Influence of design parameters on the flexural properties of a bio-inspired suture structure

ABSTRACT Among various bio-inspired structures, sutures are a prominent structure which has evolved independently to optimize their functionalities. The diabolical ironclad beetle suture-inspired structure was fabricated using multi-material additive manufacturing (3D printing) system with TangoBlackPlus (TBP) as the soft suture layer and VeroWhitePlus (VWP) as the hard material. The print quality of the specimen was assessed through the optical microscope images, and a nanoindentation test was performed to investigate the interfacial hardness between TBP and VWP. Flexural properties of the suture structure when changing the thickness of the soft layers were then studied. Experiments were continued to identify the effect of combining different sizes of suture modules to develop the suture structure. A numerical simulation model was then generated and validated using the experimental results to proceed with the parametric study. A design of experiment (DoE) was developed to analyse the effect of changing the suture geometry to optimize performance. The research concluded that gradually decreasing the size of the suture allowed the structure to withstand higher loads. It was also evident that the deformability of the structure could be increased by incorporating smaller interlocking angles and larger a:b ratios, while larger interlocking angles and smaller a:b ratios generate stiff structures.


Introduction
Recently, many research works have been conducted to develop performance-enhanced materials and structures inspired by the complex hierarchical arrangements in nature (Zhang et al. 2020;Ahamed, Wang, and Hazell 2022).However, it is not feasible to completely mimic the structural hierarchy, material chemistry and the multi-material composite arrangement in natural structures; therefore, the term bio-inspired is used in many fields.Bio-inspired designs incorporate some characteristics and functionalities from the biological structure to develop novel man-made materials and structures (Ingrole et al. 2021).These bio-inspired design approaches are becoming popular in structural engineering, protective armour development, aerospace and automobile industries (San Ha and Lu 2020;Yin et al. 2021;Budholiya et al. 2021;Ghazlan et al. 2021).Some of the widely studied bio-inspired structures include the brick and mortar structure that can be seen in nacre, concentric hexagons structure in bone osteon, layered structure in sea sponge, rotating-plywood arrangement in stomatopods, honeycomb design, and cross lamellar structure in conch shells (Wei and Xu 2021;Jia, Yu, and Wang 2019;Jia and Wang 2019;Grunenfelder et al. 2014;Li et al. 2022).
Among various bio-inspired structures, sutures are a prominent structure which has evolved independently to optimise their functionalities.They demonstrate a great toughening mechanism through crack propagation along their complex suture interface.They also enhance flexibility while maintaining the structural integrity of the structure through large deformations, impact energy, and damp shock absorption (Liu, Zhang, and Ritchie 2020;Alheit, Bargmann, and Reddy 2021;Mirkhalaf, Dastjerdi, and Barthelat 2014;Alheit, Bargmann, and Reddy 2020;Chen, Yang, and Meyers 2015).Red-bellied woodpecker beaks, ammonite shells, Pan troglodyte's cranial, turtle carapace, and deer skull are some of the biological systems where sutures can be observed, as shown in Figure 1 (Krauss et al. 2009;Lee et al. 2014;Yang et al. 2015).In a woodpecker beak, the suture structure enables extra stiffness and strength to the beak (Lee et al. 2014).The sutures in a deer skull support stress absorption during dynamic forces (Nicolay and Vaders 2006).The ammonite shells utilise sutures to reduce the stress generated due to point loads by directing the force along the suture lines.The impact strength in a turtle shell is improved by the sutures in the turtle carapace due to the interlocking of the neighbouring bones during a predatory attack (Malik, Mirkhalaf, and Barthelat 2017).The geometry, complexity and interlocking features of the suture structure vary, and they entirely depend on the specific purpose species intend to achieve.
The unique suture arrangement found in the exoskeleton of Phloeodes diabolicus, or diabolical ironclad beetles, has recently gained significant attention in research.This suture consists of interlocking blades shaped like an ellipse, providing excellent resistance against bending moments and resulting in a highly durable exoskeleton (Rivera et al. 2020).The beetle's exoskeleton is composed of two elytra that are fused together via the suture joint, which is distributed along the abdomen to safeguard the beetle's internal organs from predatory attacks.Towards the rear of the body, the joint gradually loosens, enabling the elytra to deform and absorb more energy, thereby reducing the damage caused by impacts.These flightless beetles exhibit a greater number of interlocking features and ellipsoidal blades that offer higher toughness, unlike the hemispherical and triangular blades commonly found in other terrestrial beetle species (Rivera et al. 2021;Rivera et al. 2017;Dai et al. 2008).
The currently available literature strongly suggests that in many natural sutures, a soft layer of protein at the suture interface act as a toughening and compliance mechanism (Lin et al. 2014;Yu, Liu, and Wei 2020).Biological structures utilise a combination of soft and hard materials to create durable and lightweight materials with multiple functionalities (Gao et al. 2023).The hard components provide load-bearing capabilities, while the soft materials connect and add flexibility to the structure.In nature, suture structures also incorporate a soft layer between the rigid suture lines (Malik and Barthelat 2018;Wu et al. 2022).This concept has inspired researchers to explore the impact of combining soft and hard materials in bio-inspired suture structures on their mechanical properties.The majority of the experiments are conducted to analyse the tensile properties of the structures with triangular or sinusoidal suture modules (Lin et al. 2014;Wang et al. 2021;Liu and Li 2018;Hosseini, Cordisco, and Zavattieri 2019).However, in nature, sutures are subjected to different loading conditions.Therefore, this research work explored the flexural behaviour of an interlocking ellipsoidal suture structure with a soft suture interface and provided a guideline for the interlocking angle and diameter ratios of the ellipse to obtain optimised mechanical properties.
The advancement of additive manufacturing (AM) enables mimicking these intricate structures from nature to achieve improved mechanical performances (Ingrole et al. 2021;Velasco-Hogan, Xu, and Meyers 2018;Zhang et al. 2018).However, a suitable AM technique needs to be correctly chosen to produce this biomimicry.Comparing AM techniques such as stereolithography (SLA), fused deposition modelling (FDM), and selective laser sintering (SLS), Polyjet by Stratasys offers the necessary capabilities to mimic sutures successfully.The distinctive feature of this technique is its capability to deposit multiple materials with varying mechanical properties simultaneously, setting it apart from conventional 3D printing methods.The ability to print both rigid and flexible materials simultaneously is a unique advantage of this technique, particularly in designing bioinspired materials that often exhibit composite structures or gradient material properties (Velasco-Hogan, Xu, and Meyers 2018; Bandyopadhyay and Heer 2018;Studart 2016).The inkjet technology in Polyjet printing provides the vertical resolution of layer thickness up to 14 µm when deposited with a 1200 DPI nozzle (Tee et al. 2020).After the photopolymer droplets are deposited to the build area by multiple nozzles, it is exposed to UV light for curing (Wu, Do, and Tran 2021).The autogenerated support material can be removed either by manual peeling, using a highpressure waterjet system or soaking in a sodium hydroxide (NaOH) solution (Stratasys 2022).Polyjet 3D printing is widely utilised in the healthcare industry to print medical implants and prostheses, in the motorsports industry to print functional components such as suspension valves and brake parts, and in the food industry as well (Patpatiya et al. 2022) The first section of this work focuses on experimenting with the effect of changing the suture thickness printed from a soft material, as it is a crucial factor in tolerating damage during flexural loading.The effects of combining different sizes of suture modules on the flexural properties also needed to be explored, as natural structures do not show precise dimensions throughout their whole structure.Above mentioned Secondly, a numerical simulation model was developed and validated to proceed further with the parametric study.Finally, a design of experiment (DoE) was implemented with Isight design gateway software using the Latin Hypercube to satisfy the main research gap, which is to obtain a range of interlocking angles and a:b (minor: major radius of ellipse) ratios that would provide optimum mechanical properties.

Materials and methodology
The suture design was inspired by the suture interface in diabolical ironclad beetles' (Phloeodes diabolicus) exoskeleton (Rivera et al. 2020;Rivera et al. 2017).The suture module shape was simplified to be an ellipse, as shown in Figure 2 (e), the ratio between a minor-a and major-b radii in the base design was maintained to be 1: 1.8 (a:b), and the interlocking angle (ǿ) of the two ellipses was kept to be 25°in the base design as per the literature (Rivera et al. 2020).The minor-a and major-b radii of the base ellipse used for all the designs were 1.5 and 2.7 mm. Figure 2 (a -c) shows the diabolical ironclad beetle, the elytra's cross-section, and the suture, which connects both elytra, respectively.The schematic marked with a minor-a and a major-b radii of the ellipse, and the repeat design of the suture is given in Figure 2 (d) and (e).
All the specimens were printed with the Stratasys J750 system using two materials, VeroWhitePlus RGD835 (VWP) and TangoBlackPlus FLX980 (TBP).VWP is a rigid material, and TBP is a rubbery material.

Tensile test specimen fabrication
To properly understand the above two materials for the numerical validations and parametric studies, tensile tests according to the ASTM D638 standard were conducted.Monolithic tensile test specimens were printed from both VWP and TBP separately, and the bi-material specimen was printed by replacing a part of the gauge in the monolithic VWP sample with TBP.The dimensions of the tensile test samples were maintained to be in accordance with the ASTM D638 standard but with a slight variation, according to Lumpe et al. (Lumpe, Mueller, and Shea 2019).The overall length (LO), overall width (OW), gauge length (G), width (W ) and radius at the boundary (R) for both monolithic and bimaterial tensile samples were 63.5, 11, 18, 6 and12.7 mm, respectively, and in the bi-material sample (Hybrid) 10 mm (L) from the total gauge length was assigned to be printed with TBP. Figure 3 (ab) present schematic of monolithic and bi-material coupon tensile samples.Figure 3 (c) shows all the experimental results and simulation of VWP. Figure 3 (d) exhibits the tensile test results of TBP and Hybrid samples.The tensile test results were processed to acquire material properties as inputs in numerical simulation.

Suture inspired three-point bending test specimen fabrication
VeroWhitePlus RGD835 (VWP) was used to fabricate the body part of the three-point bending test specimen (white part), and TangoBlackPlus FLX980 (TBP) was used to print the soft suture interface (black part) using Stratasys J750 system.Specimens were printed using high mix print mode with default colour and texture profiles in the tray material option and the matte finish.The schematic of the 3-point bending specimen with the suture interface is given in Figure 4, including the dimensions.
Two designs were developed as design concept -A and design concept -B according to the suture thickness (ST) variation and suture size variation, respectively.The ST variation design contains three different thickness values, 0.2 mm, 0.35 mm and 0.5 mm, for the suture part printed from TangoBlackPlus.In suture size variation, the suture part was designed by combining two sizes of suture modules to create four sub-designs.The dimensions of the inner and outer radii of the ellipses in design concept -A is given in Table 1.Schematics and the printed samples for design concept -A are shown in Figure 5 (a), and (b).
The inner and outer dimensions of the minor (a) and major (b) radii of the ellipses in design concept -B are given in Table 2.The suture thickness (ST) of all the designs was maintained to be 0.2 mm.Schematics and the printed samples for design concept -B are shown in Figure 6.

Experimental study
A three-point bending test was performed following the ASTM D790 standard (Standard 2014).The behaviour of the bio-inspired printed suture designs was then studied under flexural loading.An Instron 5900R machine with a 5 kN load cell, top-centred cylindrical roller, and two bottom rollers having a diameter of 10 mm were used for the test.A 50 mm span length was selected.The crosshead displacement of 1 mm/ min was maintained for all the specimens.A pre-load of 1 N was applied for all the specimens assure the connection between the centred top roller and the test specimen.Three specimens for each design were tested.The test setup and the test conditions are given in Figure 7 (a) and (b).

Numerical modelling
A finite element model was developed using the ABAQUS/Explicit 2020 software package.The flexural behaviour of the suture structures, when subjected to three-point bending, was simulated.3D printing imperfections were not considered in this numerical simulation, assuming VWP and TBP are bonded perfectly at the interface.The three metal rollers with a diameter of 10 mm were assigned rigid body constraints.This simplification was applied to the rollers due to their rigid nature compared to the test specimens.Based on the experimental results shown in Figure 3 (d), a simplified elastic-perfectly plastic constitutive behaviour was assigned for the hard VWP material in the body part of the flexural specimen as stress-strain fields are acquired only considering initial loading phase.The elastic modulus and yield strength of VWP was calculated from the experimental results as 802 MPa and 43.5 MPa.The densities of VWP and TBP was obtained from the Stratasys material data sheet as 1.17 g/cm 3 and 1.12 g/cm 3 (Stratasys; Stratasys).Hard contact formulation was employed to maintain normal contact behaviour between the metal rollers and the test specimen.The tangential behaviour in penalty friction formulation was given a friction coefficient of 0.3 as per the literature (Tee et al. 2020).A displacement boundary condition up to the first peak load was assigned to the top roller in the test setup.A mesh convergence study was performed to ensure the numerical results were independent of the mesh density.The suture structure was then discretised with type CPS4R -24053 four-node quadrilateral plane-stress elements.The load-displacement of the top-centred roller was recorded to analyse the maximum force, and the slope of the linear part was used to calculate the bending stiffness and the energy absorption by calculating the area under the curve using MATLAB software.

Microstructure analysis and nanoindentation
Printed specimens with 0.2 mm, 0.35 mm and 0.5 mm ST were observed under an optical microscope to analyse the print quality, interfacial transition and defects.The top surface of the specimen has a gloss effect, and the bottom surface of the specimen shows a matte effect due to the raft that holds the print part on the platform.The printed bottom surface contains visible droplets created by the nozzle, while the top surface is smooth without any distinct droplet effect.The parallel line effect caused by the polymer droplets indicates the parallel print direction in all specimens.In some areas of the printed parts, defects created by the air bubbles can be seen as given in Figure 8 (e).From the optical microscope images, it can be confirmed that there are no significant voids in the printed parts, and good surface contact between VWP and TBP can also be observed.
The nanoindentation was conducted using Hysitron Nano-indenter-TI 950 to get a proper understanding of the hardness levels of the VWP and TBP.The images given in Figure 9 (a) and (b) show the area nanoindentation was performed and the contour plot for hardness (MPa) when the material interfaces are changing from VWP to TBP to VWP (from top to bottom).The test was performed with a 50 µN load using the Berkovich tip.The hardness of TBP is less than 12 MPa, while in the VWP, the hardness value is between 75 MPa -95 MPa.The red area represents the VWP, while the blue area represents TBP.The results from nanoindentation clearly indicate that when transitioning from one material to the other, the transition interface contains both materials.In the ST area, only TBP is visible, implying there is no VWP within that soft area which would impact the mechanical tests.As both materials are visible at the interface, it implies a smooth transition from VWP to TBP as well as good interaction between them at the interface.The mixing between the two materials provides a strong connection at the interface (Liu et al. 2020).
Table 1.Inner and outer dimensions of minor (a) and major (b) radii of the ellipses in design concept -A, given in Figure 5.

Three-point bending test results of suture structures
The load-displacement graphs for design concepts -A and B are given in Figure 10 (a) and (b), respectively.In Figure 10 (a), the images show the damage of each specimen at the same displacement values, which are marked in red circles.It is evident that when the thickness of the soft TBP layer increases, the maximum load specimen can withstand reduces, but it significantly enhances the damage tolerance.Within the same displacement, 0.2 mm ST has fractured the bottom semisuture module, while 0.35 mm ST and 0.5 mm ST exhibit only separation in TBP layer; relevant images are given in Figure 10 (a).While all three graphs, as in Figure 10 (a), show similarities up to 2 mm displacement, the 0.5 mm ST specimen exhibits a larger displacement before the failure, as well as the 0.35 mm ST, compared to the 0.2 mm ST.
In design concept -B, which contains different sizes of suture modules along the suture path, all the load-displacement curves follow a similar shape, as shown in Figure 10 (b).In the cases of B-s-B and s-B-s, the difference between the two curves is minimum.This indicates that both these combinations react in a similar manner under flexural loading.The maximum force they both can withstand is also lower than the increment and decrement designs.From the force-displacement curve of the increment design, it is visible that gradually increasing the size of the suture module increases the highest load it can resist compared to the B-s-B and s-B-s designs.When the size of the suture modules is gradually decreasing, the amount of force the suture Table 2. Inner and outer dimensions of minor (a) and major (b) radii of the ellipses in design concept -B, shown in Figure 6.Increasing the thickness of the soft TBP layer between the two hard suture interfaces has caused the suture structure to gradually reduce its flexural strength, bending stiffness and energy absorption.The thickness of the TBP layer allows the structure to be more compliant and deform under flexural loading.Compared to the 0.2 mm ST, the flexural strength of 0.35 mm ST and 0.5 mm ST has reduced by 25% and 47%.Associated bending stiffness is reduced by 30% and 47%, while energy absorption is reduced by 9% and 25%, respectively.

Experimental and numerical simulation results comparison
A numerical simulation model was developed and validated by the experimental results to capture the behaviour of the structure up to the first peak load.The stressstrain data obtained for the VWP and TBP were utilised to develop the material model.The constitutive model for TBP was modelled based on the Arruda -Boyce hyperelastic model given in Eq 1 (Arruda and Boyce 1993).U -strain energy, µ-initial shear modulus, l mlocking stretch, Dmaterial incompressible parameter, I 1 -the first deviatoric strain invariant and J el -elastic volume ratio.
The stress-strain data collected from the TBP uniaxial tension test was matched to the Arruda -Boyce model, as given in Figure 11.TBP was assumed to be a material with non -compressible properties.The data for the initial shear modulus (µ) and locking stretch (l m ) were obtained as 0.213 MPa, 1.9, respectively.Figure 12 (a) exhibits the three-point bending test simulation model used to capture the force-displacement graphs up to the first peak point.As per Figure 12 (b), a good agreement could be observed between the experimental results and the simulation up to the first peak point.Figure 12 (c -d) show the comparison between the total energy, kinetic energy, and artificial strain energy from the numerical simulation of 0.2 mm ST specimen.Compared to the total energy, the kinetic energy is significantly low, confirming the quasistatic simulation's accuracy.The result comparison of total energy and the artificial strain energy confirms that the artificial inertial forces are minimum in the simulation model.The simulation result for the specimen without a TBP suture bond is also given as a reference.The 0.2 mm ST in this specimen was maintained to create an air gap.The snapshots illustrated in Figure 13 (a) and (b) show the comparison between the deformation and stress generation in 0.2 mm ST with TBP and 0.2 mm air gap specimens within 1.09 mm displacement.This is the displacement where 0.2 mm TBP ST specimen reaches the first peak load.The strain energy for both specimens within the same displacement is given in Figure 13 (c).It is evident that without the TBP ST and leaving an air gap reduces the total strain energy.The inclusion of a soft layer of TBP allows minor deformation in the beam, while in 0.2 mm air gap indicates significant deformation within the same displacement, as highlighted in red in Figure 13 (a) and (b).
It can be observed that the deformations captured by the simulation model for 0.2 mm suture thickness (ST), 0.35 mm ST and 0.5 mm ST under flexural loading up to the first peak point are consistent with the experimental as given in Figure 14 (a).The contour plot indicates the value of the stress (MPa) generation in the specimen.The maximum stress generation in TBP in all three specimens is around 0.4 MPa, similar to the hybrid tensile specimen.Data extracted from the simulation for the total strain energy is given in Figure 14    suture structure can store during deformation up to 0.8 mm, which can then be reversed.It indicates that within the same displacement, the strain energy that can be stored in each structure gradually decreases with the increment of the TBP suture layer thickness.In 0.2 mm ST, the total strain energy is 0.37 J, while 0.35 mm ST and 0.5 mm ST exhibit 0.26 J and 0.21 J strain energies, respectively.
The comparison between the plastic energy dissipation of these three designs is given in Figure 14 (c).These graphs exhibit the amount of irreversible energy stored in the suture structure in the form of permanent damage (Yang et al. 2018).The plastic energy dissipation in 0.2 mm ST starts to accelerate at around 0.3 mm displacement causing permanent damage to the suture structure.With the increment of the TBP layer thickness, the structure delays its plastic deformation in 0.35 mm ST and 0.5 mm ST compared to the 0.2 mm ST specimen.The deformation in 0.35 mm ST becomes irreversible after 0.6 mm displacement, whereas in 0.5 mm ST, plastic deformation begins at around 0.7 mm displacement.The graph shows that plastic energy dissipation in 0.2 mm ST within the same displacement is significantly higher compared to the other two designs.In both cases, the results are given only up to 0.8 mm for the comparison, as the 0.5 mm ST specimen reached the first peak load in 0.8 mm displacement.

Parametric study
Further investigations were conducted to identify the effects of changing the a:b ratio (minor: major radii of the ellipse) and the interlocking angle between the two ellipses on the mechanical properties of the suture structure.The thickness of the TBP layer was maintained to be 0.2 mm for the parametric study to eliminate additional variables.During the experiment, the first peak load in all the 0.2 mm ST specimens in design concept-A and design concept-B occurred due to the failure in the TBP layer.The failure of the TBP layer started at the same displacement range in all the 0.2 mm ST specimens.Therefore, in the parametric study, the displacement boundary condition is employed to obtain the first peak load as the thickness of the TBP layer is 0.2 mm in the parametric study.Figure 15 shows the result of changing the a:b ratio   and the interlocking angle on the shape of the suture modules.All the designs contain three suture modules on one side of the specimen and two suture modules on the other side of the specimen to form an interlocking.The number of suture modules was maintained to be the same in all the given schematics.With the increment of the interlocking angle, suture modules get compacted and reduce their height.When the ratio between the minor-a and major-b radii of the ellipse (a:b) decreases, the total height of the suture structure containing the same amount of suture modules decreases.
A parametric study was conducted to understand how changing the abovementioned parameters affects bending stiffness, energy absorption, and maximum force.The sample height (H) condition was taken into consideration for a fair comparison.When the total height of the suture component increase, the interlocking feature between each suture module becomes minimised.Therefore, the sample height H < 27 mm condition was given for the parametric study to maintain a proper interlocking feature between the suture modules.Then the volume fraction (V TBP ) of the TBP needed to be considered, as increasing, or decreasing the V TBP significantly impact on the mechanical properties of the structure.To calculate the V TBP , the area of the interface covered by the suture was calculated using a complete elliptic integral of the 2nd kind.The total circumference c was calculated using Eq 2 and the arc length at a specific given angle c`was calculated using Eq 3, and Eq 4. Minor and major radii are a and b, where θ is 90°; refer to Figure 2(f).These equations were utilised to generate the equation to calculate the volume fraction of the TBP suture part.A python code was then developed to generate suture specimens, where the thickness of the suture part, ellipse radii, and interlocking angle are variables.This code was used to calculate the volume fraction as given in Eq 5 when the a:b ratio and interlocking angles are changing from 1:1-1:2 and 1°-38°.Using MATLAB software, the V TBP of the structure which satisfies the height condition of H < 27 mm was then extracted.In Eq 5, a 1, b 1 , a 2 , b 2 are same as the a outer , b outer , a inner , b inner as shown in Table 1 and a is the interlocking angle between two ellipses.The calculated volume fraction is given in Figure 16 (a) contour plot.The area shaded as out of design space is the specimen that do not satisfy the H < 27 mm condition.The difference in the TBP volume fraction, when changing the a:b ratio and interlocking angle, is only 0.1%.This implies that the volume fraction of TBP will not significantly vary with the a:b ratio and interlocking angle when the thickness of the suture part is maintained to be 0.2 mm.Design of experiment (DoE) was then developed to identify the effect of varying a:b ratio and interlocking angle on mechanical properties of the suture structure.The DoE was conducted through Isight design gateway software using Latin Hypercube DoE technique.Python code generates the simulation model and runs it, then the MATLAB code extract and filter data that satisfy the V TBP between 0.2% -0.3% from the model and calculate the bending stiffness, energy absorption and maximum force.Interlocking angles and a:b ratios were given as the input for the DoE while maintaining the thickness of the TBP suture at 0.2 mm.Bending stiffness, energy absorption and force were then delivered as the outputs.500 cases satisfied the V TBP condition.Among those 500 cases, 115 cases satisfied the height condition of H < 27 mm; the results are given in Figure 16   It is evident that when increasing the a:b ratio, the bending stiffness, energy absorption and the maximum reaction force reduces.When decreasing the interlocking angle, the bending stiffness, energy absorption and maximum reaction force reduce.When smaller interlocking angles are combined with larger a:b ratios, the structure becomes more flexible and deformable by reducing the bending stiffness.As illustrated in Figure 15, increasing the a:b ratio and decreasing the interlocking angle reduces the interlocking areas with the adjacent ellipses.When increasing the interlocking angle and reducing the a:b ratio increases the interlocking area making them stronger and stiffer.The contour plots given in Figure 16 (b), (c), and (d) show the results of the specimens that satisfy the volume fraction of TBP (V TBP ) between 0. 2% -0.3% and height of the suture design (H) < 27 mm conditions.It clearly indicates that larger interlocking angles and smaller a:b ratios generate stiff structure while smaller interlocking angles combined with larger a:b ratios create deformable structures.These results can be carefully analysed to capture a range of a:b ratios and interlocking angles to developed structures with optimised mechanical properties.

Conclusion
In this work, the effect of changing the thickness of the soft TBP suture component and the effect of combining the different sizes of suture modules were experimented.Further analysis of the effect of changing the interlocking angles and radii of the ellipses, which create the suture modules, on the mechanical performances of the structure was conducted through numerical simulation.
. Increasing the thickness of the TBP suture component reduces the maximum load the specimen can withstand under flexural loading, but it makes the structure more flexible and allows it to deform when the load is applied.Compared to 0.2 mm ST, the flexural strength of 0.35 mm ST and 0.5 mm ST has reduced by 25% and 47%, bending stiffness has reduced by 30% and 47% and energy absorption has reduced by 9% and 25%, respectively. .The effect of combining different sizes of suture modules was then studied by utilising four designs, named as increment, decrement, s-B-s and B-s-B.
Gradually increasing the size of each suture module increases the highest load it can hold.When the size of the suture modules is gradually decreasing, the structure can hold an even higher load than the gradual increment suture structure and the structure which contains an even size of suture modules with 0.2 mm ST.
. The numerical simulation conducted via ABAQUS shows similar results to the experimental force-displacement curves up to the first peak load.Continuing on simulation, a parametric study was then conducted to identify the effect of changing the interlocking angle and radii of the ellipses, which creates the suture design, on the mechanical performances of the suture structure.A python code was developed to vary the interlocking angle from 1°-38 °and the ratio between the minor (a) and major (a) radii from 1:1-1:2 of the simulation model, and a MATLAB code to calculate the volume fraction of the TBP (V TBP ) in each design. .The design of experiment (DoE) was then developed using Isight design gateway software using Latin Hypercube DoE technique.Python code was utilised to change the interlocking angles and radii of the ellipse.MATLAB software was utilised to extract and filter data from the model that satisfy the V TBP between 0.2% -0.3% and sample height of (H) < 27 mm.Finally, to calculate the bending stiffness, energy absorption and maximum force. .From DoE, it was evident that larger interlocking angles and smaller a:b ratios generate stiff structure while smaller interlocking angles combined with larger a:b ratios create deformable structures.

Figure 4 .
Figure 4. Schematic of a 3-point bending specimen including suture interface.

Figure 8
Figure8 (ac) shows the printed suture path for 0.2 mm, 0.35 mm and 0.5 mmST, while (d)  and (e) shows the polymer droplets deposited by the nozzle and air bubble generation during printing.The top surface of the specimen has a gloss effect, and the bottom surface of the specimen shows a matte effect due to the raft that holds the print part on the platform.The printed bottom surface contains visible droplets created by the nozzle, while the top surface is smooth without any distinct droplet effect.The parallel line effect caused by the polymer droplets indicates the parallel print direction in all specimens.In some areas of the printed parts, defects created by the air bubbles can be seen as given in Figure8 (e).From the optical microscope images, it can be confirmed that there are no significant voids in the printed parts, and good surface contact between VWP and TBP can also be observed.The nanoindentation was conducted using Hysitron Nano-indenter-TI 950 to get a proper understanding
Minor (a) and major (b) radii of the base ellipse are 1.5

Figure 6 .
Figure 6.(a) Schematics of the design concept -B and (b) printed flexural specimens for (i) increment (suture module size gradually increase), (ii) decrement (suture module size gradually decrease), (iii) B-s-B (Bigger suture modules at the two ends with a smaller suture module in the middle), (iv) s-B-s (smaller suture modules at the two ends with a Bigger suture module in the middle).

Figure 7 .
Figure 7. (a) Three -point bending test setup, (b) summary of the test condition.
(b).The strain energy results indicate the amount of energy the

Figure 12 .
Figure 12.(a) Numerical simulation model for three-point bending test, (b) Experimental and simulation force-displacement resultscomparision, the results when 0.2 mm ST TBP is removed and left with an air gap is also given for the comparision, comparision of (c) total energy and kinetic energy, (d) total energy and artificial strain energy from the simulation of 0.2 mm ST.

Figure 13 .
Figure 13.Numerical simulation results for specimens with (a) 0.2 mm suture air gap, (b) 0.2 mm TangoBlackPlus (TBP) suture bond recorded at 1.09 mm displacement (the TBP bond is hidden in the image), (c) strain energy comparison of two specimens.

Figure 14 .
Figure 14.(a) Numerical simulation of the deformation up to the first peak of force-displacement graph shown in Figure 12 (b) and the deformed suture structure in experiment, (b) total strain energy, (c) plastic energy dissipation from numerical simulation.

Figure 15 .
Figure15.Effect of changing the a:b ratio (1:2, 1:1.8, 1:1.5, 1:1) and interlocking angle (1°, 15°, 25°, 38°) on the shape of the suture structure while maintaining the number of suture modules (three suture modules) same in all designs.The total length of the suture structure which contains three suture modules also varies with the a:b ratio and interlocking angle.

Figure 16 .
Figure 16.(a) Volume fraction of the TBP suture part when varying a:b ratios and interlocking angles, Results from the Design of Experiment (DoE), considering both TBP volume fraction and sample height conditions (b) Bending stiffness (N/mm), (c) maximum reaction force (N), (d) energy absorption (J).
Chenxi Peng completed his PhD at the School of Engineering, RMIT University.His research interests focus on the multifunctional properties of lattice structures fabricated by additive manufacturing.Yun Lu Tee completed her PhD at RMIT University, Melbourne, Australia.She received her Bachelor of Materials Engineering and Master of Engineering Science from the Faculty of Engineering, University of Malaya, Malaysia.Her research interests include polymeric 3D printing and bio-inspired structures.Mladenko Kajtaz is a member of sports technology and lightweight structures research groups at School of Engineering, and a research member of RMIT Centre for Additive Manufacturing.His primary research focus is on the design of optimised lightweight load-bearing structures utilising additive (3D) printing and hybrid materials.Phuong Tran is the director of the digital construction laboratory at RMIT University in Melbourne.His research interests include biomimicry design and additive manufacturing.He has published over 120 peer-reviewed journal articles in related fields.