Material response characterization of three poly jet printed materials used in a high fidelity human infant skull

Material response characterization of three poly jet printed materials used in a high fidelity human infant skull

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Materials Today: Proceedings xxx (xxxx) xxx

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Material response characterization of three poly jet printed materials used in a high fidelity human infant skull Ghaidaa A. Khalid a,⇑, H. Bakhtiarydavijani c, W.R. Whittington c, R. Prabhu c, M.D. Jones b a

College of Electrical Engineering Techniques, Middle Technical University, Baghdad, Iraq Institute of Medical Engineering and Medical Physics, Cardiff School of Engineering, Cardiff University, The Parade, Cardiff CF24,UK c Department of Agricultural and Biological Engineering and the Center for Advanced Vehicular Systems, Mississippi State University, Mississippi, USA b

a r t i c l e

i n f o

Article history: Received 15 July 2019 Received in revised form 2 September 2019 Accepted 29 September 2019 Available online xxxx Keywords: Infant skull Material response PolyJet printing PolyJet polymers Physical model

a b s t r a c t This present study seeks to improve the infant head model biofidelity by providing a range of 3D printable candidate materials tested at different temperatures and strain rates so as to tailor materials properties to match the specific age-dependent infant skull being modeled. Mechanical properties of the three polypropylene polymers named VeroWhitePlus (RGD835 VW), TangoBlackPlus (FLX980), and RigidLightGrey25 (RGD8510-DM), were applied using a PolyJet 3D printer to replicating the comparative paediatric skull (cranial bone, fontanelles, sutures) mechanical properties. Quasi-static tensile tests were carried out for VeroWhitePlus, RigidLightGrey25 and TangoBlackPlus PolyJet polymers at two different speeds, and 3 temperatures. Strain rate and temperature dependence on elastic modulus and flow stress is evident. Mechanical properties of the PolyJet polymers indicate appropriate response for biocompatibility in modeling infant skull. Temperature dependence of the mechanical properties of the PolyJet polymers indicate a unique method to fine tune the mechanical response for specific age-dependent infant skull modeling. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Materials Engineering & Science.

1. Introduction Head injury in paediatric populations is a reason for sickness and death. Thresholds of head injury are appropriately classified in the adult’s populations [1]. The materials and geometrical differences between adults and paediatric head means it inapplicable to present a scale down of adult’s structures and material properties [2]. In medical and law field, it is ordinary for paediatric abuse cases be undistinguished [3]. Thus, there is a need for a tool used for detecting children non accidental cases that provide understanding for injury mechanism and evidence used in the court. By the means of a physical and computational models, this can be achievable. However, these models were restricted as there was a demand for validation. Building and validation of a physical model will help to validate a computational model, both can act as a surrogate to replicate different impact scenarios. Recently, Additive Manufacturing (AM) was used numerously through many fields, especially in the medical field like producing hip surrogates ⇑ Corresponding author. E-mail address: [email protected] (G.A. Khalid).

[4]. The manufacturing of complex skull geometry of paediatric is now practicable through AM. Recently, an approach documented by [5–7], a physical model of a 10-day-old infant cranium see Fig. 1, constructed from Computed Tomography (CT) scans of an actual paediatric skull, have been created and validated. Consequently, a computational head model resembling the 3D printed physical head model was created [6,7]. The coupled physical – computational model was validated with the individual study which documented the paediatric newborn impact response of the whole skull [8]. However, the coupled models were comprised three acrylate polymers, two simulating bone in different regions (VeroWhitePlus, RigidLightGrey25) and one simulating suture (TangoBlackPlus). Bones and sutures properties of the 3D printed model from the manufacture sheet [9,10], shown in Table 1, were corresponded to the limited mechanical response values of paediatric skull reported by [11]. Further, the manufacture sheet provides range of values related to the three polymers rather than plain numeric values, as seen in Table 1. So testing is expected to confirm the polymers materials applied in AM, even a technology even in its immaturity. This mechanical characterization study, aimed to justify the use of the different commercially available

https://doi.org/10.1016/j.matpr.2019.09.156 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Materials Engineering & Science.

Please cite this article as: G. A. Khalid, H. Bakhtiarydavijani, W. R. Whittington et al., Material response characterization of three poly jet printed materials used in a high fidelity human infant skull, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.156

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b

a

Fig. 1. The (a) frontal view; (b) side view of paediatric skull physical surrogate [6].

Table 1 Mechanical properties of polypropylene polymers materials from the manufacture sheet [9,10]. Material

Tensile strength (MPa)

Elongation at failure (%)

Modulus of elasticity (MPa)

VeroWhitePlus RigidLightGrey25 TangoBlackPlus

50.0–65.0 40.0–60.0 3.5–5.0

10.0–25.0 15.0–25.0 65.0–80.0

2000.0–3000.0 1700.0–2300.0 not provided

materials (VeroWhitePlus, TangoBlackPlus, RigidLightGrey25) for application in a 3D printed physical model which in turn feed the computational model of paediatric skull. Quasi-static tensile tests will be carried out for the three PolyJet polymers to evaluate their sensitivity at two different speeds (1 and 300 mm min1), and 3 temperatures (cold, ambient, hot). In addition, temperature sensitivity will be investigated to determine whether it can be used to optimise material response, to better mimic cranial bones and sutures.

BS EN ISO 527-2:2012 [12,13] based on the initial distance between the specimen grips reported in Table 2. However, it was found throughout testing of the VeroWhitePlus and RigidLightGrey25 rigid materials at 1 mm min1, the machine was generated a hydraulic noise that would hide the Black material’s loading signal. As the stiffness of rubber material increased with strain rate and to improve signal to noise ratio, the speed rate of 300 mm min1 was selected for testing the Black material. The temperature chamber was maintained at 0 °C and 50 °C, using liquid nitrogen and an inbuilt heating element, respectively. The second test was to study the impact of different speeds (1 and 300 mm min1) on the VeroWhitePlus (RGD835 VW), TangoBlackPlus (FLX980), and RigidLightGrey25 (RGD8510-DM) materials properties. For each test, nine specimens of each material were used. Throughout the experiential testing, samples curve of the force with displacement were documented. Further, the raw information represented by force with extension were used to compute the engineering mechanical parameters (ultimate tensile stress ðrut Þ, ultimate maximum strain ðemax Þ, and the elastic modulus ðE) (till 0.1% strain) of both VeroWhitePlus (RGD835VW) and RigidLightGrey25 (RGD8510-DM) polymer materials; however, due to the large deformation of the TangoBlackPlus (FLX980) material samples, true stress (rut ), true strain (emax ) and stiffness (EÞ (till 1%, 10% and 20% strain) was figured assuming uniform deformation and constant density. The measured data were presented as mean ± SD. The statistical significance of the differences between the groups was determined by one-way Anova test analysis of variance. The significance level for all analyses was set as P < 0.05 and all statistical analyses were performed using SPSS 20.0 (SPSS, Inc., Chicago, IL, USA) [14].

3. Results and discussion

2. Materials and methods

3.1. Effect of temperature

To evaluate the stability of the VeroWhitePlus, RigidLightGrey25 and TangoBlackPlus materials, using the type ‘1BA’ of BS EN ISO 527-2:2012 [12,13], The experiential samples were designed as CAD models, see Table 2 using SolidWorks software and AM printed in XYZ orientation. A tensile experiment was applied to each material, VeroWhitePlus (RGD835 VW), TangoBlackPlus (FLX980), and RigidLightGrey25 (RGD8510-DM), see Fig. 2. A series of mechanical testing were conducted to investigate the effect of temperature on the three polypropylene polymers materials that comprised physical model of paediatric head. Initial tests were conducted at an ambient temperature approximately 23 °C, using the tensile test machine Zwick 100. Also, the analysis of the tensile strength, for all the three materials, at ‘‘hot” 50 °C (±0.5 °C) and ‘‘cold” 0 °C (±0.5 °C) temperatures, were carried out by tensile material testing system (MTS 858 Mini Bionix II), at a crosshead speed of 1 and 300 mm min1. The low rate testing speed of 1 mm min1 for 1BA specimens was recommended by

A series of experiments were conducted to determine if the mechanical parameters of the material specimens changed with increased or decreased specimens’ temperature by comparing their response with samples tested at ambient temperature 23 °C at a constant displacement rate of 1 mm min1 for VeroWhitePlus and TangoBlackPlus materials and 300 mm min1 for TangoBlackPlus. From Fig. 3, which illustrate the temperature dependent mechanical behavior of the different specimens. The output parameters were subsequently used to determine if these specimens appeared in the range of the reported values for the paediatric cranial bones [11]. Table 3 shows the values for the average mechanical outputs, at different testing temperatures, after implementing one-way Analysis of Variance (ANOVAs). From the statistical analysis, it appears that there was a significant effect of temperature on the polymer samples mechanical parameters (ruts ; emax , E) (p < 0.05). To study the influence of temperature on the mechanical characteristics of the polymer materials represented by VeroWhitePlus, Rigid Light Grey and Tango Black Plus, samples were experimented at three temperatures (0 ± 0.5 °C), (23 ± 0.5 °C) and (50 ± 0.5 °C). For VeroWhitePlus and Rigid Light Grey materials, at 50 °C the stress-strain curves seemed to be changeable, as shown in Fig. 3 b & c. For the VeroWhitePlus material, seen in Table 3, at 50 °C the maximum strain ðemax Þ was 82.82 ± 9.06%, approximately 4.4* ambient temperature 23 °C (13.93 ± 2.22 MPa); the tensile strength ðrutsÞ was 27.21 ± 0.81 MPa decreased by 6* compared to ambient temperature 23 °C (4.50 ± 1.85 MPa) and the stiffness ðEÞ (9.19 ± 1.55 MPa) was reduced by a factor of 250 compared to ambient temperature 23 °C (2504 ± 150 MPa). By 0 °C, there was also a significant decrease (around >7*) in stiffness ðEÞ. Also, at

Table 2 Dimensions in mm of tensile tested specimen [12,13]. Description

Dimension

Overall length Distance between broad parallel-sided portions (Z) Length of narrow parallel-sided portion Radius Width at ends Width at narrow portion Preferred thickness Gauge length Initial distance between grips

75 58 ± 2 30 ± 0.5 30 10 ± 0.5 5 ± 0.5 2 ± 0.1 25 ± 0.5 Z + 20

Please cite this article as: G. A. Khalid, H. Bakhtiarydavijani, W. R. Whittington et al., Material response characterization of three poly jet printed materials used in a high fidelity human infant skull, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.156

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a

b

c

d

Fig. 2. (a) VeroWhitePlus (RGD835 VW) material samples; (b) TangoBlackPlus (FLX980) material samples; and Mechanical testing of polymer samples (c) TangoBlackPlus (FLX980); (d) VeroWhitePlus (RGD835 VW).

Fig. 3. Mechanical response of (a) Tango Black Plus at 300 mm min1, (b) VeroWhitePlus, (c) Rigid Light Grey at 1 mm min1 at different temperatures.

Table 3 Mechanical response of VeroWhitePlus, Rigid Light Grey at 1 mm min1 and Tango Black Plus at 300 mm min1 at different temperatures. Material

Condition

E (MPa)

ruts (MPa)

emax (%)

White (Vero White Plus)

Cold Ambient Hot Cold Ambient Hot Cold Ambient Hot

330 ± 50 2504 ± 150 9.19 ± 1.55 287 ± 82 726 ± 20 9.83 ± 1.72 59.78 ± 7.46 15.46 ± 2.14 1.68 ± 0.25

59.33 ± 3.91 27.21 ± 0.81 4.50 ± 1.85 51.38 ± 3.39 22.47 ± 1.30 3.60 ± 0.80 15.75 ± 1.39 4.19 ± 0.41 1.01 ± 0.24

25.08 ± 1.86 13.93 ± 2.22 82.82 ± 9.06 28.05 ± 2.41 77.05 ± 6.76 82.82 ± 9.06 86.74 ± 8.20 148.12 ± 6.67 162.71 ± 8.76

Grey (Rigid Light Grey)

Black (Tango Black Plus)

Please cite this article as: G. A. Khalid, H. Bakhtiarydavijani, W. R. Whittington et al., Material response characterization of three poly jet printed materials used in a high fidelity human infant skull, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.156

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lower temperature, the polymers materials turned to be brittle because of the untangling reduction that reduce polymer chain unfolding. Resulting in doubling of both tensile strength ðrutsÞ and strain ðemax Þ. For the Grey material at 50 °C, as shown in Table 3, there was an increase in the maximum strain emax (82.82 ± 9.06%), to ambient temperature 23 °C (77.05 ± 6.76%). The tensile strength ðruts ) was 3.60 ± 0.80 MPa decreased by 6* compared to ambient temperature 23 °C of 22.47 ± 1.30 MPa. The stiffness ðEÞ was 9.83 ± 1.72 MPa decreased by factor 100, compared to ambient temperature 23 °C of 726 ± 20 MPa. By 0 °C, the outcomes have a significant decrease in the young’s modulus (E) represented by greater than 2.5* and 2* of the tensile strength ðrutsÞ: These latter measurements were erroneous, because of machines grips movement. Related to the TangoBlackPlus (FLX980) material, there was less impact of changing temperature on the tested samples, probably because of the worked with speed of 300 mm min1 rather than 1 mm min1 and greater appearance of elongations which in turn reduce the slippage impact on the tested samples. The stiffness (E), was significantly affected by temperature, where at at 0 °C the stiffness was increased by a factor 4 compared to ambient temperature 23 °C, also increased by a factor 36 compared to 50 °C. The tensile strength ðrutsÞ of the black tested samples was increased by a factor 4 and 16 above ambient and 50 °C respectively. While the maximum strain ðemax Þ at an ambient temperature 23 °C was decreased by around 2 factor from clod (0 °C) to hot (50 °C) temperature, even though the variance was less around 10% between cold and ambient temperature 23 °C. From this study, as illustrated in Fig. 3 b & c, it appeared from the uniaxial tensile testing at 1 mm min1 of the two polymer materials VeroWhitePlus and Rigid Light Grey, there was a significant increase in the strain ðemax Þ linked with a significant decrease in both the stiffness (E) and tensile strength ðrutsÞ at hot temperature (50 °C) compared to ambient temperature 23 °C. While the movement of the test machine grips caused unsound values of extension at cold temperature (0 °C). From Fig. 3 a, it seemed that as the temperature comes higher, the tested samples of TangoBlackPlus (FLX980) material showed a reductions in the stiffness (E) and tensile strength ðrutsÞ response combined with an increase in maximum strain ðemax Þ. 3.2. Strain rate sensitivity analysis A second series of experiments were conducted to study the impact of variations in strain rates on the mechanical response of the VeroWhitePlus (RGD835 VW), TangoBlackPlus (FLX980), and RigidLightGrey25 (RGD8510-DM) materials. The parameters response was used to determine if these specimen values appeared in the range of those reported for the paediatric cranial bones response [11]. The structure–response relationships, derived from testing of the materials at different quasi-static strain rates is shown in Fig. 4, whilst Table 4 shows the values of the average mechanical properties of the materials, using one-way Analysis of Variance (ANOVAs). From the statistical analyses the VeroWhitePlus material shows a significant variations in stiffness ðEÞ and maximum strain ðemax Þ values, tested with 1 and 300 mm min1 speeds, at higher strain rate there was a reduction in stiffness ðEÞ around 35% and the maximum strain ðemax Þ increased by 79%, while there was a significant increase in the tensile/yield strength ðruts Þ/ðryield ) around 83%. For the Grey material, there was a significant increase by 48%, and 114% in the stiffness ðEÞ and tensile/yield strength ðruts Þ/ðryield Þ respectively, compared to the reduction in maximum strain ðemax Þ by 57%. For the TangoBlackPlus material, there was a significant growth by 10*, 4*, 85% across the parameters of stiffness ðEÞ and tensile/yield strength

ðruts Þ/ðryield Þ; maximum strain ðemax Þ respectively, see Table 4. The polymers samples (VeroWhitePlus, TangoBlackPlus, and RigidLightGrey25) were experimented at different speeds. To explore the influences of strain rates on these polymers materials. It was documented from [15], that it is common at higher strain rates for polypropylene polymers to have an increase in both the stiffness ðEÞ and tensile strength ðruts Þ with a decrease in elongation ðemax Þ. From Fig. 4 b and Table 4, the RigidLightGrey25 material showed this statement, where the stiffness ðEÞ and tensile strength ðruts Þ was raised by 50% and the maximum strain ðemax Þ declined by 57%, whilst this was not apparent in the VeroWhitePlus material, illustrated in Fig. 4 a and Table 3 , where the stiffness (E) was decrease (35%) and the maximum strain (emax Þ was increased (79%). For the TangoBlackPlus material, it appeared from Fig. 4 d that the stress-strain curve, have the same appearance to the stress-strain curve of the biological tissues, the stiffness ðEeng Þ increased by a factor of 10. Therefore, by increasing the strain rate of testing resulting in increasing the stiffnessðEÞ of the RigidLightGrey25 and TangoBlackPlus (FLX980) materials while in contrast to the VeroWhitePlus (RGD835 VW) material. Further, The tensile strength ðruts Þ and the maximum strain (emax Þ of the TangoBlackPlus material was increased by 4* and 85% respectively which implying that at higher strain rates there is a larger capability for energy immersion and that was similarly observed with VeroWhitePlus material. The used strain rates in this study (1 and 300 mm min1) were three hundred times lower than those used in testing cranial bones and sutures specimens as reported by [11], the latter represent the strain rates of impact experienced by paediatric through short fall that is recommended to explore whether the VeroWhitePlus (RGD835 VW), TangoBlackPlus (FLX980), and RigidLightGrey25 (RGD8510-DM) materials close to the actual paediatric skull (cranial bones and sutures). The difference between researchers in selecting the speed and methodology of testing of studying the mechanical behavior of paediatric cranial bones samples and sutures causes comparison of mechanical response data difficult. From the results of analysing the VeroWhitePlus at an ambient temperature 23 °C, it appeared that its stiffness (2504 ± 150 MPa) at 1 mm min1 and (1618 ± 575 MPa) at 300 mm min1 joined with the tensile strength (27.21 ± 0.81 MPa) at 1 mm min1 and (49.76 ± 3.16 MPa) at 300 mm min1 can be used in the modelling of the paediatric parietal/frontal cranial bones mechanical response from birth up to 3 months because of the close mechanical response compared to [11,16]. Related to the RigidLightGrey25 used to replicate the occipital cranial bone, its stiffness (represented by 726.00 ± 20.00 MP, 1075 ± 75 MPa at speeds 1 mm min1, 300 mm min1 respectively), was higher than reported by [10] for paediatric age from newborn to three months have stiffness 29–551 MPa and more appropriate for group of age from three to six months have stiffness 318–1318 MPa, In spite of this, these mechanical values documented by [11] was from specimens have perpendicular fibres direction and it is not reported that occipital bone was tested with known fibres orientation. Further, it’s worth mentioning that both age and fibres direction impact the stiffness of paediatric parietal and frontal cranial bones [16]. Where the stiffness was increased with age, also stiffness of parietal bones has parallel fibres direction were four times more than samples of perpendicular fibre orientation. Therefore, it could be that stiffness values of 29–551 MPa (from newborn to three months) and 318–1318 MPa (from three to six months) by [11] could be used to represent the occipital cranial bones. Also, the speed of testing by [11] is higher than the strain rate used in this experiment. Likewise, the tensile strength of RigidLightGrey25 represented by 22.47 ± 1.30 MPa at 1 mm min1 was appropriate

Please cite this article as: G. A. Khalid, H. Bakhtiarydavijani, W. R. Whittington et al., Material response characterization of three poly jet printed materials used in a high fidelity human infant skull, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.156

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Fig. 4. Mechanical response of (a) VeroWhitePlus, (b) Rigid Light Grey and (c) engineering strain and (d) true strain of Tango Black Plus materials.

Table 4 Mechanical response of VeroWhitePlus, Rigid Light Grey and Tango Black Plus materials at an ambient temperature 23 °C with different strain rates. Material

Strain Rate (mm.min1)

E (MPa)

ryield (MPa)

ruts (MPa)

emax (%)

White (Vero White Plus)

1 300 1 300 1 300

2504.00 ± 150.00 1618.00 ± 575.00 726.00 ± 20.00 1075.00 ± 75.00 1.55 ± 0.23 15.46 ± 2.14

27.21 ± 0.81 49.76 ± 3.16 22.47 ± 1.30 48.02 ± 2.10 – –

27.21 ± 0.81 49.76 ± 3.16 22.47 ± 1.30 48.02 ± 2.10 1.10 ± 0.07 4.19 ± 0.41

13.93 ± 2.22 24.95 ± 6.77 77.05 ± 6.76 33.31 ± 27.22 80.47 ± 5.38 148.73 ± 7.34

Grey (Rigid Light Grey) Black (Tango Black Plus)

to the age group reported by [11] from newborn till twelve months. In case of the TangoBlackPlus replicating the sutures, its stiffness represented by 15.46 ± 2.14 MPa at 300 mm min1 was applicable to age of paediatric up to twelve months in comparison to 408.12 ± 59.08 MPa of sagittal sutures and 354.83 ± 44.86 MPa of coronal sutures reported by [17]. Next, the mechanical response values from this study with those from the manufacture sheet, shown in Table 1, where none of these values are close to those in Table 1, noticing that speed of testing was not reported from the manufacture. However, from the statistical analysis it appeared that eleven from sixteen average values have no significant distinction from the manufacture sheet. So it was proposed that 99% of manufacture reported values integrates with mechanical response values from tested samples in this study. Generally, the VeroWhi-

tePlus (RGD835 VW) replicate paediatric parietal and frontal cranial bones, TangoBlackPlus (FLX980) replicate paediatric occipital cranial bones, and RigidLightGrey25 (RGD8510-DM) replicate paediatric sutures with fontanelles. 4. Conclusion This research introduces the mechanical properties of three PolyJet printed copolymers, identified as VeroWhitePlus (RGD835 VW), TangoBlackPlus (FLX980), and RigidLightGrey25 (RGD8510DM) to be used in manufacturing of a physical model mimicking human paediatric head, validate the computational paediatric head model consecutively be used in paediatric head impact scenarios investigations. By increasing the temperature, there was a signifi-

Please cite this article as: G. A. Khalid, H. Bakhtiarydavijani, W. R. Whittington et al., Material response characterization of three poly jet printed materials used in a high fidelity human infant skull, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.156

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cant increase in strain of both the VeroWhitePlus and Rigid Light Grey materials combined by a decrease in stiffness and tensile strength compared to ambient temperature. In contrast, the TangoBlackPlus (FLX980) material showed a reduction in stiffness and tensile strength response linked with an increase in strain. By increasing the strain rate of testing resulting in increasing stiffness (E) of the RigidLightGrey25 and TangoBlackPlus (FLX980) materials while in contrast to the VeroWhitePlus (RGD835 VW) material. Predominantly, the VeroWhitePlus (RGD835 VW) represented paediatric parietal and frontal cranial bones, TangoBlackPlus (FLX980) represented paediatric occipital cranial bone, and RigidLightGrey25 (RGD8510-DM) represented paediatric sutures with fontanelles, in their mechanical response.

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Please cite this article as: G. A. Khalid, H. Bakhtiarydavijani, W. R. Whittington et al., Material response characterization of three poly jet printed materials used in a high fidelity human infant skull, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.156