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Helical and Bouligand Porous Scaffolds Fabricated by Dynamic Low Strength Magnetic Field Freeze Casting
Abstract
Porous Fe3O4 scaffolds were fabricated while subject to a low (7.8 mT) magnetic field applied in helical and Bouligand motions using a custom-built tri-axial nested Helmholtz coils-based freeze-casting setup. This setup allowed for the control of a dynamic, uniform low-strength magnetic field to align particles during the freezing process, resulting in the majority of lamellar walls aligning within ± 30° of the magnetic field direction and a decrease in porosity by up to 42%. Similar to how helical and Bouligand structures in nature promote impact resistance, these magnetic field motions produced structures with improved high strain rate mechanical properties. Strain at failure was increased by up to 2 times as cracks deflected to match the applied angles of rotation of the magnetic field.
... To achieve these weak uniform fields, a Helmholtz coils setup was utilized. [30][31][32]35,36 Helmholtz coils are a pair of symmetric electromagnets that are spaced such that the separation distance between them is equal to the radius of both coils. When spaced at such a distance, the coils are then able to apply a nearly uniform field at the center point between them and, therefore, throughout the freeze-casting setup. ...
... This setup allows the user to apply weak uniform magnetic fields in multiple directions simultaneously. This setup has allowed the user to apply complex field types during the freeze-casting process, such as rotating, 35 diagonal, 30 bouligand, 35 and oscillating fields. 32,36 All these complex field types allowed for complex control over the freeze-cast structure. ...
... This setup allows the user to apply weak uniform magnetic fields in multiple directions simultaneously. This setup has allowed the user to apply complex field types during the freeze-casting process, such as rotating, 35 diagonal, 30 bouligand, 35 and oscillating fields. 32,36 All these complex field types allowed for complex control over the freeze-cast structure. ...
Porous scaffolds can be utilized in a variety of biomedical as well as mechanical applications. The process of freeze casting is a successful method to fabricate these porous structures but with ideal characteristics in only one direction (the ice‐growth direction). The application of magnetic fields led to an increase in both the microstructural control and mechanical strength in an additional orthogonal direction. The application of these weak, uniform fields (≤20 mT), in particular oscillating fields from a Helmholtz coils setup, has led to increases in mechanical strength through microstructural alignment in multiple material types. However, structures fabricated from these uniform fields have primarily been compared to each other, with little research comparing them to structures fabricated under strong, non‐uniform fields from permanent magnet setups. Therefore, iron‐oxide scaffolds were fabricated under weak, uniform fields (≤20 mT) as well as strong, non‐uniform fields (≥20 mT), and their mechanical and microstructural properties were compared to one another. The application of weak, uniform fields led to superior mechanical properties compared to those produced from the application of strong, non‐uniform fields, no distortions in the physical structure of the freeze‐cast scaffold, and the best microstructural alignment ever seen in freeze‐cast structures.
... The application of unidirectional freezing, bidirectional freezing and radial freezing, were discussed in section 2. 4.2. In addition to the engineering of freeze cast structures by controlling the thermal environment, external magnetic [232], electric [233], and acoustic [234] fields can be used to further alter the solidification process [235]. ...
... Magnetic field aligned freeze-casting is used to fabricate anisotropic magnetic ceramic scaffolds with a hierarchy of architectural alignment in multiple directions. Nelson et al. 2020 used a weak rotating magnetic field, applied normal to the ice growth direction in a uniaxial freezing apparatus, to achieve a long-range helical architecture in porous Fe 3 O 4 scaffolds [232]. The conformity of lamellar alignment to the orientation of the applied magnetic field enables direct and specific control of macrostructural morphology. ...
... Magnetic field aligned freeze-casting is used to fabricate anisotropic magnetic ceramic scaffolds with a hierarchy of architectural alignment in multiple directions. Nelson et al. 2020 used a weak rotating magnetic field, applied normal to the ice growth direction in a uniaxial freezing apparatus, to achieve a long-range helical architecture in porous Fe 3 O 4 scaffolds [232]. The conformity of lamellar alignment to the orientation of the applied magnetic field enables direct and specific control of macrostructural morphology. ...
A range of biomaterials and fabrication methods have been explored to produce biomimetic scaffolds to facilitate cardiac tissue regeneration. Ice-templated collagen scaffolds have demonstrated translational success in other clinical applications. The ice templating technique utilizes phase separation dynamics during solidification and subsequent sublimation of ice to produce scaffolds with interconnected porosity. Although composition has been found to be key to determining cellular response, both nano-scale and micro-scale surface features of ice templated collagen scaffolds have also been found to encourage cellular ingrowth and attachment. Previous research has introduced techniques to control pore size and anisotropy. To date, however, ice-templating has not been shown to allow control of architecture to the extent that it is possible to replicate the structure of more complex tissue morphologies such as the myocardium. In this thesis, the underpinning physics of ice formation is leveraged to determine the final architecture of ice-templated collagen scaffolds. A controllable directional freezing apparatus was designed and built to enable fine control of the thermal environment during solidification. A relationship between the set thermal parameters and final pore architecture was established. This relationship enabled the production of structures with controlled pore alignment and size. The direct control and monitoring capabilities of the freezing apparatus enabled observations of intrinsic freezing kinetics. This insight allowed the solidification processes of anisotropic and isotropic ice-templating to be compared, and a link between the previously distinct fields was hypothesized. A novel thermal control technique was developed that dictated ice growth directions and achieved complex lamellar orientation of ice-templated collagen scaffolds. A new mould design was produced, with a heat-sink at the base and heat sources in the mould walls, which afforded three-dimensional thermal control during the solidification process. Ultimately, this created complex lamellar orientation of ice-templated collagen scaffolds. The technique is presented alongside a finite element model, developed as a predictive tool for the design of final lamellar orientation. Heat source moulds were used to introduce controlled thermal gradients during the solidification phase of the ice-templating process. Various heat source profiles were implemented and simulated. It was found that by introducing controlled complex thermal gradients during solidification, scaffolds with multidirectional pore orientations were produced, and the finite element simulation was found to accurately predict lamellar orientation. Taken together, the model and heat source freezing technique provide the opportunity for design and production of regenerative collagen scaffolds with tailored architectural morphologies. After establishing a control protocol for producing structures with tailored local lamellar architecture, patches were tested by observing the effects of scaffold architecture on cellular behaviour and mechanical conformation to the dynamic movements of the heart. Cardiomyocytes (H9 hESCs) were seeded onto scaffolds with aligned and isotropic pore structures and the cell signalling patterns were then compared. It was determined that the biomimetic accuracy of the aligned scaffold improved the uniformity of calcium signalling in cardiomyocytes when compared with those on isotropic structures. These results indicate that myocardial function is enhanced by defined scaffold orientation. The application of an ex vivo ovine cardiac perfusion model enabled direct observation of the native myocardial movement during the cardiac cycle. Through direct optical imaging and digital image correlation, collagen scaffolds were tested to assess their response to native myocardial deformation patterns. The strain dynamics of aligned and isotropic scaffold architectures were compared, and the efficacy of both glue and suture fixation methods were explored. It was determined that aligned scaffolds adhered with suture fixation complied with the native physio-mechanical environment. Similarly adhered isotropic scaffolds and patches adhered with glue, however, resulted in reduced deformation relative to the native myocardium. The work in this thesis has established a novel freeze casting technique to afford specific three dimensional control of collagen scaffold alignment. The resulting scaffolds with directionally aligned pore architectures were found to enhance cellular and mechanical dynamics to better replicate the native behaviour of myocardial tissue.
... Intrinsic control methods can affect the global microstructure and porosity but tend to act uniformly, thus lacking the influence to enact localized alterations in the pore structure. Extrinsic controls, which are defined as those that act upon the freezing process through external influences [3], include the use of sacrificial templates [76][77][78], changes in the freezing direction [79], multi-step freezing [80], and the use of applied energized fields such as magnetic [67,81,82], electric [83][84][85], and ultrasonic fields [86][87][88]. Extrinsic control methods allow for more complex pore structures to be made, with hierarchical and localized anisotropy that can differ across multiple length scales and locations. ...
... While there is a report on using magnetic fields to control iron (II, III) oxide (Fe 3 O 4 ) added to HA, Al 2 O 3 , ZrO 2 , and TiO 2 , the magnetic field had little influence on the bioceramics, instead the magnetic field caused distinct phase separation of the materials and Fe 3 O 4 [195]. As some bioinert ceramics such as TiO 2 are paramagnetic, applying strong magnetic fields to produce aligned pore structures could eventually prove useful in creating freeze-cast biomaterials [67,81]. Similarly, layered pore structures created through ultrasound freeze casting could mimic the laminar arrangement of osteons in bone [88]. ...
Freeze casting with bioceramics affords the opportunity to create the next generation of bone graft substitutes. Because of its versatile material fabrication process and effective methods of structural control, freeze casting helps to meet the many criteria that are required of viable bone graft substitute biomaterials. In combination with biocompatible and resorbable ceramics and ceramic composites that can offer bone growth through osteoconduction, this process helps provide a tailored pore structure while maintaining the necessary mechanical properties for bone growth. Here, the advantages of freeze casting with bioceramics for orthopedic and dental applications are summarized: in particular, these advantages include its compatibility with a large variety of bioceramics, many forms of both uniform and localized structure control, and its ability to be used in combination with other advanced manufacturing processes.
... Therefore, attempts have been made to change material layer orientation from being unidirectional in freeze-cast microstructure. For example, Nelson et al. [20] fabricated helical and Bouligand structures using dynamic low strength magnetic field freezecasting technique. In their work [20], porous iron oxide (Fe 3 O 4 ) scaffolds were fabricated using a low (7.8 mT) magnetic field applied in he-lical and Bouligand motions using a custom-built triaxial nested Helmholtz coils-based freeze-casting setup. ...
... For example, Nelson et al. [20] fabricated helical and Bouligand structures using dynamic low strength magnetic field freezecasting technique. In their work [20], porous iron oxide (Fe 3 O 4 ) scaffolds were fabricated using a low (7.8 mT) magnetic field applied in he-lical and Bouligand motions using a custom-built triaxial nested Helmholtz coils-based freeze-casting setup. ...
Structural orientation gradient in living organisms is attributed to their enhanced surface protection to penetration and deformation, providing bioinspiration to fabricate engineering materials with gradient in mechanical properties. Freeze-casting is a promising technique to fabricate bioinspired materials. However, in the conventional freeze-casting process, due to the applied unidirectional temperature gradient, all the material layers are also unidirectional, oriented in the direction of applied gradient. While bi-directional freezing conditions have been used but still preserving the characteristic unidirectional alignment. The main objective of this work was to demonstrate that bi-directional freezing conditions could be employed to create and control orientation gradient in freeze-cast materials and thereby develop multilayered composites with orientation gradient. Bi-directional freezing conditions were used to generate both horizontal freezing front (HFF) and vertical freezing front (VFF), where the ice crystals associated with VFF tilted the ice crystals associated with HFF, resulting in orientation gradient. 4 different bi-directional freezing conditions were achieved to change gradient microstructure. In situ studies were performed to investigate different growth front movements and calculate the velocities. Porous ceramics were infiltrated to develop bi-directionally freeze-cast composites. While the current literature sheds invaluable insights into the mechanical behavior of freeze-cast composites with unidirectional orientation, this investigation characterized uniaxial compressive mechanical response and damage characteristics of bi-directionally freeze-cast composites, which is another objective of this work. It was revealed that bi-directional freezing conditions can tune material layer orientation throughout freeze-cast microstructure, strongly influencing compressive mechanical response and damage characteristics in the resulting composite materials.
... The freeze-casting fabrication process has been extensively studied over the past 20 years for its ability to fabricate tailored, porous ceramic materials and composites with hierarchical microstructures. These novel materials have a variety of potential applications such as biomedical implants [1][2][3], energy materials [4][5][6], and impact resistant porous ceramics [7][8][9]. The porous structure of the freeze-cast ceramic is critical for each of these applications; however, due to their brittle nature and high porosity, their low fracture resistance is often a limiting factor for practical applications. ...
... Given that the critical fabrication step (step (2) above) in freeze casting occurs while the solid loading particles are unconstrained and suspended in a liquid medium, a number of techniques that employ external energized fields have been reported to provide additional control over the structure and properties of the resultant freeze-cast materials. These include the application of electric [23,24], magnetic [7,[25][26][27], and ultrasonic [28,29] fields. These techniques allow for user-tailored microstructures to be produced within the final freeze-cast materials. ...
The anisotropic mechanical properties of ultrasound freeze cast epoxy-ceramic composite materials were studied by measuring flexural strength and fracture resistance curves (R-curves) using both unnotched and notched three-point beam bending experiments, respectively, cut in three different orientations relative to the directional freezing axis. Three ultrasound frequencies of 0.699, 1.39 and 2.097 MHz were used in order to introduce different length scales into the microstructure, with 0 MHz used as the control samples. For all cases, the composites showed higher strength and fracture resistance when the crack plane cut across the direction of ice growth (denoted as the YX orientation). The anisotropic properties were more evident for the material produced without ultrasound (0 MHz) where the flexural strength was approximately 160% higher in the YX orientation compared to two orthogonal orientations. Most of the fracture resistance increase was found to occur within a crack extension, Δa, of ∼0.5 mm. Comparing the fracture resistance at Δa = 0.5 mm for the highly anisotropic 0 MHz samples showed that the YX orientation was approximately 86% tougher than the two orthogonal orientations. When the ultrasound operation frequencies of 0.699, 1.39 and 2.097 MHz were applied, the amount of anisotropy in the strength and fracture resistance gradually decreased as the operating frequency increased. The high strength and fracture resistance for the YX orientation was attributed to the alignment of the ceramic particles along the freeze front direction creating a barrier for crack propagation. Ultrasound modifies the material microstructure, introducing relatively dense ceramic layers perpendicular to the freezing front direction that act as an additional, orthogonal barrier to crack propagation. The addition of the denser layers acts to improve the mechanical properties in the weaker orientations and reduce the overall anisotropy.
... Doing so not only improves mechanical properties, but as indicated by Chi et al. aligned scaffolds lead to more ideal arrangements of osteoprogenitor cells and osteoblasts which leads to enhanced biological activity [19]. Electric fields, ultrasound waves, and magnetic fields are just a few of the ways this has been done in the past [11,19,[21][22][23]. Magnetic fields have been known to be especially effective in increasing the microstructural alignment, which has resulted in drastic improvements in mechanical properties, with specific reports detailing an increase of 200% in both the modulus and strength of scaffolds [11,24]. ...
Magnetic freeze-casting with a mix of particles and particles of increased aspect ratio (platelets) in surface-magnetized alumina scaffolds gives rise to the Hall effect, which increases the ability to tune scaffold mechanical properties in an unexpected way. Even after surface magnetization, alumina particles are only weakly magnetically susceptible, which has greatly limited their ability to be controlled via Helmholtz coils in magnetic freeze-casting due to their low-strength magnetic fields. The magnetic susceptibility of alumina can be increased by increasing the aspect ratio of alumina particles used. It was found that when a low-strength magnetic field was applied via Helmholtz coils perpendicular to the ice-growth direction to alumina scaffolds at a 1:1 weight ratio of platelets to particles (mixed scaffolds) during freeze-casting, the ultimate compressive strength perpendicular to the applied field and ice-growth direction increased in a statistically significant manner. Lowered viscosity in mixed slurries allowed for more particle movement which resulted in greater Hall voltages than in slurries with a uniform type of particle (either particles or platelets). Using this method, mechanical properties of porous alumina scaffolds can be controlled and tailored to best suit a given application. Given the wide scope of applications of alumina, this method has the potential to be highly impactful for uses ranging from aerospace to dental implants and medical devices.
Graphical abstract
... It has the potential to quickly produce scaffolds in a cost-efficient way. Previous research in ice templating has tried to control pore orientation with external stimulations, such as magnetic [44] and thermal [18]. The other existing approaches to building constructs that feature helical angle variations are stacking singleorientated layers [26,30,31], 3D printing [20,24], and rotary jet printing [8]. ...
The helical arrangement of cardiac muscle fibres underpins the contractile properties of the heart chamber. Across the heart wall, the helical angle of the aligned fibres changes gradually across the range of 90–180°. It is essential to recreate this structural hierarchy in vitro for developing functional artificial tissue. Ice templating can achieve single-oriented pore alignment via unidirectional ice solidification with a flat base mould design. We hypothesise that the orientation of aligned pores can be controlled simply via base topography, and we propose a scalable base design to recapitulate the transmural fibre orientation. We have utilised finite element simulations for rapid testing of base designs, followed by experimental confirmation of the Bouligand-like orientation. X-ray microtomography of experimental samples showed a gradual shift of 106 ± 10°, with the flexibility to tailor pore size and spatial helical angle distribution for personalised medicine.
... The research in [99] focuses on fabricating bioinspired Bouligand and helical structures through magnetic freeze casting using triaxial nested Helmholtz coils. Bouligand and helical structures tend to give high-impact resistance materials. ...
Freeze casting, popularly known as ice templating or freeze gelation, is a mechanical method to fabricate scaffolds of desirable properties and materials. Aerospace engineering, the healthcare sector, manufacturing department, and automotive industries are the different fields where freeze casting has been used. Bioinspiration refers to the translation of biological systems into new and innovative creations. Bioinspired materials are extensively used in freeze casting methods such as ceramide, spines of porcupine fish, and collagen. Due to the tunable properties and production of complex structures with ease, biomaterials have found numerous applications in the ice templating method. This review rigorously explains the freeze casting process and the effect of thermal conductivity, stress, and electrostatic repulsion on the porous materials. Also, we have discussed the different biomaterial polymers used in freeze casting along with different methods involved.
... 79 Review freeze casting can also be used to create helicoidal structures. 81,82 In this case, the ceramics are highly porous due to the formation of anisotropic ice crystals. 83 Finally, shear-driven alignment by brush painting of hydroxyapatite microfibers suspended in a sodium alginate hydrogel has been used to create Bouligand structures. ...
Mantis shrimps use their dactyl club to strike multiple high-velocity impacts against stiff and hard surfaces. To sustain the loads and dissipate energy, their club has evolved a complex multiscale organization segmented in an impact surface, an impact region, and a periodic region. Composed essentially of nanoparticles, mineralized chitin microfibers, and proteins, each region exhibits microstructural specificities linked to energy-dissipating mechanisms. Fabricating synthetic materials that exploit similar organizations and mechanisms could lead to the development of lightweight impact-resistant strategies for a multitude of applications. To this aim, the microstructure and properties of the natural dactyl club and its key toughening mechanisms are reviewed, as well as current and potential fabrication approaches. Challenges and limitations of those approaches are discussed to hopefully help guide future research on bioinspired impact-resistant materials.
... The optimization of the parameters on the coils pairs was needed to maintain the required field uniformity for the applications [25,26]. Based on the magnetic field by Helmholtz coils, helical and Bouligand porous Fe 3 O 4 scaffolds were fabricated by the dynamic low strength magnetic field free casting to improve the mechanical properties [27]. Untill now, the report about the manufacturing process equpied with Helmholtz coils is still not popular. ...
In this report, the effects of magnetic fields by using Helmholtz coils on the microstructures and mechanical properties of sand-casting Al-Cu alloys were firstly investigated. Due to the magnetic field stirring effect during the solidification process, the average grain size of sand-casting A201 ingots decreased, and the uniformity of α-Al grain increased. The grain refinement by the magnetic fields equipped with Helmholtz coils enhanced the mechanical properties of sand-casting A201 ingots, including hardness, yield strength, ultimate tensile strength and elongation. Meanwhile, according to the characterization of x-ray diffraction, preferred orientation (111) planes of α-Al phase was observed as the increase of the magnetic field. The magnetic field of Helmholtz coils provided the Lorenz force to agitate the melt during the solidification of sand-casting Al-Cu ingots, which had influence on the migration of solid-liquid interface and the rotation of the single-crystal nucleus. In summary, an easy and low-cost technique was proposed to improve the mechanical properties of sand-casting A201 alloys.
... Aside from above influencing factors, the coupling of assisting external fields such as electric field [136][137][138] , magnetic field [62,[139][140][141][142][143][144] , and acoustic field [145] influences the ice-growth direction, remotely controlling the microstructural patterns. Given that the water molecule is a dipole, strong electric (up to 150 kV m -1 ) and magnetic fields can alter the molecular structure and properties of water, resulting in the formation of various pore morphologies and orientations [146] . ...
The innovations of materials science and modern technologies are boosting the prosperity of polymer composites in various emerging multi-disciplinary fields. Cooperating with the conventional and emerging processing methods, the freeze-casting (ice-templating) technique is attracting interest in the assembling of three-dimensional structural materials (3D-SMs) accompanying the growth of ice crystals. These unique 3D-SMs with isotropic, cellular, lamellar and radially aligned structures have enabled to fabricate multifunctional polymer composites as diverse as mechanically reinforced materials, electrically conductive materials, thermally conductive materials, thermally insulating materials, adsorbents, energy-related materials, biomaterials, and many more. Herein, the working principles and methodologies of ice-templating strategy and its recent advances in shaping and structuring of 3D-SMs and production of corresponding multifunctional polymer composites are summarized. Finally, directions and prospects lying ahead are highlighted, involving the structure designs, processing routes and potential applications. Freeze-casting has manifested responsibilities for producing advanced functional composites. What is it being prepared to do?
... Novel fabrication methods for processing porous templated ceramics with controlled microstructural orientation have been proposed in recent years. Nelson et al 16 17 presented a method to fabricate the dense alumina ceramic with the deliberate periodic grain orientation by combining magnetic stirring assisted slip casting (MASC) and templated grain growth (TGG). They processed alumina ceramics with highly aligned grains by TGG and control the orientation of the anisotropic local particles by MASC. ...
In this study, a novel computational micromechanical framework is proposed to predict the effective mechanical properties of the ice‐templated ceramics under off‐axis compressive loading. The mechanical behavior is simulated by a computational micromechanical model and validated against experimental results. Smeared cracking approach was used to describe failure in ice‐templated alumina. The representative volume element (RVE) was developed based on the honeycomb analogy of lamellar walls considering the morphology of the material. The periodic boundary conditions were applied in RVE to simulate bulk behavior of the material. The compression testing was conducted on the ice‐templated alumina samples to obtain the effective compressive moduli and strength with different loading angles. Digital image correlation method was used to measure strain field during the experiment and quantify the effective misalignment angle corresponding to porous material. The effective stiffness and strength obtained from RVE analysis compared well with experimental results. The proposed micro‐mechanical RVE model allows for determining the properties of the ice‐templated porous ceramic for various off‐axis angles.
... Recent research has shown that small uniform magnetic fields (7.8 mT in field strength) can create discrete regions of pore orientation. However, the alterations to pore area and porosity do not mimic the characteristic changes in material composition and structure found in natural materials [17], [27], [30], [31]. Despite the higher degree of user control than with intrinsic methods, extrinsic methods such as magnetic and electrical field-assisted freeze casting are not capable of creating a layered microstructure in freeze-cast scaffolds and limit the constituent materials that can be used [17], [21], [25]- [29]. ...
Some natural materials, such as the dactyl club of the mantis shrimp, have impressive mechanical properties (e.g. strength) due to their microstructure that consists of periodic layers of high and low density material, which prevent crack propagation. Although such layered structures have the potential to increase the strength of engineered epoxy-ceramic composites relative to their constituents, synthetically replicating this class of layered structures in engineered materials has been challenging to date. To overcome this challenge, ultrasound freeze casting (UFC) was used to manufacture macroscale specimens of epoxy-ceramic composite materials with periodic layers of high and low density that mimic the structure of natural materials. The critical operating parameter of the UFC technique, the ultrasound operating frequency, was related to the resulting hardness, porosity, and flexural strength of the resultant epoxy-ceramic composite materials. Scanning electron microscopy and micro X-ray CT was used to visualize the microstructure of the specimens and connect it to the mechanical properties. The ultrasound operating frequency controlled the spacing of the layers as well as the local hardness of the epoxy-ceramic composite, which increased by up to 18%. The flexural strength of the epoxy-ceramic composite was also related to the ultrasound operating frequency, with a maximum increase of 52%.
Artificial bone graft stands out for avoiding limited source of autograft as well as susceptibility to infection of allograft, which makes it a current research hotspot in the field of bone defect repair. However, traditional design and manufacturing method cannot fabricate bone scaffold that well mimics complicated bone-like shape with interconnected porous structure and multiple properties akin to human natural bone. Additive manufacturing, which can achieve implant’s tailored external contour and controllable fabrication of internal microporous structure, is able to form almost any shape of designed bone scaffold via layer-by-layer process. As additive manufacturing is promising in building artificial bone scaffold, only combining excellent structural design with appropriate additive manufacturing process can produce bone scaffold with ideal biological and mechanical properties. In this article, we sum up and analyze state of art design and additive manufacturing methods for bone scaffold to realize shape/properties collaborative intelligent manufacturing. Scaffold design can be mainly classified into design based on unit cells and whole structure, while basic additive manufacturing and 3D bioprinting are the recommended suitable additive manufacturing methods for bone scaffold fabrication. The challenges and future perspectives in additive manufactured bone scaffold are also discussed.
Helicoidal composites have been found in shrimp club, lobster claw, beetle cuticle, crab shell, scorpion pincer, and fish scale as a natural material. The helicoidal composite possesses excellent impact resistance and extraordinary damage tolerance due to its hierarchical structure and the unique helicoidal arrangement of its reinforcement fibres. Its structure and performance have been studied through various characterisation and mechanical testing methods. Based on the structure-property relationship of the natural helicoidal composite, researchers have been able to mimic the unique fibre arrangement and develop bio-inspired helicoidal composites with enhanced impact performance. Various helicoidal composites comprising of synthetic fibrous materials such as carbon fibre (CF), glass fibre (GF), and aramid fibre, and matrix materials such as thermoset and thermoplastic polymers have been developed through biomimicry. The failure mechanisms of the bio-inspired helicoidal composites have been studied and the advantages of arranging the fibre reinforcement into helicoidal architectures have been elucidated over conventional composite constructions such as quasi-isotropic (QI) and cross-ply layups. This review systematically elaborates the recent progress of the research work on both natural and bio-inspired helicoidal composites. It sheds light on the distinctive construction of the natural helicoidal composites found in different animals such as shrimps, lobsters, crabs, beetles, scorpions, and fish, and their energy absorption mechanisms. Different manufacturing methods for developing bio-inspired helicoidal composites are discussed and various reinforcements and matrix materials used in the composites are described. The processing-structure-property interrelationship of the bio-inspired helicoidal composites is summarised. This review will contribute to the advancement of the knowledge of the natural helicoidal composite and potentially help researchers to develop highly efficient bio-inspired damage tolerant helicoidal composites.
Hydroxyapatite (HA) is commonly used as a bone substitute material, but it lacks mechanical strength when compared to native bone tissues. To improve the efficacy of HA as a bone substitute by improving the mechanical strength and cell growth attributes, porous composite scaffolds of HA and titania (HA‐TiO 2 ) were fabricated through a freeze‐casting process. Three different compositions by weight percent, 25–75 HA‐TiO 2 , 50–50 HA‐TiO 2 , and 75–25 HA‐TiO 2 , were custom‐made for testing. After sintering at 1250°C, these composite scaffolds exhibited improved mechanical properties compared to porous HA scaffolds. Substrate mixing was observed, which helped reduce crystal size and introduced new phases such as β‐TCP and CaTiO 3 , which also led to improved mechanical properties. The composition of 50–50 HA‐TiO 2 had the highest ultimate compressive strength of 3.12 ± 0.36 MPa and elastic modulus 63.29 ± 28.75 MPa. Human osteoblast cell proliferation assay also increased on all three different compositions when compared to porous HA at 14 days. These results highlight the potential of freeze casting composites for the fabrication of bone substitutes, which provide enhanced mechanical strength and biocompatibility while maintaining porosity.
Manufacturing processes yielding stronger, yet lighter structures are sought for in many industries and scientific applications. Freeze casting is a fabrication process that offers a way to achieve these strong, lightweight structures, but only in a single direction (the direction of the templating-ice growth). Applying a uniform magnetic field to these structures allows for increased strength in an additional direction, thus allowing for them to be applied in a variety of complex loading environments. Using a Helmholtz coil, it is possible to apply weak, uniform fields in any direction, magnitude, or frequency. Previous research using Helmholtz coils has shown that an applied field can increase strength through microstructural alignment, but the limited field strength reduces the applicability of these materials. To mitigate this, an oscillating field (i.e., a stronger magnetic field in a single direction with a weaker alternating field in an orthogonal direction) of various magnitudes of oscillation during the fabrication of freeze-cast materials was applied using Helmholtz coils. These oscillating magnetic fields led to an increase of strength of up to 2.5x compared to materials fabricated with either no applied field or a non-oscillating applied field due to increased alignment and thickness of the lamellar walls. This demonstrates that increased material response can be induced through the application of an oscillating field without increasing the maximum magnetic field strength.
With the increasing demand for orthopedic and dental reconstruction surgeries, there comes a shortage of viable bone substitutes. This study was therefore designed to assess the efficacy of fluorohydroxyapatite (FHA) as a potential bone substitute. For this, porous FHA scaffolds were fabricated using the freeze casting technique. They were then sintered at 1250, 1350 and, 1450 °C, and surface, mechanical, and in vitro properties were analyzed. The microstructure analyses revealed the porosity remained constant within the temperature range. However, the pore size decreased with increasing sintering temperature. The greatest compressive strength and elastic modulus were obtained at 1450 °C, which were 13.5 ± 4.0 MPa and 379 ± 182 MPa, respectively. These are comparable values to human trabecular bone and other porous scaffolds made using hydroxyapatite. This analysis has thus helped to attain an understanding of the mechanical and material properties of freeze-cast FHA scaffolds that have not been presented before. In vitro studies revealed an increasing rate of human osteoblast cell proliferation on freeze-cast FHA scaffolds with increasing sintering temperature, suggesting improved osteogenic properties. Additionally, osteoblasts cells were also shown to proliferate into the interior pores of all freeze-cast FHA scaffolds. These results indicate the potential of porous FHA scaffolds fabricated using the freeze casting technique to be utilized clinically as bone substitutes.
Despite extensive research, the manufacture in the bulk of high‐performance flake‐based magnetic composites with a highly aligned, nacre‐like structure remains challenging. Many challenges can be overcome by freeze casting in an externally applied, uniform magnetic field, which causes both the flakes and the composite walls of the cellular solid to align parallel to the B‐field lines. When appropriately sized, the flakes experience a second alignment parallel to the freezing direction because of a shear flow that occurs due to both the volumetric expansion of the ice phase and mold contraction during the directional solidification. The resulting orthotropic structure of the freeze‐cast magnetic composite is reflected in orthotropic mechanical and magnetic properties of the material. The magnetic composites manufactured by magnetic‐field assisted freeze casting outperform by a factor of 2–4 in terms of stiffness, strength, and toughness materials that are processed in the absence of a magnetic field and do not exhibit a monodomain architecture. Because of the highly aligned microstructure, it is possible to compact the initially lamellar composite with 90% porosity to at least 80% strain. The results presented in this study illustrate the tremendous potential for magnetic freeze casting of magnetic composites for use in power conversion.
This study investigated the compressive response of ice-templated composites and provides an understanding of their mechanical behavior based on the properties of templated ceramic and epoxy. Results suggested a dependence of properties on the microstructure of the templated porous ceramic, whereas more interestingly composites exhibited catastrophic and progressive types of failure. Compressive strength was found to be markedly greater relative to the strength of templated ceramic and polymer, and irrespective of the failure type, strength was greatly enhanced under dynamic loading relative to quasistatic loading. Compressive strength was also calculated based on the rule of mixtures and mode of failure in ice-templated ceramic. The analysis suggested that the axial mode of failure was not dominant in composites, and failures resulted from the fracture of lamella walls, possibly due to elastic instability. Fragments of the composite specimens were analyzed using scanning electron microscopy to study the fracture characteristics and rationalize the catastrophic and progressive types of failure.
Research is conducted into freeze‐casting of surface‐magnetized Fe3O4 particles under uniform, low‐strength magnetic fields (5.2 mT) to mimic the mechanical characteristics of natural human bone. Freeze‐casting is a technique that fabricates porous materials by directionally freezing and sublimating an aqueous slurry. A novel, Helmholtz coil‐based freeze‐caster is developed and it is shown that, during freeze‐casting, the use of this Helmholtz coil generates a more uniform magnetic field than permanent magnets. This uniform magnetic field, applied in the direction of ice growth, keeps particles from agglomerating and results in an increase of 55% in both the ultimate compressive strength and the elastic modulus of porous surface‐magnetized Fe3O4 scaffolds. These increases can be linked to a reduction in the porosity that occurs due to magnetic interactions between particles in the presence of the field. These results offer a novel method for the fabrication of bone‐inspired biomaterials and structural materials.
High performance lead zirconate titanate (PZT) ceramics with aligned porosity for sensing applications were fabricated by an ice-templating method. To demonstrate the enhanced properties of these materials and their potential for sensor and hydrophone applications, the piezoelectric voltage constants (g33 and g31), hydrostatic parameters (dh, gh, −d33/d31, dh·gh and dh·gh/tanδ) and AC conductivity as a function of the porosity in directions both parallel and perpendicular to the freezing temperature gradient were studied. As the porosity level was increased, PZT poled parallel to the freezing direction exhibited the highest dh, −d33/d31 and figures of merit dh·gh, dh·gh/tanδ compared to the dense and PZT poled perpendicular to the freezing direction. The gh, g33 and g31 coefficients were highest for the PZT poled perpendicular to the freezing direction; the gh was 150%–850% times higher than dense PZT, and was attributed to the high piezoelectric activity and reduced permittivity in this orientation. This work demonstrates that piezoelectric ceramics produced with aligned pores by freeze casting are a promising candidate for a range of sensor applications and the polarisation orientation relative to the freezing direction can be used to tailor the microstructure and optimise sensitivity for sensor and hydrostatic transducer applications.
We investigated and thoroughly analyzed compressive response of ice-templated ceramics in quasistatic and dynamic regimes of strain rate. In the high pore volume regime, sintered scaffolds exhibited highly lamellar pore morphology and pore architecture transitioned to dendritic with the decreasing pore volume. Mechanical property measurements in the quasistatic regime of strain rate revealed that with increasing density, compressive response transitioned from a damageable, cellular-like failure to a brittle-like failure. We rationalized the measured results in terms of the propensity of the lamella walls to undergo buckling. Our conjecture is that in the high pore volume regime, lamella walls of the scaffolds are prone to buckling-induced elastic instability, which leads to a compressive response that manifests a gradual decrease of stress beyond peak stress. In contrast, we suggest that in the low pore volume regime thick lamella walls and extensive transverse bridging exhibit marked resistance to buckling-induced instability and scaffolds undergo a global failure, which is manifested in the measured quasistatic compressive response as a sharp drop of stress beyond peak stress. Dynamic compressive response of the scaffolds exhibited measurable differences relative to the quasistatic compressive response. Scaffolds exhibited a relatively gradual decrease of dynamic compressive stress beyond peak stress, and an overall improvement of compressive response and energy absorption capacity was measured under the dynamic loading conditions. We rationalized the measured differences in between the strain rate regimes in terms of the micro-inertia and other effects. This study is of critical significance for applications of ice-templated structures in dynamic environments.
The Bouligand structure, which is found in many biological materials, is a hierarchical architecture that features uniaxial fiber layers assembled periodically into a helicoidal pattern. Many studies have highlighted the high damage-resistant performance of natural and biomimetic Bouligand structures. One particular species that utilizes the Bouligand structure to achieve outstanding mechanical performance is the smashing Mantis Shrimp, Odontodactylus Scyllarus (or stomatopod). The mantis shrimp generates high speed, high acceleration blows using its raptorial appendage to defeat highly armored preys. The load-bearing part of this appendage, the dactyl club, contains an interior region [16] that consists of a Bouligand structure. This region is capable of developing a significant amount of nested twisting microcracks without exhibiting catastrophic failure. The development and propagation of these microcracks are a source of energy dissipation and stress relaxation that ultimately contributes to the remarkable damage tolerance properties of the dactyl club. We develop a theoretical model to provide additional insights into the local stress intensity factors at the crack front of twisting cracks formed within the Bouligand structure. Our results reveal that changes in the local fracture mode at the crack front leads to a reduction of the local strain energy release rate, hence, increasing the necessary applied energy release rate to propagate the crack, which is quantified by the local toughening factor. Ancillary 3D simulations of the asymptotic crack front field were carried out using a J-integral to validate the theoretical values of the energy release rate and the local stress intensity factors.
The use of weak magnetic fields to control the microstructural evolution of colloidal‐based systems in conjunction with directional solidification is demonstrated as a convenient processing route to fabricate anisotropic ceramic scaffolds with complex microarchitectures. A variety of graded and aligned microstructures were formed by applying external static magnetic fields oriented radially, axially, and transversely with respect to the solidification direction of freezing slurries containing micro/nanoparticles of ZrO2 and Fe3O4. The graded structures, formed by the radial and axial fields, resemble core–shell architectures composed of dense outer perimeters surrounding porous inner cores. The aligned structures, formed by transverse fields, exhibit two modes of microstructural alignment: lamellar walls aligned by the growing ice crystals and mineral bridges aligned by the magnetic fields. The alignment of mineral bridges that connect adjacent lamellae, provide these scaffolds enhanced strength and stiffness when compressed parallel to their orientation (parallel to the direction of the magnetic field).
The structure of the stomatopod dactyl club-an ultrafast, hammer-like device used by the animal to shatter hard seashells-offers inspiration for impact-tolerant ceramics. Here, we present the micromechanical principles and related micromechanisms of deformation that impart the club with high impact tolerance. By using depth-sensing nanoindentation with spherical and sharp contact tips in combination with post-indentation residual stress mapping by Raman microspectroscopy, we show that the impact surface region of the dactyl club exhibits a quasi-plastic contact response associated with the interfacial sliding and rotation of fluorapatite nanorods, endowing the club with localized yielding. We also show that the subsurface layers exhibit strain hardening by microchannel densification, which provides additional dissipation of impact energy. Our findings suggest that the club's macroscopic size is below the critical size above which Hertzian brittle cracks are nucleated.
Scaffolds with potential biological applications having a variety of microstructural and mechanical properties can be fabricated by freezing colloidal solutions into porous solids. In this work, the structural and mechanical properties of TiO2 freeze cast with different soluble additives, including polyethylene glycol, NaOH or HCl, and isopropanol alcohol, are characterized to determine the effects of slurry viscosity, pH, and alcohol concentration on the freezing process. TiO2 powders mixed with water and these different additives are directionally frozen in a mold, then sublimated and sintered to create the porous scaffolds. The different scaffolds are characterized to compare the compressive strength, modulus, porosity, and pore morphology. For all scaffolds, the overall porosity remains constant (80–85%). By changing the concentration of each additive, the lamellar thickness, pore area, and aspect ratio vary significantly, showing inverse relationships to both the compressive strength and modulus. The strength is predicted from the pore aspect ratio of the scaffolds when subjected to compressive loading with the primary failure mode identified as Euler buckling. TiO2 scaffolds freeze cast with different soluble additives are suitable for biomedical applications, such as bone replacements, requiring high porosity and specific pore morphologies.
Through evolutionary processes, biological composites have been optimized to fulfil specific functions. This optimization is exemplified in the mineralized dactyl club of the smashing predator stomatopod (specifically, Odontodactylus Scyllarus). This crustacean's club has been designed to withstand the thousands of high-velocity blows that it delivers to its prey. The endocuticle of this multiregional structure is characterized by a helicoidal arrangement of mineralized fiber layers, an architecture which results in impact resistance and energy absorbance. Here, we apply the helicoidal design strategy observed in the stomatopod club to the fabrication of high-performance carbon fiber-epoxy composites. Through experimental and computational methods, a helicoidal architecture is shown to reduce through-thickness damage propagation in a composite panel during an impact event and result in an increase in toughness. These findings have implications in the design of composite parts for aerospace, automotive and armor applications.
This paper investigates the behavior of colloidal suspensions of alumina particles during directional solidification, by in situ high-resolution observations using X-ray radiography and tomography. This second part is focussed on the evolution of ice crystals during steady-state growth (in terms of interface velocity) and on the particle redistribution taking place in this regime. In particular, it is shown that particle diffusion cannot determine the particle concentration profile in this regime of interface velocities (20–40 μm/s). Particles are redistributed by a direct interaction with the moving solidification interface. Several parameters controlling the particle redistribution were identified, namely the interface velocity, the particle size, the shape of the ice crystals, and the orientation relationships between the crystals and the temperature gradient.
Hammering Home the Lesson
Stomatopods are marine crustaceans that use hammerlike claws for defense and to attack their prey. The claws undergo repeated high-velocity and high-force impacts. Weaver et al. (p. 1275 ; see the Perspective by Tanner ) used a variety of techniques to examine the structure, mechanical behavior, and toughening mechanisms of the claw of the Peacock Mantis shrimp. The claw's composite structure is optimized for toughness, which helps to prevent the complete failure that might arise from the claw's repetitive hammering.
Directional freeze-casting – a process used to create foams with elongated, aligned pores applied so far exclusively for ceramics – is demonstrated for titanium foams. An aqueous slurry of <45 μm titanium powders was directionally solidified, resulting in a powder preform consisting of elongated, aligned dendrites of pure ice separated by interdendritic regions with high powder content. After freeze-drying to remove the ice dendrites and sintering to densify the powders, the resulting titanium foams exhibited 57–67% aligned pores (∼0.1 mm wide and several millimeters long) replicating the ice dendrites. Because of the high powder oxygen content, the foams display high compressive strength and signs of embrittlement. Lower contamination was achieved by using purer <125 μm powders, but their larger size prevents the formation of pure ice dendrites (and thus elongated pores in the foam), in agreement with a model considering particle pushing and engulfment by a moving ice front.
The functional requirements for synthetic tissue substitutes appear deceptively simple: they should provide a porous matrix with interconnecting porosity and surface properties that promote rapid tissue ingrowth; at the same time, they should possess sufficient stiffness, strength and toughness to prevent crushing under physiological loads until full integration and healing are reached. Despite extensive efforts and first encouraging results, current biomaterials for tissue regeneration tend to suffer common limitations: insufficient tissue-material interaction and an inherent lack of strength and toughness associated with porosity. The challenge persists to synthesize materials that mimic both structure and mechanical performance of the natural tissue and permit strong tissue-implant interfaces to be formed. In the case of bone substitute materials, for example, the goal is to engineer high-performance composites with effective properties that, similar to natural mineralized tissue, exceed by orders of magnitude the properties of its constituents. It is still difficult with current technology to emulate in synthetic biomaterials multi-level hierarchical composite structures that are thought to be the origin of the observed mechanical property amplification in biological materials. Freeze casting permits to manufacture such complex, hybrid materials through excellent control of structural and mechanical properties. As a processing technique for the manufacture of biomaterials, freeze casting therefore has great promise.
In this research, Fe 3 O 4 particles were magnetically manipulated to create porous scaffolds using a tri-axial nested Helmholtz coils-based freeze-casting setup. This novel setup allowed for a uniform magnetic field to be applied in any direction and for it to effectively change directions at any time. Applying a uniform low magnetic field of 7.8 mT in various directions was investigated to fabricate a variety of tailored microstructures and mechanical properties in the resultant scaffolds. It was observed that using the magnetic field aligned up to 81% of the lamellar walls and also altered the area and shape of the pores of the resultant scaffolds. This lamellar wall alignment occurred at every applied magnetic field direction due to the Fe 3 O 4 particles aligning during the freeze-casting process. As a result of this alignment, increases in the mechanical properties of up to 4.1× were observed. The results provide a novel experimental technique for the fabrication of user-defined microstructures in Fe 3 O 4 -based freeze-cast materials that provides significant advantages over previous experimental setups for magnetic freeze casting.
Lightweight porous engineered materials are advantageous in a broad range of research fields because they combine desirable mechanical properties with the ability to leverage their porous structure. Existing techniques for fabricating porous material structures are limited by material choice, require multiple steps, and/or additional post-processing to create regions of varying material properties within the material structure, and are not easily scalable. In contrast, we implement and characterize a fabrication process for macroscale porous engineered material samples with a user-specified microstructure, by combining freeze casting, which allows fabrication of porous materials samples, with ultrasound directed self-assembly, which allows controlling the microstructure of the porous materials. We refer to this process as “ultrasound freeze casting (UFC),” and employ it to fabricate bioinspired materials that mimic the concentric rings of natural materials such as osteons and Liesegang rings. Specifically, we employ the UFC process to create material samples with three, four, and five concentric rings of alternating dense and porous TiO2 material. We find statistically significant differences of both the porosity and Vickers hardness when comparing the porous and dense regions of the material samples. These results will provide a new pathway to fabricate porous engineered materials with user-specified microstructure.
Magnetic freeze casting utilizes the freezing of water, a low magnetic field and surface magnetized materials to make multi-axis strengthened porous scaffolds. A much greater magnetic moment was measured for larger magnetized alumina platelets compared with smaller particles, which indicated that more platelet aggregation occurred within slurries. This led to a more lamellar wall alignment along the magnetic field direction during magnetic freeze casting at 75 mT. Slurries with varying ratios of magnetized particles to platelets (0:1, 1:3, 1:1, 3:1, 7:1, 1:0) produced porous scaffolds with different structural features and degrees of lamellar wall alignment. The greatest mechanical enhancement in the magnetic field direction was identified in the synergistic condition with the highest particle to platelet ratio (7:1). Magnetic freeze casting with varying ratios of magnetized anisotropic and isotropic alumina provided insights about how heterogeneous morphologies aggregate within lamellar walls that impact mechanical properties. Fabrication of strengthened scaffolds with multi-axis aligned porosity was achieved without introducing different solid materials, freezing agents or additives. Resemblance of 7:1 particle to platelet scaffold microstructure to wood light-frame house construction is framed in the context of assembly inspiration being derived from both natural and synthetic sources.
Freeze casting of traditional ceramic suspensions and freeze casting of preceramic polymer solutions were directly compared as methods for processing porous ceramics. Alumina and polymethylsiloxane were freeze cast with four different organic solvents (cyclooctane, cyclohexane, dioxane, and dimethyl carbonate) to obtain ceramics with ∼70% porosity. Median pore sizes were smaller for solution freeze casting than for suspension freeze casting under identical processing conditions. The pore structures, which range from foam-like to lamellar, were correlated to the Jackson α-factor of the solvent; solvents with low α-factors yielded nonfaceted pore structures, while high α-factors produced more faceted structures. Intermediate α-factors resulted in dendritic pore structures and were most sensitive to the processing method. Small suspended particles ahead of a solid–liquid interface are hypothesized to destabilize the dendrite tip in suspension freeze casting resulting in more foam-like structures. Differences in processing details were highlighted, particularly regarding the improved freezing front observation possible with solution-based freeze casting.
Bone consists of a hard mineral phase and a compliant biopolymer phase resulting in a composite material that is both lightweight and strong. Osteoporosis that degrades spongy bone preferentially over time leads to bone brittleness in the elderly. A porous ceramic material that can mimic spongy bone for a one-time implant provides a potential solution for the future needs of an aging population. Scaffolds made by magnetic freeze casting resemble the aligned porosity of spongy bone. A magnetic field applied throughout freezing induces particle chaining and alignment of lamellae structures between growing ice crystals. After freeze drying to extract the ice and sintering to strengthen the scaffold, cubes from the scaffold center are mechanically compressed along longitudinal (z-axis, ice growth direction) and transverse (y-axis, magnetic field direction) axes. The best alignment of lamellar walls in the scaffold center occurs when applying magnetic freeze casting with the largest particles (350 nm) at an intermediate magnetic field strength (75 mT), which also agrees with stiffness enhancement results in both z and y-axes. Magnetic moments of different sized magnetized alumina particles help determine the ideal magnetic field strength needed to induce alignment in the scaffold center rather than just at the poles.
A fibrous herringbone-modified helicoidal architecture is identified within the exocuticle of an impact-resistant crustacean appendage. This previously unreported composite microstructure, which features highly textured apatite mineral templated by an alpha-chitin matrix, provides enhanced stress redistribution and energy absorption over the traditional helicoidal design under compressive loading. Nanoscale toughening mechanisms are also identified using high load nanoindentation and in-situ TEM picoindentation.
The processing technique of freeze casting has been intensely researched for its potential to create porous scaffold and infiltrated composite materials for biomedical implants and structural materials. However, in order for this technique to be employed medically or commercially, it must be able to reliably produce materials in great quantities with similar microstructures and properties. Here we investigate the reproducibility of the freeze casting process by independently fabricating three sets of eight ZrO2–epoxy composite scaffolds with the same processing conditions but varying solid loading (10, 15 and 20 vol.%). Statistical analyses (One-way ANOVA and Tukey's HSD tests) run upon measurements of the microstructural dimensions of these composite scaffold sets show that, while the majority of microstructures are similar, in all cases the composite scaffolds display statistically significant variability. In addition, composite scaffolds where mechanically compressed and statistically analyzed. Similar to the microstructures, almost all of their resultant properties displayed significant variability though most composite scaffolds were similar. These results suggest that additional research to improve control of the freeze casting technique is required before scaffolds and composite scaffolds can reliably be reproduced for commercial or medical applications.
Eight structural elements in biological materials are identified as the most common amongst a variety of animal taxa. These are proposed as a new paradigm in the field of biological materials science as they can serve as a toolbox for rationalizing the complex mechanical behavior of structural biological materials and for systematizing the development of bioinspired designs for structural applications. They are employed to improve the mechanical properties, namely strength, wear resistance, stiffness, flexibility, fracture toughness, and energy absorption of different biological materials for a variety of functions (e.g., body support, joint movement, impact protection, weight reduction). The structural elements identified are: fibrous, helical, gradient, layered, tubular, cellular, suture, and overlapping. For each of the structural design elements, critical design parameters are presented along with constitutive equations with a focus on mechanical properties. Additionally, example organisms from varying biological classes are presented for each case to display the wide variety of environments where each of these elements is present. Examples of current bioinspired materials are also introduced for each element.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
This paper provides an optimal parametric design for tri-axial nested Helmholtz coils, which are used to generate a uniform magnetic field with controllable magnitude and direction. Circular and square coils, both with square cross section, are considered. Practical considerations such as wire selection, wire-wrapping efficiency, wire bending radius, choice of power supply, and inductance and time response are included. Using the equations provided, a designer can quickly create an optimal set of custom coils to generate a specified field magnitude in the uniform-field region while maintaining specified accessibility to the central workspace. An example case study is included.
This work investigated the behavior of TiO2-containing α-Al2O3 samples prepared by the freeze-casting technique. Camphene and liquid nitrogen were used as the solvent and cooling fluid, respectively. Camphene resulted in the formation of dendritic pores, in the direction of the freeze-casting cold front during sample preparation. The formation of β-Al2TiO5 phase occurred at 1300 °C, and became more evident as the sintering temperatures reached 1500 °C. The TiO2 loading did not affect the sample porosity at a given temperature, but it was detrimental in the case of mechanical properties under certain conditions. For instance, the flexural strength slightly improved with increasing the TiO2 loading and sintering temperature from 1100 to 1300 °C. This effect was attributed to the occurrence of a more effective sintering of alumina. However, as the heat treatment temperature was raised from 1300 to 1500 °C, the flexural strength did not increase as a function of the TiO2 loading, even though the densification occurred with loss of porosity. The loss of mechanical strength was found to be associated with the formation of microcracks which stemmed from the formation of β-Al2TiO5 phase for TiO2 loadings in excess of 4 wt% at these high sintering temperatures.
Ice templating is able to do much more than macroporous, cellular materials. The underlying phenomenon—the freezing of colloids—is ubiquitous, at a unique intersection of a variety of fields and domains, from materials science to physics, chemistry, biology, food engineering, and mathematics. In this review, I walk through the seemingly divergent domains in which the occurrence of freezing colloids can benefit from the work on ice templating, or which may provide additional understanding or inspiration for further development in materials science. This review does not intend to be extensive, but rather to illustrate the richness of this phenomenon and the obvious benefits of a pluridisciplinary approach for us as materials scientists, and for other scientists working in areas well outside the realms of materials science.
Structured xylan-based hydrogels, reinforced with cellulose nanocrystals (CNCs), have successfully been prepared from water suspensions by cross-linking during freeze-casting. In order to induce cross-linking during the solidification/sublimation operation, xylan was first oxidized using sodium periodate to introduce dialdehydes. The oxidized xylan was then mixed with CNCs after which the suspension was frozen unidirectionally in order to control the ice crystal formation and by that the pore morphology of the material. Finally the ice crystal templates were removed by freeze-drying. During the freeze-casting process hemiacetal bonds are formed between the aldehyde groups and hydroxyl groups, either on other xylan molecules or on CNCs, which cross-links the system. The proposed cross-linking reaction was confirmed by using cross-polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy. The pore morphology of the obtained materials was analyzed by scanning electron microscopy (SEM). The materials were also tested for compressive strength properties, both in dry and water swollen state. All together this study describes a novel combined freeze-casting/cross-linking process which enables fabrication of nanoreinforced biopolymer-based hydrogels with controlled porosity and 3-D architecture.
Freeze casting of aqueous suspension was investigated as a method for fabricating hydroxyapatite (HA) porous ceramics with lamellar structures. The rheological properties of HA suspensions employed in the ice-templated process were investigated systematically. Well aligned lamellar pores and dense ceramic walls were obtained successfully in HA porous ceramics with the porosity of 68–81% and compressive strength of 0.9–2.4 MPa. The results exhibited a strong correlation between the rheological properties of the employed suspensions and the morphology and mechanical properties of ice-templated porous HA ceramics, in terms of lamellar pore characteristics, porosities and compressive strengths. The ability to produce aligned pores and achieve the manipulation of porous HA microstructures by controlling the rheological parameters were demonstrated, revealing the potential of the ice-templated method for the fabrication of HA scaffolds in biomedical applications.
Magnetic field aligned freeze casting is a novel method to fabricate porous, anisotropic ceramic scaffolds with a hierarchy of architectural alignment in multiple directions. A weak rotating magnetic field applied normal to the ice growth direction in a uniaxial freezing apparatus allowed the manipulation of magnetic nanoparticles to create different pore structures and channels with long-range order in directions parallel and perpendicular to the freezing direction. Porous scaffolds consisting of different host ceramics (hydroxyapatite (HA), ZrO2, Al2O3, or TiO2) mixed with varying concentrations (0–9 wt%) of Fe3O4 nanoparticles were fabricated by freeze casting under three different conditions: (1) no magnetic field, (2) a static magnetic field of 0.12 T, or (3) a rotating magnetic field of 0.12 T at 0.05 rpm. The HA, ZrO2, and Al2O3 scaffolds showed biphasic material properties with separate Fe3O4-rich and Fe3O4-poor regions. The TiO2 scaffolds showed homogeneous distributions of Fe3O4 throughout the macrostructures, which resulted in aligned pore channels parallel to the magnetic field, normal to the ice growth direction. In the magnetic field direction, the compressive strength and stiffness of the TiO2 scaffolds containing Fe3O4 was doubled. The enhanced mechanical performance of the field aligned TiO2 scaffolds are the result of the long-range microstructural order in multiple directions—(1) the magnetic field direction and (2) the ice growth direction.
One of the most intriguing protein materials found in nature is bone, a material composed of assemblies of tropocollagen molecules and tiny hydroxyapatite mineral crystals that form an extremely tough, yet lightweight, adaptive and multifunctional material. Bone has evolved to provide structural support to organisms, and therefore its mechanical properties are of great physiological relevance. In this article, we review the structure and properties of bone, focusing on mechanical deformation and fracture behavior from the perspective of the multidimensional hierarchical nature of its structure. In fact, bone derives its resistance to fracture with a multitude of deformation and toughening mechanisms at many size scales ranging from the nanoscale structure of its protein molecules to the macroscopic physiological scale.
Surprisingly few reports have been published, to date, on the structure-property-processing correlations observed in freeze-cast materials directionally solidified from polymer solutions, or ceramic or metal slurries. The studies that exist focus on properties of sintered ceramics, thus materials whose structure was altered by further processing. In this contribution, we report first results on correlations observed in alumina-chitosan-gelatin composites, which were chosen as model system to test and compare the effect of particle size and processing parameters on their mechanical properties at a specific composition. Our study reveals that highly porous (>90%) hybrid materials can be manufactured by freeze casting, through the self-assembly of a polymer and a ceramic phase that occurs during directional solidification without the need of additional processing steps like sintering or infiltration. It further illustrates that the properties of freeze-cast hybrid materials can independently be tailored at two levels of their structural hierarchy, allowing for the simultaneous optimization of both mechanical and structural requirements. An increase in freezing rate resulted in a decrease in lamellar spacing, cell wall thickness, pore aspect ratio and cross-sectional area, as well as an increase in both Young's modulus and compressive yield strength. The mechanical properties of the composite scaffolds increased with an increasing particle size. The results show that both structure and mechanical properties of the freeze-cast composites can be custom-designed and that they are thus ideally suited for a large variety of applications that require high porosity at low or medium load-bearing capacity.
The solidification behavior of suspensions of alumina particles during directional solidification is investigated here by in situ observations using X-ray radiography and tomography. The objective of this study was to assess the influence of particle size on the solidification behavior of the suspensions during the early stages of solidification. Four powders with particle size in the range of 0.2–3.4 μm (median size) were investigated. Solidification is obtained by cooling at a constant rate, starting from room temperature. Attention is specifically paid to the nucleation and growth behavior of the ice crystals in these suspensions. We propose that the nucleation of ice crystals is controlled by the particle size, the surface of the particles acting as nucleation sites. Smaller particle size leads to a lower degree of supercooling because nucleation and growth can proceed at a higher temperature than with larger particles. The initial interface velocity is dependent on the degree of supercooling, and controls the extent of the initial structural gradient in the resulting porous materials.
This paper reports a novel method for producing porous Ti scaffolds with a gradient in porosity and pore size using the freeze casting method, in which TiH2/camphene slurries with various TiH2 contents (40, 25, and 10 vol.%) were cast sequentially into a mold, followed by freeze drying and heat-treatment in a vacuum at 1300 °C for 3 h. This simple sequential freeze casting method produced good bonding between the layers with different porosities of 35, 53, and 75 vol.% obtained using the TiH2 contents of 40, 25 and 10 vol.%, respectively. In addition, the pore size could be increased significantly by increasing the freezing time. The pore sizes obtained in the regions produced using 40, 25, and 10 vol.% TiH2 after freezing for 7 days were 96, 166, and 270 μm, respectively.
The split Hopkinson pressure bar (SHPB) is being widely used to determine the dynamic compressive strength of ceramics and ceramic composites. However, extreme caution needs to be exercised while testing these highstrength ceramics at high strain rates. The highest strain rate at which ceramics can be tested using an SHPB without violating the underlying assumptions is found to be in the range of 2500–3000/s. It is also shown that at very high loading rates, dispersion in the transmitted pulse can lead to discrepancies in measuring the dynamic failure strength of ceramics.
The formation of regular patterns is a common feature of many solidification processes involving cast materials. We describe here how regular patterns can be obtained in porous alumina by controlling the freezing of ceramic slurries followed by subsequent ice sublimation and sintering, leading to multilayered porous alumina structures with homogeneous and well-defined architecture. We discuss the relationships between the experimental results, the physics of ice, and the interaction between inert particles and the solidification front during directional freezing. The anisotropic interface kinetics of ice leads to numerous specific morphological features in the structure. The structures obtained here could have numerous applications, including ceramic filters and biomaterials, and could be the basis for dense multilayered composites after infiltration with a selected second phase.
Innovative metal/ceramic composites produced by melt infiltration of ceramic preforms prepared by a freeze-casting technique are examined in this study. These composites exhibit a characteristic hierarchical structure: On a mesoscopic length scale, lamellar domains with sizes up to several millimetres are observed. The aim of the present study was to analyze the anisotropic elastic properties of such a composite on different length scales. Experimental studies were carried out on samples containing about 44% alumina ceramic and 56% aluminium-based alloy (Al-12Si). Ultrasonic wave velocity measurements were carried out on macroscopic poly-domain samples as well as on miniature single-domain samples. The elastic constants derived from these measurements are discussed in light of elasticity models established for lamellar and for fibrous composites. (c) 2007 Elsevier Ltd. All rights reserved.
Silica aerogels are highly porous solid materials consisting of three-dimensional networks of silica particles and are typically obtained by removing the liquid in silica gels under supercritical conditions. Several unique attributes such as extremely low thermal conductivity and low density make silica aerogels excellent candidates in the quest for thermal insulation materials used in space missions. However, native silica aerogels are fragile at relatively low stresses. More durable aerogels with higher strength and stiffness are obtained by proper selection of silane precursors and by reinforcement with polymers. This paper first presents a brief review of the literature on methods of silica aerogel reinforcement and then discusses our recent activities in improving not only the strength but also the elastic response of polymer-reinforced silica aerogels. Several alkyl-linked bis-silanes were used in promoting flexibility of the silica networks in conjunction with polymer reinforcement by epoxy.
A comparison is made between certain fibrous and regularly twisted biological materials and certain ordered liquids commonly called 'esteric liquid crystals' Three examples of twisted arrangements (Arthropod cuticle, Ascidian tunica, Dinoflagellate chromosomes) and typical textures of cholesteric mesophases are studied for their optical properties and their defects. These materials are strongly different. Very long polymer chains occur in the organic matrix of skeletal tissues or in chromosomes. On the contrary, in a cholesteric liquid crystal, the molecules are free to move one along the other. However the geometry of such systems is similar. The objections to the twisted model are reviewed and evidence is presented to support a generalized twisted model. A list of the known biological cholesteric analogues is given.
For bone to remodel adaptively, the cells responsible should follow some algorithm. Nine different loading situations and structures are discussed. It seems that either algorithm must be extremely complex, or cells in different structures must follow different algorithms.
A novel method which employs water present in swollen hydrogels as a porogen for shape template was suggested for preparing porous materials. Biodegradable hydrogels were prepared through crosslinking of gelatin with glutaraldehyde in aqueous solution, followed by rinsing and washing. After freezing the swollen hydrogels, the ice formed within the hydrogel network was sublimated by freeze-drying. This simple method produced porous hydrogels. Irrespective of any rinsing and washing processes, water was homogeneously distributed into the hydrogel network, allowing the hydrogel network to uniformly enlarge and the ice to act as a porogen during the freezing process. Different porous structures were obtained by varying the freezing temperature. Hydrogels frozen in liquid nitrogen, had a two-dimensionally ordered structure, while the hydrogels prepared at freezing temperatures near -20 degrees C, showed a three-dimensional structure with interconnected pores. As the freezing temperature was lowered, the hydrogel structure gradually became more two-dimensionally ordered. These results suggest that the porosity of dried hydrogels can be controlled by the size of ice crystals formed during freezing. It was concluded that the present freeze-drying procedure is a bio-clean method for formulating biodegradable sponges of different pore structures without use of any additives and organic solvents.
Because of many suitable properties, collagen sponges are used as an acellular implant or a biomaterial in the field of tissue engineering. Generally, the inner three-dimensional structure of the sponges influences the behavior of cells. To investigate this influence, it is necessary to develop a process to produce sponges with a defined, adjustable, and homogeneous pore structure. Collagen sponges can be produced by freeze-drying of collagen suspensions. The pore structure of the freeze-dried sponges mirrors the ice-crystal morphology after freezing. In industrial production, the collagen suspensions are solidified under time- and space-dependent freezing conditions, resulting in an inhomogeneous pore structure. In this investigation, unidirectional solidification was applied during the freezing process to produce collagen sponges with a homogeneous pore structure. Using this technique the entire sample can be solidified under thermally constant freezing conditions. The ice-crystal morphology and size can be adjusted by varying the solute concentration in the collagen suspension. Collagen sponges with a very uniform and defined pore structure can be produced. Furthermore, the pore size can be adjusted between 20-40 microm. The thickness of the sponges prepared during this research was 10 mm.
The covariation of a number of mechanical of properties, and some physical characteristics, of compact bones from a wide range of bones were examined. Young's modulus was well predicted by a combination of mineral content and porosity. Increasing Young's modulus was associated with: increasing stress at yield, increasing bending strength, and a somewhat higher resilience, tensile strength and fatigue strength. Contrarily, in the post-yield region a higher Young's modulus (and more clearly, a higher mineral content) was associated with: a reduced work to fracture in tension, a reduced impact strength and an increased notch sensitivity in impact. Increasing porosity is associated with deleterious effects in the pre-yield region, but has little effect in the post-yield region. Bone, like many other materials, is unable to have good qualities in both the pre- and post-yield regions. Since an increase in mineral or Young's modulus is more potent, that is deleterious, in the post-yield than it is advantageous in the pre-yield region, it is likely that mineral content will be selected to be slightly lower than would be the case if it were equally potent in both regions. As is usual in biology, different adaptive extremes are incompatible.
The current state of materials systems used in total hip replacement is presented in this paper. An overview of the various material systems used in total hip replacement reported in literature is presented in this paper. Metals, polymers, ceramics and composites are used in the design of the different components of hip replacement implants. The merits and demerits of these material systems are evaluated in the context of mechanical properties most suitable for total joint replacement such as a hip implant. Current research on advanced polymeric nanocomposites and biomimetic composites as novel materials systems for bone replacement is also discussed. This paper examines the current research in the materials science and the critical issues and challenges in these materials systems that require further research before application in biomedical industry.
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