Polycaprolactone (PCL) is drawing increasing attention in the field of medical 3D printing and tissue engineering because of its biodegradability. This study developed polycaprolactone prepolymers that can be cured using visible light. Three PCL acrylates were synthesized: polycaprolactone-530 diacrylate (PCL530DA), glycerol-3 caprolactone triacrylate (Glycerol-3CL-TA), and glycerol-6 caprolactone triacrylate (Glycerol-6CL-TA). PCL530DA has two acrylates, whereas Glycerol-3CL-TA and Glycerol-6CL-TA have three acrylates. The Fourier transform infrared and nuclear magnetic resonance spectra suggested successful synthesis of all PCL acrylates. All are liquid at room temperature and can be photopolymerized into a transparent solid after exposure to 470 nm blue LED light using 1% camphorquinone as photoinitiator and 2% dimethylaminoethyl methacrylate as coinitiator. The degree of conversion for all PCL acrylates can reach more than 80% after 1 min of curing. The compressive modulus of PCL530DA, Glycerol-3CL-TA, and Glycerol-6CL-TA is
Biodegradable materials are drawing increasing attention in the field of 3D printing because they can be used to print body implants, tissue engineering scaffolds, and even drug-releasing capsule [
The biodegradability of common polymers, such as polyglycolide (PGA), PLA, and polycaprolactone (PCL) polyesters, depends on their structures [
PCL is a popular biodegradable material for resin additives, small scale modeling, and bone tissue engineering [
The properties of PCL depend on its molecular weight [
In this study, three PCL-based acrylates were synthesized. The first prepolymer is a linear molecule with two acrylates at each end, named polycaprolactone-530 diacrylate (PCL530DA). In addition, two branched molecules, glycerol-3 caprolactone triacrylate (Glycerol-3CL-TA) and glycerol-6 caprolactone triacrylate (Glycerol-6CL-TA), were also prepared through ringopening polymerization of
For the synthesis of PCL530 diol, diethylene glycol was used as the starting material. Diethylene glycol was dried at 130°C for 1 hour; then, an appropriate amount of
PCL530DA, Glycerol-3CL-TA, and Glycerol-6CL-TA were prepared by the reaction of acryloyl chloride with PCL530-diol, Glycerol-3 caprolactone triol, and Glycerol-6 caprolactone triol, respectively (Figures
Synthesis procedure for PCL530 diol (a), glycerol-3 caprolactone triol, and glycerol-6 caprolactone triol (b).
Synthesis procedure for PCL530DA, Glycerol-3-CL-TA, and Glycerol-6-CL-TA.
The infrared (IR) spectra were used to determine whether the materials were well synthesized. The chemical bonds of the specimens tested could be found and are shown as peaks on the diagram. All specimens were evaluated using a Nicolet™ iS5TM Fourier transform infrared spectrometer (ThermoFisher, MA, USA) equipped with an attenuated total reflectance (ATR) device comprising a horizontal ZnSe crystal. The samples prepared were large enough to cover the whole surface of the ATR crystal and were scanned at least 16 times. Collected spectra were acquired in the range of 800–4000 cm−1, with a resolution of 4 cm−1.
Chemical bonds could be investigated through the IR spectra, but the peaks may be overlaid on the diagram to affect the interpretation of the results. The quantity of each element in a specimen could be determined through 1H-nuclear magnetic resonance (NMR) spectroscopy. For the NMR measurement, 10 mg of samples was dissolved in 1 ml of deuterated chloroform with tetramethylsilane as an internal standard. 1H-NMR spectra were acquired at a temperature of 300 K using a Bruker Ascend 400-MHz spectrometer. Each specimen was scanned 32 times to obtain an average spectrum. Chemical shifts were referenced relatively to chloroform at 7.26 ppm in the 1H-NMR spectra. Peaks on the spectra were delimited and integrated on Topspin (version 3.0).
Viscosity is a critical property of 3D printing materials. Therefore, the viscosity of PCL530DA, Glycerol-3CL-TA, and Glycerol-6CL-TA was measured with a cone-plate viscometer (Brookfield DV3TRV Rheometer, Wells-Brookfield Cone/Plate; Brookfield Engineering Laboratories Inc., Middleboro, MA, USA) at room temperature. To determine the best compromise between viscosity and mechanical properties of the materials after photopolymerization, the viscosity of the materials mixtures was further investigated. A constant volume (1 cm3) of each resin was dispensed between the cone and plate, and the viscosity was collected at 10 rpm with a shear rate of 38.4 s−1 for 2 min. The data was collected every second, and the readings between 10 and 95% torque were recorded and expressed in centipoise (cps).
To estimate of the degree of conversion (DC) of the PCL-based acrylates, five light-cured resin solutions were exposed to 470 nm LED blue lights (6 mW cm−2) for 10, 20, 30, and 60 s. The IR spectra of the light-cured resins were obtained using the same Fourier transform IR (FTIR) spectrometer. Before calculating of the DC of the samples, all sample spectra were normalized to the C=O peak at 1720 cm−1 because the number of the carbonyl bonds did not change after the light induced polymerization. The DC for each sample was then determined by comparing the intensity of the aliphatic C=C stretching vibration at 1640 cm−1 of the polymerized resin and the negative control, the unpolymerized resin. DC (%) was calculated by subtracting the percentage of remaining aliphatic C=C from 100%.
All three PCL-based acrylates containing 1.2% CQ and 2.4% DMAEMA were prepared and light-cured under 6 mW cm−2 for 1 min. A universal testing machine (Shimadzu AGS-500G) equipped with a 100 kg or 500 kg load cell in compression mode was used to measure the compressive strength and modulus of the samples at room temperature. Light-cured samples with a diameter of 14 mm and a thickness of 8 mm were used. All tests were conducted until the samples were crushed. The crosshead speed was 2 mm min−1. To obtain statistically reliable results, all measurements were repeated at least three times.
Experimental data was processed using on GraphPad Prism (version 6). Quantitative data were analyzed using one-way analysis of variance. The data are presented as mean ± standard deviation. A
Figure
(a) FTIR spectra of Glycerol-3CL-TA at different steps of the synthesis. Top: glycerol; middle: Glycerol-3CL triol; bottom: Glycerol-3CL-TA. (b) NMR spectrum of Glycerol-3CL-TA. Three groups of peaks within 5.8–6.5 ppm correspond to vinyl protons in the acrylate groups.
FTIR spectrum of the three PCL acrylates, including PCL530DA (top), Glycerol-3CL-TA (middle), and Glycerol-6CL-TA (bottom).
At room temperature, PCL530DA is a transparent liquid, with a viscosity of 176.8 cps (Figure
Viscosity of PCL530DA, Glycerol-3CL-TA, Glycerol-6CL-TA, and their mixtures at 3 : 2, 1 : 1, and 2 : 3 ratios.
After light exposure, all PCL acrylates became transparent solids (Figure
The arrows point to the yellowish materials identified as photopolymerized Glycerol-6CL-TA, Glycerol-3CL-TA, and PCL530DA after curing (left to right).
DC of PCL530DA, Glycerol-3CL-TA, and their mixtures at 3 : 2, 1 : 1, and 2 : 3 ratios under blue light exposure.
DC of PCL530DA, Glycerol-6CL-TA, and their mixtures at 3 : 2, 1 : 1, and 2 : 3 ratios under blue light exposure.
After curing under blue light, the compression tests showed that PCL530DA and Glycerol-6CL-TA had a much lower compressive modulus and compressive strength than Glycerol-3CL-TA for all curing times tested (Figures
Compressive modulus of PCL530DA, Glycerol-3CL-TA, and Glycerol-6CL-TA for different curing times.
Compressive stress of PCL530DA, Glycerol-3CL-TA, and Glycerol-6CL-TA for different curing times.
In this study, three PCL prepolymers, namely, PCL530DA, Glycerol-3CL-TA, and Glycerol-6CL-TA, were prepared. All these prepolymers have a common characteristic: their viscosity is smaller than 800 cps—important for 3D printing materials. A smaller viscosity enables the molecules to flow and separate efficiently in the resin tank, which makes the prepolymers suitable for most commercial DLP and SLA 3D printers. This low viscosity was similar to that of commercially available 3D printing resins from Formlabs, the 3D printing company with the largest market share.
PCL530DA has two acrylates in its structure, whereas Glycerol-3CL-TA and Glycerol-6CL-TA have three. During photopolymerization, prepolymers that have at least two acrylates can form a cross-linked network. In other words, all three prepolymers studied can form a cross-linked network after exposure to light. Nevertheless, the DC of 100% PCL530DA is lower at all curing times compared with (1) Glycerol-3CL-TA, (2) Glycerol-6CL-TA, and (3) the mixture of PCL530DA with Glycerol-3CL-TA or Glycerol-6CL-TA. This can be caused by the lower density of acrylate in the linear PCL530DA. The DC of Glycerol-3CL-TA increased rapidly during 10–20 s light exposure. Then, it increased in a flat slope until 60 s curing time. By contrast, Glycerol-6CL-TA retained a high DC from 10 to 60 s. The rate of the DC is determined by the frequency of an acrylate to meet another acrylate [
DC also plays an important role for the mechanical properties of materials [
In this study, three PCL-based photopolymerizable materials were prepared. Because these materials can be light-cured to form a solid and have low viscosity, they have great potential for soft tissue engineering and medical 3D printing.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This study was supported by the Ministry of Science and Technology, Taiwan, Grant MOST106-2221-E-010-001. This manuscript was edited by Wallace Academic Editing.