Layer-dependent properties of material extruded biodegradable Polylactic Acid

journal of the mechanical behavior of biomedical materials 104 (2020) 103654

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Layer-dependent properties of material extruded biodegradable Polylactic Acid Alper Ekinci a, Andrew A. Johnson b, Andy Gleadall a, Daniel S. Engstrøm a, Xiaoxiao Han c, * a

Wolfson School of Mechanical, Electrical and Manufacturing Engineering, Loughborough University, Loughborough, LE113TU, UK School of Design and Creative Arts, Loughborough University, Loughborough, LE113TU, UK c State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, 410082, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Additive manufacturing Material extrusion Polylactic acid Crystallinity and molecular weight Mechanical properties Number of layers

Polylactic acid (PLA) is a biodegradable, biocompatible and non-toxic biopolymer with good mechanical properties, and is commonly used for the additive manufacture of PLA-based biomedical devices. Such devices are available in a range of sizes and thicknesses, with smaller devices capable of being realised via additive manufacturing in just a few layers. Due to their thermal history and thermal degradation, the thermal, molecular weight and mechanical properties of each layer was different when the raw material was melted, and the incourse layer was deposited to the previous layer. This study investigated the effect of the number of layers on mechanical, thermal and molecular weight properties, and the relationship between them. Material extruded ISO 527–2 type 5A specimens with 1-, 2-, 3-, 4-, 5-, 7- and 10-layers were prepared with the cutting die. Results indicated that the degree of crystallinity was found to decrease from 8% to 0.5% with an increasing number of layers. This was likely due to different cooling rates, where the molecular weight was lowest for 1-layer and increased with the increasing number of layers until it almost reached that of the bulk material. Additionally, ultimate tensile strength and strain increased with an increasing number of layers, while Young’s Modulus decreased due to heterogeneous material structure. Of all obtained results, there was no significant difference between 5- and 10-layer in terms of mechanical and thermal properties.

1. Introduction Temporary biomedical components that assist the body in repair are being replaced with alternatives manufactured from biodegradable materials (Naghieh et al., 2016; Nair and Laurencin, 2007). Biode­ gradable polymers are commonly used as they degrade during their function and eliminate the need for secondary surgeries (Temenoff and Mikos, 2000). Specifically, the practical preparation and synthesis of biodegradable polymers ensure a range of appropriate physical prop­ erties and functions can be realised, thus making their use highly desirable (Kulshrestha and Mahapatro, 2008). Biodegradable polymers are divided into two categories: natural and synthetic. While natural biodegradable polymers are highly biocompatible and degradable, they are often expensive and weak in terms of their physical and mechanical properties. Therefore, synthetic polymers are commonly utilised in order to eliminate these weaknesses (Bose et al., 2017). Of all of the synthetic biodegradable polymers available, Polylactic Acid (PLA) based materials are often preferred in biomedical applications due to its

enhanced mechanical properties, biocompatibility, biodegradability, and non-toxicity (Luckachan and Pillai, 2011). Biodegradable polymers for biomedical applications are extensively being utilised in the field of orthopaedics (Ciccone et al., 2001), drug delivery (Huang and Fu, 2010), tissue engineering (Xiao et al., 2012), and ureteral stents (Pawar et al., 2014). Biomedical devices can vary in size and shape depending on their specific application (Kumar et al., 2016; Vert, 2005), for example, 0.05–0.15 mm thin films are used for controlled drug delivery (Karki et al., 2016); 0.3 mm skin graft devices are used for damage remedy (Greenwood, 2017); and 0.5–2 mm fixation plates are used for skeletal repair (Weisberger and Eppley, 1997). The properties of PLA can vary depending on its synthesis and manufacture. Properties such as the glass transition temperature, the tensile strength, the Young’s Modulus (E), strain, and the degree of crystallinity can range from 50-64 � C, 28–50 MPa, 1.2–3 GPa, 2–6%, and 0–37% respectively (Saini et al., 2016). The degradation of PLA is influenced by a number of factors including the molecular weight, the crystallinity, the dimension and size required from the application, and

* Corresponding author. E-mail address: [email protected] (X. Han). https://doi.org/10.1016/j.jmbbm.2020.103654 Received 1 October 2019; Received in revised form 9 January 2020; Accepted 23 January 2020 Available online 25 January 2020 1751-6161/© 2020 Elsevier Ltd. All rights reserved.

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degradation temperature (Domingos et al., 2010; Madhavan Nampoo­ thiri et al., 2010; Shasteen and Choy, 2011). Degradation temperature is often maintained at the normal human body temperature of 37 � C, while degradation can be accelerated at temperatures above and below a materials glass transition temperature (Musioł et al., 2018; Weir et al., 2004a, 2004b). It has also been observed that degradation time can vary according to device thickness, with degradation occurring rapidly in thicker biomedical applications in comparison to thinner applications (Grizzi et al., 1995). The degree of crystallinity, which indicates the fractional quantity of crystallinity within a polymer, is an important parameter when evaluating the properties of biodegradable polymers as it can impact properties such as their modulus, tensile strength, glass transition temperature, and melting points (Karamanlioglu et al., 2017). Due to the structured crystals, the crystalline region enables water penetration during hydrolysis when compared to the amorphous region, therefore, the susceptibility of degradation rate declines (Tsuji and Ikada, 1998). As for molecular weight, degradation time depends on the initial molecular weight, where a high molecular weight has a greater degradation time since it is more robust and less sensitive to degradation in comparison to a lower molecular weight polymer (Karamanlioglu et al., 2017; Liu et al., 2016). Through the utilisation of Additive Manufacturing (AM), the design cycle time can be decreased this enabling devices to be manufactured to specific customer and patient requirements in a shorter time frame in comparison to conventional methods (Kumar et al., 2016). AM is the term used to describe a group of manufacturing technologies that join material layer-by-layer to create objects from 3-dimensional (3D) digital data (ASTM ISO/ASTM52900-15, 2015). Material Extrusion (MEX), a key AM process technology, is a highly customisable, relatively fast and low cost process that uses common polymers such as Polylactic Acid (PLA) and Acrylonitrile-Butadiene-Styrene (ABS) to realise objects that feature complex geometries that are often too difficult or impossible to manufacture by conventional manufacturing technologies (Calignano et al., 2017; Gibson et al., 2010; Ngo et al., 2018). When using MEX, feed-stock material is melted and extruded thorough a nozzle to form 3D objects under the influence of various process parameters such as raster �n et al., 2017; orientation, layer thickness, and infill percentage (Chaco Dizon et al., 2018; Lanzotti et al., 2015). MEX manufactured devices undergo a change in their thermal history when the in-course layer is deposited on to the layer beneath due to the difference in heat distri­ bution, therefore, each added layer demonstrates a different thermal history after each layer is deposited and connected due to a heteroge­ neous material structure (Wolszczak et al., 2018). This study sets out to understand the effect of the number of MEX deposited layers in terms of their thermal, molecular weight, and mechanical properties, and to determine the critical number of layers over which the local thermal history is stabilised with a view to inform the future design and manu­ facture of biomedical devices.

direction were printed via MEX in 1-, 2-, 3-, 4-, 5-, 7- and 10-layers at a layer resolution of 0.1 mm each in the z-direction, and created using NX 11.0 CAD software. Planar specimen CAD data was saved in STL format, prepared using Cura 3.4.1, and printed on an Ultimaker3 system using the parameters outlined in Table 1. The 3D printer was enclosed with a chamber as presented in Fig. 1a to ensure that the ambient environmental temperature during printing was kept constant for all specimen sheets. The ambient temperature was recorded at 30 � C during all printed sheets due to heated bed and extruder. The nozzle moved 100 mm in x-direction using zigzag pattern and 0.36 mm in y-direction until finishing the first layer as demonstrated in Fig. 1b. Following the first layer, the build platform descended 0.1 mm until completing the specimen sheet as presented in Fig. 1c. The extrusion and bed temperature were kept at 210 � C and 60 � C recom­ mended from the company to have a better quality and adhesion. The completed printed specimen sheets were then removed immediately from the printed bed and cooled to room temperature. 2.3. Specimen preparation Following the preparation of the CAD data and the print process parameters, the specimen sheets were manufactured as demonstrated in Fig. 2a. Following printing, test specimen sheets were punched longi­ tudinally in x direction in accordance with ISO 527–2 type 5A specifi­ cation as presented in Fig. 2b. Seven tensile test specimens, as shown in Fig. 2c, were generated from each sheet after cutting. Five of these specimens, as demonstrated in Fig. 2d, were used for tensile testing to ascertain mechanical property information, while the remaining two specimens were used to obtain thermal and molecular weight characterisation. 2.4. Characterisation of 3D printed specimens 2.4.1. Thermal properties Thermal properties evaluated within this research included the glass transition temperature, the melting temperature, the cold crystallisation temperature, the cold crystallisation enthalpy, and the melting enthalpy of the PLA specimens, all of which were found using a TQ(200) Differ­ ential Scanning Calorimetry (DSC) testing machine with a 50 mL/min nitrogen flow rate. A cross-section of material was taken from two specimens for testing and weighed between 5.5 - 6.0 mg. The heating cycle during DSC testing increased from 20 � C to 200 � C at a rate of 10 � C/min with results analysed using TA Universal Analysis software. The glass transition temperature, the melting temperature, the cold crystal­ lisation temperature (Tcc), the cold crystallisation enthalpy (ΔHcc in J/g) and melting enthalpy (ΔHmelt in J/g) were obtained using a DSC ther­ mogram. The reference enthalpy of fusion (ΔHref in J/g) for 100% crystallinity in PLLA is reported to be 93.64 J/g, and was used to calculate the degree of crystallinity (%χ) by using the following the equation (1) (Fischer et al., 1973);

2. Materials and methods 2.1. Materials

%χ ¼ ððΔHmelt

The biodegradable polymers studied within this research was 2.85 mm diameter natural PLA filament (3DXTECH® branded NatureWorks® polylactide 4043D, Sigma Aldrich, LOT: 85633). The mechanical prop­ erties of this material using ISO 527 were measured at 56 MPa, 2.86 GPa, and 8% for tensile strength, tensile modulus and elongation at break respectively; with filament printed using a 0.2 mm layer height and 50 mm/s print speed. The glass transition temperature and printing tem­ peratures were supplied from the manufacturer as 60 � C and 190–210 � C respectively.

2.4.2. The molecular weight The molecular weight properties included number average

(1)

ΔHcc ÞÞ ⁄ ΔHref

Table 1 Ultimaker3 print process parameters.

2.2. Printing technology and process parameters A series of planar specimens measuring 100 � 100 mm in the x and y 2

Process Parameters

Value

Nozzle Diameter Line Width Layer Thickness Printing Speed Raster Angle Extrusion Temperature Bed Temperature

0.4 mm 0.36 mm 0.1 mm 80 mm/s 0� to the x-direction 210 � C 60 � C

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specimens using load (N) - extension (mm) curves. Five tensile test specimens pulled longitudinally were used at each thickness, with mean and standard deviation values for Young’s Modulus, strain at UTS, and UTS established. The width and thicknesses of each specimen were measured from the centre of the specimens. The samples were gripped from 12.5 mm distance on both sides with a gauge length of 20 mm used for calculation. Young’s Modulus was calculated from 0.25% to 0.5% of the stress-strain curves for five tensile test specimens. 3. Result and discussion The thermal, molecular weight and mechanical properties of the printed specimens in 1-, 2-, 3-, 4-, 5-, 7- and 10-layer configurations were determined to understand how the initial properties may affect the degradation of biomedical applications with thickness of 0.1, 0.2, 0.3, 0.4, 0.5, 0.7 and 1 mm.

Fig. 1. Schematic presentation of printer and printing toolpath: a) the printer; b) the top view of the printing toolpath; and c) the side view of the printing toolpath with layer thickness.

3.1. Analysing the raw material The thermal and molecular weight properties of raw material were found using data gathered from DSC and GPC. The glass transition temperature, melting temperature, cold crystallisation temperature, degree of crystallinity, melting enthalpy, and cold crystallisation enthalpy were calculated to be 58.83 � 0.11 � C, 156.96 � 0.30 � C, 132.13 � 0.16 � C, 8.19 � 0.15%, 18.03 � 0.28 J/g and 10.36 � 0.14 J/g respectively. The glass transition temperature at which the PLA became rubbery by getting more heat capacity is found in the first region of the DCS thermogram. The cold crystallisation temperature and the cold crystal­ lisation enthalpy can be calculated by the peak heat flow and the area of the second region, respectively. Above the glass transition temperature, the material gains mobility and rearranges the crystalline by releasing heat. The cold crystallisation temperature and enthalpy indicates that the material can crystallise. Likewise, the third region identifies the melting temperature and melting enthalpy with the highest heat flow and area respectively. When the material is heated to above the crys­ tallisation temperature, the material reaches a temperature where the crystals disperse and melt. The Mn, Mw, and PDI of the raw PLA material were found to be 251.68 � 0.53, 358.06 � 24.83 and 1.42 � 0.10 respectively. The raw material data were identified to understand the effect of thermal degradation occurred during printing and were used as a reference to compare against printed specimens. However, the mechanical proper­ ties of raw material were not tested due to its fragility especially when using bollard style grips.

Fig. 2. Specimen preparations.

molecular weight (Mn), weight average molecular weight (Mw) and polydispersity index (PDI) (Mw/Mn), and were determined using a Waters Breeze2 Gel Permeation Chromatography (GPC) system (1525 pump, 2414 differential detector, and 2707 autosampler). 8–12 mg of materials were cut from the cross-section of the test specimens and dissolved into 1 ml of High-Performance Liquid Chromatography (HPLC) grade Dimethylformamide (DMF). All molecular weight data was averaged across two measurements. The GPC sample was uniformly mixed on a shaker at 500-rpm for 48 h to ensure that it was fully dis­ solved. Once dissolved the solution was extracted with a syringe using a 0.22 μm needle filter and loaded into a sample vial. The solution was injected from the autosampler with a flow phase rate of 1 mL/min. The oven temperature was set at 60 � C and the reference material was polystyrene (Agilent’s EasiVial PS-M 2 ml kit). The unit of Mn and Mw were found to be kilodalton (kDa).

3.2. Printed specimens 3.2.1. Thermal analysis The thermal properties obtained from DSC, as demonstrated in Fig. 3, and the results of 1-, 2-, 3-, 4-, 5-, 7- and 10-layer printed spec­ imens are given in Table 2. In Table 2, the thermal properties including the glass transition temperature, the melting temperature, the cold crystallisation temper­ ature, and the degree of crystallinity decreased following material deposition. The glass transition temperature decreased from 58.83 � C for the raw PLA material to 56.20, 57.06, 57.92, 58.08, 58.20, 58.07 and 58.07 � C for 1-, 2-, 3-, 4-, 5-, 7- and 10-layer printed specimens respectively. In addition, the melting temperature decreased from 156.96 � C for the raw PLA material to 147.94, 149.79, 149.39, 149.69, 149.91, 149.74 and 150.15 � C for 1-, 2-, 3-, 4-, 5-, 7- and 10-layer printed specimens respectively. The decrease in glass transition tem­ perature following deposition may have occurred due to a change in their lamellar morphology, such as crystalline thickness, in comparison to raw PLA material. This lamellar structure occurs where the folding chain arranged related to cooling time (Balani et al., 2015) after the raw

2.4.3. Mechanical properties Printed PLA tensile specimens were assessed using an Instron 3345 Universal Testing System equipped with a 5 kN load cell and tested at a constant strain rate of 1 mm/min. The tested mechanical properties included Young’s Modulus, strain at ultimate tensile strength (UTS) and UTS were calculated for 1-, 2-, 3-, 4-, 5-, 7- and 10-layer printed 3

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decrease in the cold crystallisation enthalpy during printing with an increasing number of layers due to their different thermal histories. After reaching the glass transition temperature, the increased heating allows the material to undergo crystallisation, resulting in cold crystal­ lisation. This is caused due to increase in mobility as the molecular chains reorganised (Senatov et al., 2016). Cold crystallisation increased with the increasing number of layers due to the in-course layer con­ nected to the deposited layer cooling slowly. Additionally, the cold crystallisation temperature increased with an increasing number of layers from 120 � C to up to 125 � C, thus resulting in a higher cold crystallisation enthalpy. In Fig. 5, the increase in the cold crystallisation enthalpy continued until 5-layer printed specimens and then became constant from 5- to 10-layer printed specimens. 3.2.2. Molecular weight analysis The molecular weight distribution obtained from GPC is presented within Fig. 6. The Mn and Mw of 1-, 3-, and 5-layer printed specimens are illus­ trated in Fig. 7. Mn reduced by approximately 24% at 192 kDa, 15% at 214 kDa, and 7% at 233 kDa for 1-, 3-, and 5-layer printed specimens respectively in comparison to raw PLA material. Likewise, the Mw decreased by around 18% at 295 kDa, 5% at 339 kDa, and 0.21% at 359 kDa for 1-, 3-, and 5layer printed specimens respectively. The molecular weight is strongly associated with the viscosity of the material (Speranza et al., 2014), while viscosity is affected by temperature and shear rate (Dealy and Wang, 2013). The viscosity decreased within seconds of shearing because of chain scission and thermal degradation (Spoerk et al., 2019). Therefore, such a reduction in Mn and Mw is likely to have occurred following deposition due to molecular deterioration caused by thermal degradation and shearing (Farah et al., 2016). In an addition research by (Gr�emare et al., 2018), the molecular weight decreased 48% after deposition as a results of shortening in polymer chains. The decrease in the molecular weight of the printed specimens may also be attributed holding the print bed temperature at 60 � C and their cooling rate. When printing using MEX, the bed temperature can have an impact on mo­ lecular weight due to holding the material above glass transition tem­ perature, thus likely to cause a greater level of degradation (Wang et al., 2017). It is recognised that when the material is cooled quickly, mole­ cules freeze before reaching the equilibrium volume state; thus, decreasing the molecular motion and resulting in the freezing of the chains. The connection between layers is when the macromolecules entangle in the interface; therefore, the molecular diffusion is lower when the layer is cooled fast (Gao et al., 2019). Since the bottom layers cooled quicker after deposition, the shearing action during deposition disentangles the polymers chains and likely caused lower viscosity due to chain scission; however, the macromolecules begin to return to their equilibrium state resulted in different mechanical and thermal

Fig. 3. DSC thermogram for 1-, 2-, 3-, 4-, 5-, 7- and 10-layer printed specimens. Table 2 The thermal properties for 1-, 2-, 3-, 4-, 5-, 7- and 10-layer printed specimens. Number of Layers

Tg(oC)

Tm(oC)

Tcc(oC)

%χ

ΔHmelt(J/ g)

ΔHcc(J/ g)

1

56.20 � 0.14

147.94 � 0.71

119.89 � 0.42

11.68 � 0.49

7.18 � 0.26

2

57.06 � 0.09

149.79 � 0.35

122.10 � 0.08

11.30 � 0.23

7.99 � 0.20

3

57.92 � 0.08

149.39 � 0.42

122.79 � 0.57

11.27 � 0.17

9.28 � 0.11

4

58.08 � 0.02

149.69 � 0.25

124.50 � 0.64

10.32 � 0.27

9.34 � 0.16

5

58.20 � 0.21

149.91 � 0.38

125.00 � 0.71

10.33 � 0.11

9.65 � 0.33

7

58.07 � 0.06

149.74 � 0.25

124.82 � 0.10

10.21 � 0.08

9.60 � 0.05

10

58.07 � 0.08

150.15 � 0.27

124.69 � 0.23

4.81 � 0.17 3.69 � 0.27 2.12 � 0.06 1.05 � 0.12 0.72 � 0.23 0.66 � 0.03 0.53 � 0.03

10.17 � 0.07

9.67 � 0.04

PLA material deposited. For the melting temperature, the reduction may have been influenced by the crystal size of PLA after deposition. The degree of crystallinity decreased to 4.81%, 3.69%, 2.13%, 1.05%, 0.72%, 0.66% and 0.53% for 1-, 2-, 3-, 4-, 5-, 7- and 10-layer printed specimens as demonstrated in Fig. 4 from 8.19% in the raw PLA mate­ rial. Similar observations have been made where an increase in layers have caused a reduction in the degree of crystallinity due to heat dissipation in the first layer (Drummer et al., 2012; Srinivas et al., 2018). It is evident that the reduction in crystallinity occurred as a result of a

Fig. 5. Cold crystallisation enthalpy of 1-, 2-, 3-, 4-, 5-, 7- and 10-layer prin­ ted specimens.

Fig. 4. Degree of crystallinity for 1-, 2-, 3-, 4-, 5-, 7- and 10-layer specimens. 4

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Fig. 6. Molecular weight distribution of 1-, 3-, and 5-layer printed specimens.

Fig. 8. Relationship between glass transition temperature and number average molecular weight.

specimens demonstrated in Fig. 9 reached their highest tensile strengths and then stretched continually at a constant stress due to their visco­ elastic behaviour under slow testing speeds (Dearmitt, 2017). The data collected from the curves determine the mechanical prop­ erties of 1-, 2-, 3-, 4-, 5-, 7- and 10-layer printed specimens. The Young’s Modulus and strain at UTS are shown in Fig. 10. The mean Young’s Modulus of the 1-layer printed specimens was recorded at 1.44 � 0.04 GPa. With an increasing number of layers, the Young’s Modulus decreased to 1.37 � 0.01, 1.32 � 0.04, 1.28 � 0.04, 1.23 � 0.06, 1.18 � 0.02 and 1.14 � 0.05 GPa for 2-, 3-, 4-, 5-, 7- and 10layer printed specimens respectively. However, the strain at UTS of the printed specimens increased with an increasing number of layers from 1 to 10. The strain at UTS were recorded as 3.87 � 0.15%, 4.56 � 0.20%, 5.00 � 0.08%, 5.26 � 0.14%, 5.98 � 0.17%, 6.01 � 0.07% and 6.05 � 0.25% for 1-, 2-, 3-, 4-, 5-, 7- and 10-layer printed specimens respec­ tively. The Young’s Modulus decreased as a result of a decreasing degree of crystallinity. As such, the decrease in both Young’s Modulus and degree of crystallinity were almost similar. The increase in strain at UTS resulted from the various thicknesses that would also explain the decrease in the Young Modulus. The UTS was 43.32 � 0.93 MPa for 1-layer printed specimens while for the 1-, 2-, 3-, 4-, 5-, 7- and 10-layer printed specimens their UTS was recorded at 45.90 � 0.99, 47.73 � 1.24, 48.09 � 0.63, 48.71 � 1.13, 49.24 � 0.47 and 49.55 � 1.37 MPa respectively. The results shows that the mechanical performance of the printed specimens increased with an increasing number of layers, as shown in Fig. 11. In polymers, the me­ chanical properties are associated with the molecular weights and crystallinities (Wang et al., 2010; Zhang et al., 2019), and the reason of change in molecular weight and impact on mechanical properties dis­ cussed in section 3.2.2. In MEX, when a deposited layer is connected to the previous layer, the interface disappears due to molecular diffusion;

Fig. 7. Mn, Mw and PDI of 1-, 3-, and 5-layer specimens.

properties of printed specimens (Sanchez et al., 2019). Conversely, the PDI of 1-, 3-, and 5-layer printed specimens increased to 1.54, 1.59, and 1.54 respectively. The increasing molecular weight and decreasing de­ gree of crystallinity had almost the same trend with the increasing number of layers. Therefore, the lower molecular weight resulted in a higher degree of crystallinity because of chain mobility (Perego et al., 1996). The glass transition temperature of a polymer is dependent on molecular weight of the material by following the Flory-Fox equation (2) where T∞ is the glass transition temperature at the infinite molecular weight, Mn is the number average molecular weight, and K is a constant based on the free volume of polymer chains (Fox and Flory, 1954, 1950). This method has been used to predict the glass transition temperature of PLA, depending on its purity, within traditional manufacturing pro­ cesses (Müller et al., 2015). � � K Tg ¼ ðTg∞ Þ (2) Mn It has been concluded that the glass transition temperature increased up to a critical point with the increasing molecular weight which then become constant (Saeidlou et al., 2012). In their critical review, the different trend following the Flory-Fox equation was due to the purity of the PLA material and the constant K and the T∞ can be varied and range between 5.5–7.3 � 104 and 57–58 � C depending on material purity (Jamshidi et al., 1988). However, there is no current work for material extruded PLA and further research is needed using different process parameters. In this study, the number average molecular weight was increased with an increasing number of layers, while the glass transition temperature increased up to a point and then became constant, as pre­ dicted by using the Flory-Fox equation. This is demonstrated in Fig. 8. 3.2.3. Mechanical properties The stress-strain curves of 1-, 2-, 3-, 4-, 5-, 7- and 10-layer printed

Fig. 9. The stress-strain curves of 1-, 2-, 3-, 4-, 5-, 7- and 10-layer prin­ ted specimens. 5

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The Young’s Modulus, strain at UTS, and UTS ranged from 1.46 to 1.14 GPa, 3.92%–6.05% and 44.39–49.55 MPa. The results of the thermal, molecular weight, and mechanical properties of the printed specimens clearly demonstrated that 10-layer printed specimens almost have no significant difference in comparison to 5-layer printed specimens. Therefore, the thermal histories of the printed specimens with 5 or less layers need to be carefully considered for biomedical applications. Additionally, identifying the appropriate mechanical, thermal, and molecular weight properties are crucial in order to design degradable devices for medical applications using material extruded biodegradable PLA and this study provides the prerequisite work for such degradation studies. For further future research, mechanical, thermal and physical properties of MEX biodegradable PLA with different process parameters including extrusion speed, bed and extrusion temperature should be investigated to understand the effect of process parameters before and after degradation.

Fig. 10. The Young’s Modulus and strain at UTS of 1-, 2-, 3-, 4-, 5-, 7- and 10layer printed specimens.

Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. CRediT authorship contribution statement Alper Ekinci: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Andrew A. Johnson: Funding acquisition, Supervision, Writing - review & editing. Andy Gleadall: Supervision, Writing - review & editing. Daniel S. Engstrøm: Writing review & editing. Xiaoxiao Han: Conceptualization, Supervision, Project administration, Funding acquisition, Writing - review & editing.

Fig. 11. The UTS and its relationship with molecular weight of 1-, 3-, 5-, and 10-layer printed specimens.

Acknowledgement

therefore, rheological properties including the tensile stress increases (Mackay, 2018). For UTS (Odian, 2004), discovered that the strength increased rapidly with the increase of the molecular weight to a critical point and then increases slowly to its limit. The results from this study show the same behaviour that the UTS increased rapidly up to 5-layers and then increased steadily up to 10-layers, with increasing number of layers and increasing molecular weight, as demonstrated in Fig. 11. The mechanical properties of PLA are highly dependent on the de­ gree of crystallinity, molecular weight properties, and their distribution (Perego et al., 1996). While crystallinity decreased with an increasing number of layers, the molecular weight properties increased. This resulted in higher tensile strength due to the entangled and bulky chains complicating chain movement. The 5- and 10-layer printed specimens demonstrated very similar properties in terms of their thermal and mechanical properties, and the molecular weight increased almost to the raw PLA properties with 5-layer printed specimens. Therefore, there was no need to increase the number of layers any further.

This work is supported by the PhD Scholarship for Alper Ekinci from the Ministry of National Education (Republic of Turkey), and also sup­ ported by the Huxiang High-level Talent Gathering Project (No. 2019RS1019) from China Hunan Provincial Science and Technology Department. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jmbbm.2020.103654. References ASTM ISO/ASTM52900-15, 2015. Standard Terminology for Additive Manufacturing – General Principles – Terminology. ASTM International, West Consh, pp. 1–9. https:// doi.org/10.1520/ISOASTM52900-15. Balani, K., Verma, V., Agarwal, A., Narayan, R., 2015. Physical, thermal, and mechanical properties of polymers. Biosurfaces 329–344. https://doi.org/10.1002/ 9781118950623.app1. Bose, S., Ke, D., Sahasrabudhe, H., Bandyopadhyay, A., 2017. Additive manufacturing of biomaterials. Prog. Mater. Sci. 93, 45–111. https://doi.org/10.1016/j. pmatsci.2017.08.003. Calignano, F., Manfredi, D., Ambrosio, E.P., Biamino, S., Lombardi, M., Atzeni, E., Salmi, A., Minetola, P., Iuliano, L., Fino, P., 2017. Overview on additive manufacturing technologies. In: Proceedings of the IEEE, vol. 105, pp. 593–612. https://doi.org/10.1109/JPROC.2016.2625098. Chac� on, J.M., Caminero, M.A., García-Plaza, E., Nú~ nez, P.J., 2017. Additive manufacturing of PLA structures using fused deposition modelling: effect of process parameters on mechanical properties and their optimal selection. Mater. Des. 124, 143–157. https://doi.org/10.1016/j.matdes.2017.03.065. Ciccone, W.J., Motz, C., Bentley, C., Tasto, J.P., 2001. Bioabsorbable implants in orthopaedics: new developments and clinical applications. J. Am. Acad. Orthop. Surg. 9, 280–288. https://doi.org/10.5435/00124635-200109000-00001.

4. Conclusion The thermal properties and the molecular weight of test specimens manufactured via MEX using PLA change depending on the number of layers deposited due to their different thermal histories. The crystallinity of the raw PLA material decreased from 4.0% to 0.5% following printing in comparison to raw material. However, the number average and weight average molecular weight of raw PLA decreased from 251 and 358 kDa to 192 and 295 kDa respectively following deposition and then both Mn and Mw increased with an increasing number of layers approximately up to the raw PLA value. Both the degree of crystallinity and molecular weight properties impacted the mechanical properties. 6

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Journal of the Mechanical Behavior of Biomedical Materials 104 (2020) 103654 Naghieh, S., Karamooz Ravari, M.R., Badrossamay, M., Foroozmehr, E., Kadkhodaei, M., 2016. Numerical investigation of the mechanical properties of the additive manufactured bone scaffolds fabricated by FDM: the effect of layer penetration and post-heating. J. Mech. Behav. Biomed. Mater. 59, 241–250. https://doi.org/ 10.1016/j.jmbbm.2016.01.031. Nair, L.S., Laurencin, C.T., 2007. Biodegradable polymers as biomaterials. Prog. Polym. Sci. 32, 762–798. https://doi.org/10.1016/j.progpolymsci.2007.05.017. Ngo, T.D., Kashani, A., Imbalzano, G., Nguyen, K.T.Q., Hui, D., 2018. Additive manufacturing (3D printing): a review of materials, methods, applications and challenges. Compos. B Eng. 143, 172–196. https://doi.org/10.1016/j. compositesb.2018.02.012. Odian, G., 2004. Introduction. In: Principles of Polymerization. John Wiley & Sons, Inc., Hoboken, NJ, USA, pp. 1–38. https://doi.org/10.1002/047147875X.ch1. Pawar, R.P., Tekale, S.U., Shisodia, S.U., Totre, J.T., Domb, A.J., 2014. Biomedical applications of poly(lactic acid). PLA Appl. 4, 40–51. https://doi.org/10.2174/ 2210296504666140402235024. Perego, G., Cella, G., Bastioli, C., 1996. Effect of molecular weight and crystallinity on poly (lactic acid) mechanical properties. J. Appl. Polym. 59, 37–43. https://doi.org/ 10.1002/(SICI)1097-4628(19960103)59:1<37::AID-APP6>3.0.CO;2-N. Saeidlou, S., Huneault, M.A., Li, H., Park, C.B., 2012. Poly(lactic acid) crystallization. Prog. Polym. Sci. 37, 1657–1677. https://doi.org/10.1016/j. progpolymsci.2012.07.005. Saini, P., Arora, M., Kumar, M.N.V.R., 2016. Poly(lactic acid) blends in biomedical applications. Adv. Drug Deliv. Rev. 107, 47–59. https://doi.org/10.1016/j. addr.2016.06.014. Sanchez, L.C., Beatrice, C.A.G., Lotti, C., Marini, J., Bettini, S.H.P., Costa, L.C., 2019. Rheological approach for an additive manufacturing printer based on material extrusion. Int. J. Adv. Manuf. Technol. 2403–2414. https://doi.org/10.1007/ s00170-019-04376-9. Senatov, F.S., Niaza, K.V., Zadorozhnyy, M.Y., Maksimkin, A.V., Kaloshkin, S.D., Estrin, Y.Z., 2016. Mechanical properties and shape memory effect of 3D-printed PLA-based porous scaffolds. J. Mech. Behav. Biomed. Mater. 57, 139–148. https:// doi.org/10.1016/j.jmbbm.2015.11.036. Shasteen, C., Choy, Y. Bin, 2011. Controlling degradation rate of poly(lactic acid) for its biomedical applications. Biomed. Eng. Lett. 1, 163–167. https://doi.org/10.1007/ s13534-011-0025-8. Speranza, V., De Meo, A., Pantani, R., 2014. Thermal and hydrolytic degradation kinetics of PLA in the molten state. Polym. Degrad. Stabil. 100, 37–41. https://doi.org/ 10.1016/j.polymdegradstab.2013.12.031. Spoerk, M., Arbeiter, F., Ragu�z, I., Holzer, C., Gonzalez-Gutierrez, J., 2019. Mechanical recyclability of polypropylene composites produced by material extrusion-based additive manufacturing. Polymers 11. https://doi.org/10.3390/polym11081318. Srinivas, V., van Hooy-Corstjens, C.S.J., Harings, J.A.W., 2018. Correlating molecular and crystallization dynamics to macroscopic fusion and thermodynamic stability in fused deposition modeling; a model study on polylactides. Polymer 142, 348–355. https://doi.org/10.1016/j.polymer.2018.03.063. Temenoff, J.S., Mikos, A.G., 2000. Review: tissue engineering for regeneration of articular cartilage. Biomaterials 21, 431–440. https://doi.org/10.1016/S0142-9612 (99)00213-6. Tsuji, H., Ikada, Y., 1998. Properties and morphology of poly(L-lactide). II. hydrolysis in alkaline solution. J. Polym. Sci. Polym. Chem. 36, 59–66. https://doi.org/10.1002/ (SICI)1099-0518(19980115)36:1<59::AID-POLA9>3.0.CO;2-X. Vert, M., 2005. Aliphatic polyesters: great degradable polymers that cannot do everything. Biomacromolecules 6, 538–546. https://doi.org/10.1021/bm0494702. Wang, L., Gramlich, W.M., Gardner, D.J., 2017. Improving the impact strength of Poly (lactic acid) (PLA) in fused layer modeling (FLM). Polymer 114, 242–248. https:// doi.org/10.1016/j.polymer.2017.03.011. Wang, Y., Han, X., Pan, J., Sinka, C., 2010. An entropy spring model for the Young’s modulus change of biodegradable polymers during biodegradation. J. Mech. Behav. Biomed. Mater. 3, 14–21. https://doi.org/10.1016/j.jmbbm.2009.02.003. Weir, N.A., Buchanan, F.J., Orr, J.F., Dickson, G.R., 2004a. Degradation of poly-L-lactide. Part 1: in vitro and in vivo physiological temperature degradation. In: Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine, vol. 218, pp. 307–319. https://doi.org/10.1243/0954411041932782. Weir, N.A., Buchanan, F.J., Orr, J.F., Farrar, D.F., Dickson, G.R., 2004b. Degradation of poly-L-lactide. Part 2: increased temperature accelerated degradation. In: Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine, vol. 218, pp. 321–330. https://doi.org/10.1243/ 0954411041932809. Weisberger, E.C., Eppley, B.L., 1997. Resorbable fixation plates in head and neck surgery. Laryngoscope 107, 716–719. https://doi.org/10.1097/00005537-19970600000003. Wolszczak, P., Lygas, K., Paszko, M., Wach, R.A., 2018. Heat distribution in material during fused deposition modelling. Rapid Prototyp. J. 24, 615–622. https://doi.org/ 10.1108/RPJ-04-2017-0062. Xiao, L., Wang, B., Yang, G., Gauthier, M., 2012. Poly (Lactic Acid) -based biomaterials : synthesis , modification and applications. Biomed. Sci., Eng. Technol. 247–282. https://doi.org/10.5772/23927. Zhang, T., Jin, G., Han, X., Gao, Y., Zeng, Q., Hou, B., Zhang, D., 2019. Multiscale modelling for the heterogeneous strength of biodegradable polyesters. J. Mech. Behav. Biomed. Mater. 90, 337–349. https://doi.org/10.1016/j. jmbbm.2018.10.018.

Dealy, J.M., Wang, J., 2013. Melt Rheology and its Applications in the Plastics Industry, Engineering Materials and Processes. Springer Netherlands, Dordrecht. https://doi. org/10.1007/978-94-007-6395-1. Dearmitt, C., 2017. 23 functional fillers for plastics. In: Applied Plastics Engineering Handbook, second ed. Elsevier Inc. https://doi.org/10.1016/B978-0-323-39040-8/ 00023-7. Dizon, J.R.C., Espera, A.H., Chen, Q., Advincula, R.C., 2018. Mechanical characterization of 3D-printed polymers. Addit. Manuf. https://doi.org/10.1016/j. addma.2017.12.002. Domingos, M., Chiellini, F., Cometa, S., De Giglio, E., Grillo-Fernandes, E., B� artolo, P., Chiellini, E., 2010. Evaluation of in vitro degradation of pcl scaffolds fabricated via bioextrusion. part 1: influence of the degradation environment. Virtual Phys. Prototyp. 5, 65–73. https://doi.org/10.1080/17452751003769440. Drummer, D., Cifuentes-Cu� ellar, S., Rietzel, D., 2012. Suitability of PLA/TCP for fused deposition modeling. Rapid Prototyp. J. 18, 500–507. https://doi.org/10.1108/ 13552541211272045. Farah, S., Anderson, D.G., Langer, R., 2016. Physical and mechanical properties of PLA, and their functions in widespread applications — a comprehensive review. Adv. Drug Deliv. Rev. 107, 367–392. https://doi.org/10.1016/j.addr.2016.06.012. Fischer, E.W., Sterzel, H.J., Wegner, G., 1973. Investigation of the structure of solution grown crystals of lactide copolymers by means of chemical reactions. Kolloid-Z. Z. Polym. 251, 980–990. https://doi.org/10.1007/BF01498927. Fox, T.G., Flory, P.J., 1954. The glass temperature and related properties of polystyrene. Influence of molecular weight. J. Polym. Sci. 14, 315–319. https://doi.org/10.1002/ pol.1954.120147514. Fox, T.G., Flory, P.J., 1950. Second-order transition temperatures and related properties of polystyrene. I. Influence of molecular weight. J. Appl. Phys. 21, 581–591. https:// doi.org/10.1063/1.1699711. Gao, X., Zhang, D., Qi, S., Wen, X., Su, Y., 2019. Mechanical properties of 3D parts fabricated by fused deposition modeling: effect of various fillers in polylactide. J. Appl. Polym. Sci. 136, 1–10. https://doi.org/10.1002/app.47824. Gibson, I., Rosen, D.W., Stucker, B., 2010. Extrusion-based systems. In: Additive Manufacturing Technologies. Springer US, Boston, MA, pp. 160–186. https://doi. org/10.1007/978-1-4419-1120-9_6. Greenwood, J.E., 2017. The evolution of acute burn care - retiring the split skin graft. Ann. R. Coll. Surg. Engl. 99, 432–438. https://doi.org/10.1308/rcsann.2017.0110. Gr�emare, A., Guduric, V., Bareille, R., Heroguez, V., Latour, S., L’heureux, N., Fricain, J. C., Catros, S., Le Nihouannen, D., 2018. Characterization of printed PLA scaffolds for bone tissue engineering. J. Biomed. Mater. Res. 106, 887–894. https://doi.org/ 10.1002/jbm.a.36289. Grizzi, I., Garreau, H., Li, S., Vert, M., 1995. Hydrolytic degradation of devices based on poly(dl-lactic acid) size-dependence. Biomaterials 16, 305–311. https://doi.org/ 10.1016/0142-9612(95)93258-F. Huang, S., Fu, X., 2010. Naturally derived materials-based cell and drug delivery systems in skin regeneration. J. Contr. Release 142, 149–159. https://doi.org/10.1016/j. jconrel.2009.10.018. Jamshidi, K., Hyon, S.H., Ikada, Y., 1988. Thermal characterization of polylactides. Polymer 29, 2229–2234. https://doi.org/10.1016/0032-3861(88)90116-4. Karamanlioglu, M., Preziosi, R., Robson, G.D., 2017. Abiotic and biotic environmental degradation of the bioplastic polymer poly(lactic acid): a review. Polym. Degrad. Stabil. 137, 122–130. https://doi.org/10.1016/j.polymdegradstab.2017.01.009. Karki, S., Kim, H., Na, S., Shin, D., Jo, K., Lee, J., 2016. Thin films as an emerging platform for. Asian J. Pharm. Sci. 11, 559–574. https://doi.org/10.1016/j. ajps.2016.05.004. Kulshrestha, A.S., Mahapatro, A., 2008. Polymers for biomedical applications. In: Polymers for Biomedical Applications, pp. 1–7. https://doi.org/10.1021/bk-20080977.ch001. Kumar, A., Nune, K.C., Murr, L.E., Misra, R.D.K., 2016. Biocompatibility and mechanical behaviour of three-dimensional scaffolds for biomedical devices: process-structureproperty paradigm. Int. Mater. Rev. 61, 20–45. https://doi.org/10.1080/ 09506608.2015.1128310. Lanzotti, A., Grasso, M., Staiano, G., Martorelli, M., 2015. The impact of process parameters on mechanical properties of parts fabricated in PLA with an open-source 3-D printer. Rapid Prototyp. J. 21, 604–617. https://doi.org/10.1108/RPJ-09-20140135. Liu, R., Wang, Y., Ma, Y., Wu, Y., Guo, Y., Xu, L., 2016. Effects of the molecular weight of PLGA on degradation and drug release in vitro from an mPEG-PLGA nanocarrier. Chem. Res. Chin. Univ. 32, 848–853. https://doi.org/10.1007/s40242-016-6155-x. Luckachan, G.E., Pillai, C.K.S., 2011. Biodegradable polymers- A review on recent trends and emerging perspectives. J. Polym. Environ. 19, 637–676. https://doi.org/ 10.1007/s10924-011-0317-1. Mackay, M.E., 2018. The importance of rheological behavior in the additive manufacturing technique material extrusion. J. Rheol. 62, 1549–1561. https://doi. org/10.1122/1.5037687. Madhavan Nampoothiri, K., Nair, N.R., John, R.P., 2010. An overview of the recent developments in polylactide (PLA) research. Bioresour. Technol. 101, 8493–8501. https://doi.org/10.1016/j.biortech.2010.05.092. � Müller, A.J., Avila, M., Saenz, G., Salazar, J., 2015. Crystallization of PLA-based materials. RSC Polym. Chem. Ser. https://doi.org/10.1039/9781782624806-00066. Musioł, M., Jurczyk, S., Kwiecie� n, M., Smola-Dmochowska, A., Doma� nski, M., Janeczek, H., Włodarczyk, J., Klim, M., Rydz, J., Kawalec, M., Sobota, M., 2018. The impact of shape memory test on degradation profile of a bioresorbable polymer. J. Mech. Behav. Biomed. Mater. 81, 39–45. https://doi.org/10.1016/j. jmbbm.2018.02.020.

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