Polycaprolactone/polysaccharide functional composites for low-temperature fused deposition modelling.

Fused deposition modelling (FDM) is a commonly used 3D printing technology. The development of FDM materials was essential for the product quality of FDM. In this work, a series of polycaprolactone (PCL)-based composites for low-temperature FDM were developed. By melt blending technique, different ratios of starch were added into PCL to improve the performances of FDM, and the printability, tensile strength, rheological properties, crystallization behaviors and biological performances of the composites were studied. The PCL/starch composite had the best performance in FDM process with the starch ratio of 9 ph at 80-90 °C. The melting strength and solidification rate of PCL/starch composites were improved. The starch also increased the crystallization temperature, degree of crystallinity and crystallization rate of PCL/starch composites, while had no negative effects on the tensile strength of PCL. Due to the low printing temperature, various kinds of bioactive components were added into PCL/starch composites for preparation of antibacterial and biocompatible materials for FDM. The present work provides a new method to develop novel low-temperature FDM materials with various functions.

Introduction

Three-dimensional (3D) printing is an advanced technology for customized manufacturing [[1], [2], [3]]. By stacking materials layer by layer, objects could be produced based on computer-aided design models by 3D printing, which has been widely applied in medical field [4,5], part manufacturing [6], automobile industry [7], and so on. The 3D printing technology has various types, including fused deposition modelling (FDM) [8], stereo lithography appearance (SLA) [9], selective laser sintering (SLS) [10], laminated object manufacturing (LOM) [11], patternless casting manufacturing (PCM) [12], and so on. Among all the types of 3D printing, FDM is commonly used because of its low the cost and easy operation [13,14]. In the FDM process, the material has the very significant influence on the quality and function of the printed products [[15], [16], [17]]. Therefore, it has high theoretic meaning and realistic value to develop high-performance materials for FDM.

The common-used thermoplastic materials for FDM, such as polylactic acid [18], poly(acrylonitrile-co-butadiene-co-styrene) [19] and polycarbonate [20], were manufactured at the temperature higher than 200 °C. The high manufacturing temperature leads to high energy consumption and low safety, especially for small-scale household FDM printer. Moreover, it was reported that volatile organic compounds (VOC) and air-borne micro-particles of plastics were produced continuously during the FDM processes under high temperature, which will be harmful to the health of the users [21,22]. Moreover, the high manufacturing temperature hinders the addition of bioactive components into the FDM materials, limiting the functionalization of these materials.

Polycaprolactone (PCL) is a kind of linear aliphatic polyester with good flexibility, machinability, biodegradability and biocompatibility, which is semi-crystalline under room temperature, and the melting temperature of PCL ranges from 59 to 64 °C. The above-mentioned properties makes PCL a potential candidate of functional FDM materials [23]. However, pristine PCL possesses many disadvantages for FDM, including low solidifying rate and insufficient melt strength, which are obstacles of large-scale application apply of PCL as a FDM material [24,25]. Recently, many types of PCL-based 3D printing materials were developed [26,27], but the raw materials of these materials were mainly slurry and the manufacturing process needed dedicated devices for modelling.

In this work, we developed a series of functional PCL/starch composites for low-temperature FDM by melt blending technique. Different ratios of starch were blended with PCL to prepare PCL/starch composites for FDM. The 3D-printability, tensile strength, rheological properties, crystallization behaviors and biological performances of the composites were studied. This work provides a novel method to develop functional materials for low-temperature FDM.

Experimental section

Materials

PCL (Perstorp 6800) was purchased from Solvay, USA. Soluble starch was purchased from Sigma-Aldrich, USA. Corn starch (Mw = 14.4 × 103 kDa) and potato starch (Mw = 25.2 × 103 kDa) were purchased from Aladdin, China. Quaternary ammonium-73 was purchased from Tianjin Ringpu Biotechnology Co., Ltd., China. Polyhexamethylene biguanidine (PHMB) was purchased from Guangdong AONA New Materials Co. Ltd., China. Nano-hydroxyapatite (nHA, with the length of ~100 nm and diameter of 20 nm) was purchased from Beijing DK Nano Technology Co. Ltd., China.

Preparation of PCL/starch composites

PCL, soluble starch, corn starch and potato starch were placed in an air-blower-driver dryer at 50 °C for 2 h. After drying, 100 g of PCL was respectively mixed with 1 g, 3 g, 5 g, 7 g, 9 g and 11 g of each kind of starch. The mixtures were thoroughly blended with a high-speed mixer and then extruded by a twin-screw extruder. The parameters of the twin-screw extruder were shown as Table 1. After being pelletized, the composites were dried completely.

Table 1

Extruder parameter setting.

	Zone 1/°C	Zone 2/°C	Zone 3/°C	Zone 4/°C	Screw speed/(mm/s)
Twin-screw extruder	50	70	90	70	20
Single-screw extruder	80	–	–	75	30

Preparation of antibacterial PCL/starch composites

The PCL/starch composite, quaternary ammonium-73 and PHMB were placed in an air-blower-driver dryer at 50 °C for 2 h. After drying, 0.01 g, 0.1 g and 1 g of quaternary ammonium-73 were respectively mixed with 100 g of dried PCL/starch composite, and then treated with high-speed mixer. The mixtures were extruded by a twin-screw extruder with the parameters shown in Table 1. Similarly, 0.1 g, 1 g and 10 g of PHMB were also mixed with PCL/starch composite as described above. The prepared antibacterial composites were pelletized and dried completely.

Preparation of nHA-modified PCL/starch composites

The PCL/starch composite and nHA were placed in an air-blower-driver dryer at 50 °C for 2 h. After drying, 10 g of nHA was mixed with 100 g of dried PCL/starch composite, and then treated with high-speed mixer. The mixtures were extruded by a twin-screw extruder with the parameters shown in Table 1. Finally, the samples were pelletized and dried completely.

Preparation of filaments for 3D printing

The filaments of each kind of composite were prepared by a single-screw extruder, and the parameters of preparation were shown in Table 1. The diameter of the filaments was confined as 1.75 ± 0.05 mm by controlling the rotating speed of tractor. The prepared filaments were collected on take-up reels by a rotary thread rolling machine.

Characterization

The 3D-printing performances were evaluated by a FDM printer (Replicator X2, Makerbot, USA). The filament samples were used to prepare a cubic modal with the size of 20 mm × 20 mm × 10 mm. The parameters of FDM printer were shown in Table 2. ABS resin was taken as a reference.

Table 2

Parameter setting of FDM printer.

Nozzle temperature	Plate Temperature	Printing resolution	Printing speed	Feeding speed	Filling ratio
90 °C	25 °C	0.2 mm	40 mm/s	90 mm/s	100%

The tensile strength was measured by a universal testing machine (UTM-1422, Jinjian, China). The pellet samples were dried at 50 °C for 2 h and then prepared into thin sheets with the thickness of 1 mm via hot-press by a plate vulcanizing machine. After cooling, the sheets were cut into standard dumbbell-shaped samples with the effective length of 25 mm and effective width of 4 mm. The testing of tensile strength was performed at the stretching speed of 100 mm/min following GB/T 1040–2006, and five duplicate samples were tested in each group.

The rheological properties were characterized by rheometer (HR-1, Waters, USA). The samples for rheological testing were prepared by a plate vulcanizing machine and cut into round slices with the diameter of 25 mm and the thickness of 1 mm. For dynamic frequency scanning, the temperature was set as 90 °C, while the strain was 5% and angular velocity (ω) was 0.5–100 rad/s. For viscosity-temperature curve, the temperature was set as 60–90 °C with the heating rate of 3 °C/min, and the strain rate was 1/s.

The crystallization behaviors were studied by a differential scanning calorimetry (DSC, Q20, TA Instruments, USA) in nitrogen atmosphere. For isothermal crystallization, the samples were heated to 100 °C with the heating rate of 10 °C/min and kept for 3 min to eliminate the heat history. Then, the samples were cooled to 40 °C at the cooling rate of 100 °C/min and kept for 20 min. For non-isothermal crystallization, the samples were heated to 100 °C with the heating rate of 10 °C/min and kept for 3 min to eliminate the heat history. Subsequently, the samples were cooled from 100 °C to 0 °C and then heated from 0 °C to 100 °C.

For polarizing microscope (POM) observation, a pellet of each sample were placed onto a glass slide and heated to 90 °C by a heating stage. Then, the heated sample was pressed by another glass slide into a thin sheet. The sample along with the heating stage were moved to POM. The initial temperature was set as 75 °C, and then cooled to 47 °C with the cooling rate of 2 °C/min. The crystallization process was observed by a 10 × objective lens.

Antibacterial test

For inhibition zone test, the samples with quaternary ammonium-73 and PHMB were cut into round slices with the diameter of 5 mm and sterilized by 75% ethanol. S. aureus was chosen as the representative pathogen. In a six-well culture plate, the solid lysogeny broth (LB) media were placed. In each well, 0.5 mL of S. aureus suspension with the bacterial density of 105 CFU/mL was inoculated and spread. Then the samples were placed into the center of each well, and the culture plates were incubated for 24 h at 37 °C. After incubation, the inhibition zones were observed.

Based on QB/T 2591-2003, E. coli and S. aureus were used for film sticking test. The samples were cut into sheets with the size of 20 mm × 20 mm and placed into culture dishes. On each sample, 32 μL of bacterial suspension with the density of 105 CFU/mL was inoculated, and then a glass slice with the same size was covered on the sample. The samples were incubated for 24 h at 37 °C. After incubation, the samples and the glass slice were thoroughly rinsed by 5 mL of normal saline. Then, 20 μL of the rinsed solution was moved onto solid LB culture media and cultured for 24 h at 37 °C. After incubation, the bacterial colonies on the culture media were observed and counted.

In vitro cytotoxicity test

A fibroblast cell line, L929, was taken for in vitro cytotoxicity test. In a 96-well culture plate, 5 × 103 cells were seeded into each well and incubated for 24 h at 37 °C under 5% of CO2. Following ISO 10993-2009, the sterilized samples with nHA were soaked in DMEM media supplemented with 10% fetal bovine serum, 100 U/mL of penicillin and 100 mg/mL of streptomycin for 24 h at 37 °C to prepare extracts. After 24 h incubation, the culture media were removed. The extracts were added into the wells and cultured for another 24 h. The fresh culture medium was taken as a control group. After culture, the cell viabilities were evaluated by CCK-8 method [28]. The OD values of CCK-8 were determined at the wavelength of 450 nm by a microplate reader (Bio-rad 680, USA). The relative cell viability of control group was defined as 100%.

Statistical analysis

At least three samples were tested in each time point. The results were reported as mean ± standard deviation (SD). When two groups were compared, the differences were assessed by the paired t-test. A significance level of p = 0.05 were performed using Origin 8.1 software.

Results and discussion

3D printing performances

To determine the temperature range of FDM process of PCL, cubic models were first printed by pristine PCL at 70 °C, 80 °C, 90 °C and 100 °C. As shown in Fig. 1, the integrity of the printed PCL models at 80 °C and 90 °C was higher than that at 70 °C and 100 °C. When the printing temperature was lower than 80 °C, the fluidity of melt PCL was low, which was difficult to pass through the nozzle. At the printing temperature higher than 90 °C, the melt strength solidification rate of PCL decreased, leading to collapse of the printed model. Therefore, the printing temperature was determined between 80 °C and 90 °C. However, the quality of 3D-printed pristine PCL models was still lower than that of ABS.

Fig. 1

General views of 3D-printed ABS and pristine PCL samples at different temperature (scale bar = 1 cm).

Due to the appropriate cost, particle diameter, thermostability and nucleation promoting property, starch was chosen as the additive. In order to improve the quality of 3D-printed PCL products, starch was used to prepare PCL/starch composite. To optimize the ratio of starch, different amounts of starch (3 phr, 5 phr, 7 phr, 9 phr and 11 phr) were added into PCL. For quantitative evaluation of the qualities of 3D-printed models, we introduced an index of completeness (C),where V* was the defected volume and V was the total volume of the preset model. After addition of starch, the qualities of 3D-printed models were improved (Fig. 2a). Furthermore, the quality of the printed products increased with from 3 phr to 9 phr (Table 3). The completeness of printed model reached 99 with the starch ratio of 9 phr. When 11 phr of starch was added, the viscosity of the melt composite was too high and blocked the nozzle. Therefore, 9 phr was the optimal ratio of starch for 3D printing of PCL. The composite with 9 phr of starch had good performance in FDM process, which could be precisely manufactured into complicated constructions (Fig. 2b).

Fig. 2

(a) Representative images of 3D-printed PCL/starch composites with various ratios of starch (scale bar = 1 cm); (b) FDM products prepared by PCL composite with 9 phr of starch (scale bar = 2 cm).

Table 3

Completeness of 3D-printed PCL/starch composites.

Starch ratio/phr	PCL	1	3	5	7	9
C	83.3	94.3	95.9	96.8	97.5	99.0

Mechanical property

When the filaments were applied in the FDM 3D printer, they were pulled by a tractor. Therefore, the tensile strength was essential for FDM materials. To evaluate the effects of starch on the mechanical property of PCL/starch composite, the tensile strength was studied. As shown in Fig. 3, the starch had no negative effects on the tensile strength of the composites, indicating the good compatibility between starch and PCL matrix. When the adding ratio was 7 and 9, the strength of PCL/starch composites showed slight increasing, which might due to the higher degree of crystallinity induced by starch [29]. The result of tensile strength demonstrated the good mechanical property of PCL/starch composites.

Fig. 3

Tensile strengths of PCL/starch composites at the additive ratios for 1–9 phr.

Rheological behaviors

The printable properties of the PCL/starch composites were evaluated by rheological behaviors. When the samples were printed by a FDM 3D printer, the printing speed (v) was 40 mm/s (Table 2). The radius (r) of the round sample for rheological test was 12.5 mm, therefore, the angular frequency (ω) was determined as 4.5 rad/s by the formula . Under this angular frequency, the printability was assessed by the storage modulus (G′) and loss modulus (G″). As shown in Fig. 4, the G″ of the samples was higher than G′, which indicated the strong interlayer adhesive force. While starch was added, both G′ and G″ increased with the amount of starch, which might due to that the melt strength of the composite was improved by starch [30]. Therefore, the printability of PCL/starch composites was improved due to the enhanced melt strengths. Based on the results of dynamic frequency scanning, the complex viscosities (η*) of the samples increased with the adding amounts of starch. When the adding amounts were less than 7 phr, the increasing levels of η* were relatively low, because the adding amounts were small and had little effects on η* of the composites. At the adding amounts of 7 phr and 9 phr, the effects of starch on η* of the composites were significant. While starch was added more than 9 phr, the increasing of η* slowed down. Moreover, the viscosity-temperature curves (Fig. 5) also demonstrated the viscosifying effects of starch on melt PCL composites. These rheological results indicated that the PCL/starch composites with adding ratios of 7 phr and 9 phr could be printed at 80–90 °C.

Fig. 4

Storage modulus, loss modulus and complex viscosity of PCL/starch composites at the additive ratios for 1–11 phr by frequency scanning.

Fig. 5

Viscosity-temperature curves of PCL/starch composites at the additive ratios for 1–11 phr.

Thermal and crystallization properties

The crystallization temperature, degree of crystallinity and crystallization rate were studied by DSC. The isothermal crystallization properties were shown in Fig. 6a. After the addition of starch, the crystallization rates increased, which leaded to complete crystallization in a short time. The results of non-isothermal crystallization (Fig. 6b and Table 4) indicated that starch could raise the crystallization temperature of PCL composites. The high crystallization temperature increased the solidification rate of melt PCL composites, which was beneficial for FDM process.

Fig. 6

Crystalline curves of PCL/starch composites at the additive ratios for 1–11 phr: (a) isothermal crystallization; (b) non-isothermal crystallization.

Table 4

DSC data of non-isothermal crystallization of PCL/starch composites.

Starch ratio (phr)	0	1	3	5	7	9	11
T0/°C	26.76	30.18	29.59	29.06	29.43	33.20	29.10
Tc/°C	32.17	35.74	33.51	33.34	33.20	33.06	33.20
θ/%	100.0	150.2	147.7	133.2	142.0	137.2	139.6

The crystallization process was observed by POM (Fig. 7). The POM images illustrated that starch could increase the crystallization rate of PCL composites. Along with the increasing of starch ratios, the crystallization rate also increased, which was consistent with the DSC results. In addition, the POM images also showed that the crystal cores appeared around the starch particles, which demonstrated that starch played the role of nucleation promoter during the crystallization process of PCL/starch composites.

Fig. 7

Crystallization process of PCL/starch composites at the additive ratios for 1–11 phr by representative POM images.

Antibacterial performances and biocompatibility

Compared with the traditional FDM materials, such as PLA and ABS, PCL/starch composites could be printed under relative low temperature (80–90 °C). Therefore, various kinds of organic components, which would decompose under high temperature, could be added into the composites for functionalization. For antibacterial functionalization, two kinds of organic antibacterial agents, quaternary ammonium-73 and PHMB, were added (Fig. 8a). When the ratios of quaternary ammonium-73 and PHMB were 0.1 phr and 1 phr, respectively, the composites possessed high antibacterial efficiency (Fig. 8b). The diameters of the inhibition zones increased with the adding ratio of the antibacterial agents. Furthermore, the antibacterial performances were also evaluated by film sticking test to simulate direct contacting (Fig. 9). For S. aureus and E. coli, the samples showed good antibacterial efficiencies, which was coincident with the results of inhibition zones. When the additive ratio of quaternary ammonium-73 was 0.05 phr, the antibacterial ratios were higher than 99.99% against S. aureus and 90% against E. coli, respectively. For PHMB, the antibacterial ratios were higher than 99.99% against both S. aureus and E. coli when the additive ratio was higher than 1 phr. These results indicated that the organic antibacterial agents were not affected by the low temperature in FDM process of PCL/starch composites. Therefore, antibacterial FDM materials could be prepared by direct addition of organic antibacterial agents into the PCL/starch composites.

Fig. 8

(a) Images of antibacterial PCL/starch composite samples manufactured by FDM; (b) inhibition zones of antibacterial PCL/starch composites with different contents of quaternary ammoniym-73 and PHMB.

Fig. 9

Bacterial colonies of antibacterial PCL/starch composites with different contents of quaternary ammoniym-73 and PHMB by film sticking test.

Moreover, PCL is a biodegradable polyester, which could be used in bone tissue engineering [31]. The nHA was also added to improve the osteogenic activity of the composites [32]. Because the additive ratio of nHA was low, it did not affect the processability. Biocompatibility is an essential property for PCL/starch composites as tissue engineering materials. As a preliminary study, the cytotoxicity of PCL/starch composites was evaluated. As shown in Fig. 10, the PCL/starch (PCL/ST) composite and PCL/starch with nHA (PCL/ST/HA) had no toxicity to cells. Therefore, the low-cytotoxic PCL/ST/HA composites might be used as FDM materials for the personalized therapy of bone defects.

Fig. 10

Cytotoxicity of PCL, PCL/ST and PCL/ST/HA composites represented by relative cell viability. Blank groups was defined as 100%.

Conclusion

In summary, a series of PCL starch composites for low-temperature FDM 3D printing were developed and further functionalized. By blending modification, the qualities of 3D-printed products were improved. When the ratio of starch was 9 phr, the PCL/starch composite had the best performance in FDM process. The addition of starch enhanced the melting strength and solidification rate of PCL/starch composites. The starch increased the crystallization temperature, degree of crystallinity and crystallization rate of PCL/starch composites, which was beneficial for FDM process. The starch components also had no negative effects on the mechanical properties of PCL. Based on the low printing temperature, various kinds of bioactive components were added into PCL/starch composites for preparation of antibacterial and biocompatible materials for FDM, providing additional functions for FDM materials.

CRediT authorship contribution statement

Yu-Qing Zhao: Data curation, Formal analysis, Writing - original draft. Ji-Hao Yang: Methodology. Xiaokang Ding: Validation. Xuejia Ding: Validation. Shun Duan: Conceptualization, Writing - review & editing. Fu-Jian Xu: Project administration.

Declaration of competing interest

The authors declare no conflict of interest.