Nature has taught us fascinating strategies to design materials such that they exhibit superior and novel properties. Shells of mantis club have protein fibres arranged in a 3D helicoidal architecture that give them remarkable strength and toughness, enabling them to absorb high-impact energy. This complex architecture is now possible to replicate with the recent advances in additive manufacturing. In this paper, we used melt electrospinning to fabricate 3D polycaprolactone (PCL) fibrous design to mimic the natural helicoidal structures found in the shells of the mantis shrimp’s dactyl club. To improve the tensile deformation behavior of the structures, the surface of each layer of the samples were treated with carboxyl and amino groups. The toughness of the surface-treated helicoidal sample was found to be two times higher than the surface-treated unidirectional sample and five times higher than the helicoidal sample without surface treatment. Free amino groups (NH2) were introduced on the surface of the fibres and membrane via surface treatment to increase the interaction and adhesion among the different layers of membranes. We believe that this represents a preliminary feasibility in our attempt to mimic the 3D helicoidal architectures at small scales, and we still have room to improve further using even smaller fibre sizes of the modeled architectures. These lightweight synthetic analogue materials enabled by electrospinning as an additive manufacturing methodology would potentially display superior structural properties and functionalities such as high strength and extreme toughness.
Natural biological composites have captured tremendous scientific interest in recent years among researchers to develop high-performance synthetic composites [
In a recent study, Weaver et al. attributed the high toughness and strength of mantis shrimp’s dactyl club to its microstructure and unique arrangement of protein fibres within the structures. Investigations have also attributed the mechanical strength and toughness of the material to the complex interplay between structure, stiffness, strength, and impact mechanics of the dactyl club [
Methodology, design, and SEM images of one layer of 3D Helicoidal Structure. (a) Schematics of near-field melt electrospinning (NFES)—the process used to first lay down patterns of electrospun fibres on top a collector; (b) illustration of a top view of the hierarchical helicoidal design of the basic building blocks found in shells of mantis shrimp dactyl club, as reported in the literature; (c) scanning electron microscope (SEM) image of a layer of highly aligned, dense fibre-structured samples of melt electrospinning fibres with diameters around 10
Mimicking this structured design could lead to the development of synthetic impact resistant materials. Although some studies have characterized the mechanical deformation behavior of the natural material, very few studies have concentrated their efforts on fabricating synthetic structures [
We believe that the helicoidal structure adapts to tensile stress by allowing the plane of fibres to rotate toward or away from the applied tensile load, prolonging the final catastrophic events of failures. The reason behind the unique strength and toughness of the structure is its ability to adapt to the loading stress. Thus, we establish a fundamental understanding of the fracture-delaying mechanisms in the 3D hierarchy microarchitectured advanced materials enabled by additive manufacturing to achieve remarkable structural properties and functionalities.
Strengthening the biomimetic 3D hierarchical nano- and microscale architectures, enabled by additive manufacturing, is the unique aspect of this article. The development of such low-cost, lightweight, high-performance materials will be highly useful for protective technologies such as in soldier body armors, sports/athletic gears, and aerospace/aircraft applications.
A custom-built near-field melt electrospinning setup was used to fabricate the samples. Briefly, polycaprolactone (PCL, molecular weight = 45 kDa, Sigma-Aldrich, Singapore) pellets were fed into a metallic syringe surrounded by a hot water jacket system (NanoNC, Korea). The temperature was raised to 80°C to melt the polymer. Following this step, a high voltage power supply was connected to the needle. The flow rate and the voltage used during the electrospinning were 20
SEM image of the 3D helicoidal structure with 15° rotations between each layer. (a) SEM image showing cross section of the samples with a few stacked layers of fibre aligned at different direction at 95x magnification (the red-dashed lines are to indicate the boundary between layers), demonstrating of the basic feasibility to synthesize the full 3D helicoidal fibre structures similar to the natural material; (b) SEM image of the sample viewed from the top.
Another set of samples were prepared by functionalizing the surface of the fibres before they were hot-pressed. Briefly, once the aligned arrays of fibres were obtained, carboxyl groups were introduced on the surface of the fibres by treating the fibres with low-pressure air plasma for 5 minutes using the electrodeless, inductively coupled RFGD instrument (PDC-002; Harrick Plasma, Ithaca, NY, USA) [
The cross section of the helicoidal samples is investigated by fracturing the samples under liquid nitrogen and imaging them using a scanning electron microscope (SEM). To view the microstructure of the samples, the samples were sputter-coated with gold (18 mA, 90 seconds) before their structures were examined using a scanning electron microscope (Tescan, accelerating voltage of 10 kV).
The tensile deformation behavior of the samples was investigated using a Zwick Roell (Zwick Roell, Ulm, Germany) Z0.5 static testing machine (Figure
Tensile test setup and sample preparation. (a) Zwick Roell static testing machine for tensile test. (b) Partial magnification of sample placing. (c) Side view of tensile test sample, four pieces of acrylic slabs with super glue were used to attach the samples. (d) Top view of tensile test sample.
A differential scanning calorimeter (DSC) from TA Instruments, DSC Q100, was used to determine the crystallinity and melting behavior of the PCL samples. The temperature was ramped at 3°C/min from 0°C temperature to 80°C under a nitrogen atmosphere. The thermal degradation temperature of the helicoidal PCL sample and bulk PCL was determined using thermogravimetric analyzer (TGA, TA Q50). The weight loss of the samples as a function of temperature was recorded at a heating rate of 10°C/min in the N2 atmosphere.
Figure
Our objective is to investigate the mechanisms leading to the enhanced mechanical properties of the 3D hierarchical microarchitectured materials inspired by nature while maintaining other desirable properties and/or functionalities.
The tensile tester is used to investigate the tensile deformation behavior of the samples. Digital images of the samples are taken during the deformation process. Figure
Various stages of tensile deformation of polycaprolactone (PCL) samples. (a1–a4) Bulk PCL. (b1–b4) Unidirectionally oriented PCL sample, composed of fibres aligned along the loading axis. (c1–c4) Helicoidal PCL sample, composed of fibres aligned at different angles forming helicoidal structure.
Stress-strain curves and toughness measurements. (a) Bulk PCL in comparison with unidirectionally oriented PCL fibre sample and helicoidal PCL fibre sample without surface treatment. (b) Bulk PCL in comparison with surface-treated unidirectionally oriented PCL fibre sample and surface-treated helicoidal PCL fibre sample. The experimental error bars are provided here to indicate variations within each group of the samples at max stress values and subsequently at strain values of 200%, 400%, and 600%. The variations of the max stress values in each of the groups are not more than ±6%. The toughness, as defined by the area under the stress-strain curve, for each of the groups of samples has been provided in the legend (the toughness is presented here with the unit of MPa × unitless strain).
The stress vs. strain curve of the helicoidal PCL fibre sample (without surface treatment) shows that the yield stress is lower than the bulk sample, as shown in Figure
Although the helicoidal PCL fibre samples showed lower yield strength, it is evident from Figure
The areas under the stress vs. strain curve (i.e., the toughness) of the bulk PCL, unidirectionally oriented PCL, and the PCL helicoidal structures are measured to be 1.65, 5.27, and 8.07 MPa (the area under the stress-strain curve is presented here with the unit of MPa x unitless strain), respectively. Thus, the toughness of the PCL helicoidal structures is at least 5 times and 1.5 times higher than those of the bulk PCL and the unidirectionally oriented PCL samples, respectively. The helicoidal PCL fibre sample achieved nearly 650% strain deformation while unidirectionally oriented PCL fibre sample reached 100% strain deformation and bulk PCL only appeared to achieve about 30% strain deformation, as shown in Figure
The effect of the surface treatment as alluded earlier in this manuscript is shown in Figure
These results show that the surface treatment of the fibres improved the overall toughness of the sample during tensile testing. The toughness of surface-treated unidirectionally oriented PCL sample increased by 2.6 times compared to the unidirectionally oriented PCL sample. The surface-treated helicoidal PCL sample showed an increase in toughness by almost 3 times compared to the 3D helicoidal PCL sample without surface treatment and by almost 1.6 times higher than surface-treated unidirectionally oriented PCL samples, accompanied by a substantial increment of engineering stress.
Although the surface treatment seems to result in improvements in both the unidirectionally oriented PCL samples and helicoidal PCL samples (as represented by the red and blue curves in Figure
This unique characteristic is also evident from the tortuosity of the breaking paths as shown in images of Figure
The stress-strain curves here thus showed promising results of the feasibility of the helicoidal PCL samples over both bulk and unidirectionally oriented PCL samples. As the PCL sample is being pulled in tension, a crack is initiated and propagated in the direction perpendicular to the loading axis. In theory, fibres in the transverse direction are least likely to break over longitudinal direction along the loading. For the unidirectionally oriented sample, since all the fibres were in the same direction as the loading axis, it was easy for the crack to be propagated through the fibres. The individual fibres were easy to rip due to the same orientation. Due to the rotated fibre alignment directions in the layers in the helicoidal samples, it is more difficult for the crack to propagate and further proceed to the final catastrophic events. As the layers of fibres keep changing directions, crack propagation from one layer to the next is effectively delayed as it keeps losing its primary driving force. Each layer of fibres is essentially propagating the crack only after further tensile straining effectively rotates its fibre alignment to become normal to the loading axis (i.e., the weakest configuration). But at the same time, as these layers rotate to weaker configurations, other layers rotate to stronger configurations (when the fibre alignment is parallel to the loading axis). These mechanisms are evident from the tortuosity of the breaking paths as shown in images of Figure
Surface adhesion between the individual PCL fibres and layers plays a significant role in the mechanical performance of these biomimetic materials. This is evident from the fracture behavior of surface-treated versus the non-surface-treated samples, as illustrated in Figures
The thermal properties of the samples are also investigated and the effect of helicoidal architecture on the crystallinity, melting temperature, and thermal degradation is determined. Figure
DSC scans of bulk, helicoidal, and unidirectionally oriented PCL samples.
TGA curves of bulk and helicoidal PCL.
In summary, we have proven the technical feasibility to create a synthetic analogue of 3D helicoidal fibre architecture mimicking the structures found in mantis dactyl club. Using electrospinning, we have produced uniform and highly dense microscale fibres in 3D helicoidal manner. The toughness of the helicoidal PCL sample is found to be nearly 5 times and 1.5 times higher than the bulk PCL and unidirectionally oriented PCL sample, respectively. Surface treatment of the helicoidal sample further enhances toughness by almost 3 times as compared to the normal 3D helicoidal sample. Mechanical testing has indeed shown some evidence of enhanced toughness in the case of the PCL helicoidal samples.
The (stress vs. strain) data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
The authors would like to gratefully acknowledge the funding from the Ministry of Education (MOE) Academic Research Funds Tier 2 titled “Materials with Tunable Impact Resistance via Integrated Additive Manufacturing” (MOE2017-T2-2-175). KA acknowledges Prof. Roland Bouffanais, Assistant Professor, Engineering Product Development (EPD) Pillar, at Singapore University of Technology and Design, for his encouragement and mentorship during her PhD study. The authors gratefully acknowledge the critical support and infrastructure provided by Singapore University of Technology and Design (SUTD) especially through the Engineering Product Design (EPD) Pillar during research work as well as during the manuscript preparation. YZ, IR, HPAA, and ASB gratefully acknowledge the critical infrastructure and capabilities in nanomechanical characterization such as provided by the Xtreme Materials Laboratory (XML). The XML Nanomechanical Characterization Laboratory capability was built through the grant funding and support provided by SUTD-MIT International Design Centre (IDC), Singapore, through the project under IDC Grant “(IDG31400102)—Designing Nanomaterials Through Atomic Engineering of Interfaces.” YZ, HPAA, IR, AB, and ASB also gratefully acknowledge receipt of funding and support from TEMASEK LAB@SUTD Singapore, through its SEED grant program for the project IGDS S15 01011: titled “Biomimetic, Strong yet Tough Composite through 3D Printing.” KA, AB, and ASB also gratefully acknowledge receipt of funding and support from SMART (Singapore-MIT Alliance for Research and Technology) through its Ignition grant program for the project SMART ING-000067 ENG IGN: titled “Development of Novel Impact-Resistant Bio-Inspired Materials using Novel 3D Fabrication Technique.” This work was supported by the SUTD-MIT International Design Centre (IDC), Singapore (IDG31400102); TEMASEK LAB@SUTD Singapore, Singapore (IGDS S15 01011); and the Singapore-MIT Alliance for Research and Technology, Singapore (SMART ING-000067), also by the Ministry of Education (MOE) Academic Research Funds Tier 2 titled “Materials with Tunable Impact Resistance via Integrated Additive Manufacturing” (MOE2017-T2-2-175).