymer (CFRP) rods to construct complex curved surfaces, establishing an overall design and construction process. Such component consists of a bunch of slender and easily bendable CFRP rods that are preformed and then bounded by connecting joints. Then, those components form free-form spatial structures. The proposed system offers several advantages: strong designability, low transportation cost, short construction period, polished appearance and low maintenance requirements during service. For component design, to determine the shape of such component, a two-stage mechanical analysis is proposed based on the large deformation theory and finite element analysis (FEA): a form-finding process to determine the deformed component shape through its cross-sections and boundary-load conditions; and a reverse form-finding process to obtain these above-mentioned parameters from a given component shape. The reliability of both methods is verified through in-plane bending tests. A design demonstration of a saddle-shaped curved spatial structure is performed using this method. Overall, the proposed system can guide the actual construction projects of complex curved surfaces with self-shaping bunched CFRP rods.

Hierarchical Composites Patterned via 3D Printed Cellular Fluidics

AbstractAdditive manufacturing of freeform structures containing multiple materials with deterministic spatial arrangement and interactions remains a challenge for most 3D printing processes, due to complex fabrication tool requirements and limitations in printability of some material classes. Here, a versatile method is reported to produce architected composites using the concept of cellular fluidics, in which lattices of unit cells are used as templating scaffolds to guide flowable infill materials in a programmed spatial pattern, upon which they are cured in place to produce a deterministically ordered multimaterial solid. The lattice design relies on the unit cell size, type, strut diameter, surface wetting, and distribution of cellular structures to control liquid flow and retention. Individual unit cells are tuned to achieve reliable infilling and combined into higher‐order architectures to achieve multiscale composite materials with disparate mechanical properties, including those considered non‐printable. Lattice design considerations for leveraging capillary phenomena and demonstrate several methods of patterning polymers in 3D‐printed cellular fluidic structures are presented. The concept of tuning the compressive response of an architected composite using a flexible‐elastomer as the lattice and a stiff‐epoxy as the infill material is illustrated.

An enhanced spatial extrusion method to manufacture large-scale thermoplastic lattice structures

Recent advances in additive manufacturing have enabled a rapid and cost-effective fabrication of complex geometries such as lattices. At the architectural scale, the emerging robotic arm spatial material extrusion method with a customized extrusion head has been used to build free-form lattice structures, but most existing studies primarily focused on the realization of geometric forms other than their mechanical properties. This work developed an enhanced extrusion-based robotic free-hanging printing method with a well-planned printing path, proper process parameters, and a specific robotic control strategy to manufacture thermoplastic lattice cubes and beams. The proposed method improved the lattice quality in terms of better strut alignment and stronger bonding on the joints, leading to increased compressive and flexural strengths by 230 % and 685 % compared to the existing method. We envision that the proposed method can be applied to build more large-scale lattice structures with diverse and tailorable geometries from art installations to structural skeletons.

The influence of invariant horizontal projection area constraint on shape optimization of free-form surface structure with openings

Free-form curved surface shell roof is favored by architects, but the openings for daylighting will decline its mechanical properties. In the shell shape optimization under the vertical load, when the outer contour of the openings or the shell changes, the total load of the structure also changes with the changes in its horizontal projection area. This contradicts the premise of structural optimization, that is, to maintain the principle of unchanging total loads. To solve this problem, the invariant horizontal projection area of the shell was taken as constraints in the shell shape optimization in which the objective function was structural strain energy and optimization design variables were internal and external boundary points of the curved surfaces with openings, and the key points on the curved surface which were design variables of the overall shape of the curved surface. The numerical examples verified the validity of the method, and the influence of the invariant horizontal projection area constraint of the curved surface with openings on the optimization results of the minimum strain energy, optimization with the frequency constraint, and the initial surface with the different support distribution were discussed. The results of the numerical examples showed that the invariant horizontal projection area constraint has a greater impact on the results of the curved surface with opening optimization, and its influence should be considered in the engineering design.

Simulation and analysis of Automated Generalised Interleaved Layered Execution (AGILE) process for concrete shell structures

There has been increased research and interest in new methods of automated construction. This study presents a novel computational model for an augmented and generalised version of an automated construction process that was previously tested experimentally only for vertical concrete walls. The Automated Generalised Interleaved Layer Execution (AGILE) utilises a robotic arm for the alternating deposition of formwork and concrete, aiming to facilitate the creation of free-form concrete shell structures with enhanced design and construction versatility, overcoming the constraints of previous methods. The paper proposes an FEA analysis framework for this construction methodology, evaluating its implications, limitations, and challenges via a detailed case study that includes calculating lateral pressures and displacements throughout the construction process. Moreover, it introduces new failure criteria specific to this construction method, enabling the optimisation of the design and geometry of the shell and formwork, as well as the construction process details and parameters. The proposed computational analysis method allows designers to improve the performance of a broader range of shell structures while reducing construction costs, time, complexity, and environmental impact by replacing early-stage experimental methods with finite-element analysis. In conclusion, apart from generalising the limited process and identifying its design options and parameters, this study proposes a new and straightforward computational method for analysis of the AGILE construction method in its general form in the absence of any other analysis method and where only the experimental method in one particular case was used before, identifies four different construction failure modes, and paves the way for experimental verification and calibration of this analysis method for different specific setups and applications of this general construction process.

In-process textile reinforcement method for 3D concrete printing and its structural performance

3D concrete printing (3DCP) is an innovative technology for constructing complex freeform structures that are difficult or expensive to build using conventional construction methods. The printing of double-curved shells and aesthetically pleasing geometries for façade panels and roof elements requires a detailed investigation of the load-carrying capacity and failure mode during complex loading. Moreover, integrating reinforcement into printed elements is one of the major challenges in 3DCP for manufacturing structural components. Textile reinforcement has high formability and tensile strength and can be used as an in-process reinforcing method during printing. In this study, one layer of alkali-resistant glass textile was used to reinforce the 3D printed and mould-cast high-performance concrete curved members. The commonly used concrete printing nozzle was modified to allow ease in textile placement along the printing path. Further, the feasibility of the modified nozzle was evaluated by printing two different curved structures and was validated. In addition, two different curvatures of curved elements, with and without textile reinforcement were printed to study the effect of textile reinforcement and geometry when subjected to a point load in the middle. The deformation behaviour, crack development, and propagation were monitored using digital image correlation. It was observed that the textile reinforcement enhances the interlayer bonding leading to slower crack propagation and enhanced load-carrying capacity. An increase of about 10 % in the peak load-carrying capacity of 3D printed specimens was observed with the addition of textile reinforcement when compared to their mould-cast. Also, 3D printed specimens was observed to have larger deformation ductility compared to mould-cast specimens. Further, the textile aligns better with increasing curvature which enhances the resistance of membrane forces more uniformly resulting in a 15.5 % increase in the ultimate moment capacity. In addition, the first crack load and the ultimate load were predicted based on the arch equations and the results showed that the current method predicts the capacity with reasonable accuracy.

Stiffening patterns for freeform composite shell structures

This paper investigates innovative approaches for enhancing the structural integrity of form-found shell structures with a focus on rib-based reinforcements. It discusses the significance of ribs in enhancing stability and addressing the sensitivity of form-found structures to various loads. Traditional methods such as increasing shell thickness or introducing supports often compromise efficiency and aesthetics. A more intelligent approach involves reinforcing the shell with ribs and advanced materials. Diverse rib patterns are presented, including geometric, biomimetic, and topological optimization-inspired designs, each adhering to a 50 % volume constraint, when compared to the original shell. Physical testing and numerical simulations demonstrate that these rib patterns significantly increase stiffness and buckling resistance. The findings presented in the paper suggest that combining form-finding methodologies with well-designed rib patterns can create sustainable and resilient structures, contributing to a reduction in both the consumption of materials and the environmental impact of these structures.

Morphogenesis of free-form surface structures based on geometrically nonlinear isogeometric analysis

Free-form surface structures have taken the forefront in space structure research owing to their free and flexible expressiveness and aesthetic visual effects. Yet, their irregular shapes and large span of the initial structure often lead to the problem of large displacement, necessitating the incorporation of geometric nonlinearity in the analysis. In this paper, we employ structural optimization design principles to investigate the morphogenesis of free-form surface structures while considering their geometric nonlinearity. Particularly, we explore both single-objective and double-objective shape optimization of free-form surface structures by employing structural strain energy and the first natural frequency as optimization indexes and the vertical coordinates of control points as optimization variables. The effectiveness of the proposed method is finally verified by several examples.

Porous structures inspired by porcupine quill: multiscale design optimization approach

This paper presents a novel approach for designing a freeform bending-resistant structure from the combination of explicit discrete component-based topology optimization (TO) and the porcupine quill-inspired features. To embed the porcupine quill’s features into the TO formulations, the method involves constructing discrete components at various scales to imitate features including solid shell, stochastically distributed pores, and graded stiffeners. The components are iteratively updated, and the optimization process allows for the grading of quill-inspired features while achieving optimal structural compliance under bending loads. The proposed approach is demonstrated to be effective through the resolution of Messershmitt–Bolkow–Blohm (MBB) beam designs, parameterized studies of geometric parameters, and numerical validation of long-span and short-span quill-inspired beam designs. By examining the von Mises stress distribution, the study highlights the mitigation of material yielding at the shell region brought by the geometric features of porcupine quills, leading to the potential theory support for the bending resistance. The optimized MBB beams are manufactured using the material extrusion technique, and three-point bending tests are conducted to explore the failure mitigation capability of the quill-inspired beam under large deformation. Consequently, the study concludes that the proposed quill-inspired component-based TO approach can design a structure with excellent bending resistance according to the improved energy absorption as well as increased deformation after reaching 75% peak load.

Ultra-Thin GaAs Single-Junction Solar Cells for Self-Powered Skin-Compatible Electrocardiogram Sensors.

GaAs thin-film solar cells have high efficiency, reliability, and operational stability, making them a promising solution for self-powered skin-conformal biosensors. However, inherent device thickness limits suitability for such applications, making them uncomfortable and unreliable in flexural environments. Therefore, reducing the flexural rigidity becomes crucial for integration with skin-compatible electronic devices. Herein, this study demonstrated a novel one-step surface modification bonding methodology, allowing a streamlined transfer process of ultra-thin (2.3µm thick) GaAs solar cells on flexible polymer substrates. This reproducible technique enables strong bonding between dissimilar materials (GaAs-polydimethylsiloxane, PDMS) without high external pressures and temperatures. The fabricated solar cell showed exceptional performance with an open-circuit voltage of 1.018V, short-circuit current density of 20.641mAcm-2, fill factor of 79.83%, and power conversion efficiency of 16.77%. To prove the concept, the solar cell is integrated with a skin-compatible organic electrochemical transistor (OECT). Competitive electrical outputs of GaAs solar cells enabled high current levels of OECT under subtle light intensities lower than 50mWcm-2, which demonstrates a self-powered electrocardiogram sensor with low noise (signal-to-noise ratio of 32.68dB). Overall, this study presents a promising solution for the development of free-form and comfortable device structures that can continuously power wearable devices and biosensors.