3.1 Fabrication, structure and properties of PTMC/TPU scaffolds
The high stretchability of human uterus is mainly due to the intrinsic
elasticity of myometrium. Elastomeric TPU has been extensively used to
restore damaged vascular and skeletal muscles[22]. Moreover, TPU elastomers possess
controllable mechanical properties that can be used to match the
targeted body tissues [23]. In the current study,
to mimic the functions and properties of myometrium, TPU, as a highly
elastic material, is a good biomaterial for scaffold fabrication. On the
other hand, given the curved shape of the uterus, it is important to
fabricate scaffolds that have the shape morphing ability to form curved
or tubular shape to match the curvature of the repair site in uterus.
Previous studies indicated that biodegradable PTMC polymer exhibited
programmed shape morphing from the planar shape to tubular structure
after being incubated at the human body temperature (37 ℃)[24]. Therefore, in the current study, PTMC was
chosen to mix homogeneously with TPU for fabricating PTMC/TPU scaffolds
via FDM. To determine the optimal properties of PTMC/TPU scaffolds
(i.e., comparable mechanical strength with the uterus and good
shape-morphing ability), mixtures of different PTMC:TPU ratios were made
under mechanical stirring at 170 ℃.
Subsequently, PTMC/TPU mixtures were 3D printed to form polymer blend
scaffolds. As shown in Fig.2(A), when the PTMC:TPU ratio was below
0.25:1, PTMC/TPU scaffolds were unable to transform from planar shape to
tubular structures after immersion in water at 37 ℃. On the contrary, at
higher PTMC:TPU ratios, PTMC/TPU scaffolds exhibited shape morphing
ability [Fig.2(B), Video S1]. The shape memory effect of PTMC/TPU
scaffolds could be attributed to the suitable glass transition (Tg)
temperature of PTMC. DSC results shown in Fig.2(C) indicated that Tg
temperatures of PTMC and PTMC/TPU scaffolds were at about 36 ℃. The
shape morphing behavior of these scaffolds could be attributed to the
amorphous nature of the PTMC polymer. After 3D printing, scaffolds were
shaped into tubular structures using a stainless-steel rod in an oven at
80℃ for 30min. The gradient increase of temperature from the surface to
interior during the heating process affected the degree of molecular
orientation, thereby resulting in anisotropic birefringence and
inhomogeneous transparency. Therefore, PTMC/TPU polymer blend scaffolds
could retain their temporary planar shape at room temperature and could
completely recover their permanent tubular structure at 37 ℃ when the
glass transition of PTMC/TPU scaffolds started to take place.
Furthermore, TGA results [Fig.2(D) and Table S1] showed that the
real PTMC percentage in PTMC/TPU scaffolds was almost the same as the
nominal percentage, suggesting that PTMC and TPU were homogenously mixed
under mechanical stirring at 170 ℃.
Since the PTMC/TPU scaffolds were made to mimic the functions of
myometrium, suitable mechanical properties from these scaffolds are
essential. Previous ex vivo studies indicated that the ultimate
tensile strength of porcine uterine tissues was 320 ± 176 kPa with a
corresponding strain of 30 ± 9.0% and that the human uterus exhibited a
better mechanical performance with an average ultimate strength of
656.3 ± 483.9 kPa at a strain of 32 ± 11.2% [25].
Another study also claimed that the mechanical strength of native
uterine tissues was 0.258 ± 0.071 MPa [26].
Moreover, in vivo tensile tests suggested that the strain of
human uterus could be up to 110 - 130 % [27]. In
this context, for adequately biomimicking the mechanical performance of
uterus, PTMC/TPU scaffolds should have an ultimate strength over 200 KPa
and a strain over 100 % at the human body temperature. In the current
study, the mechanical strength of PTMC/TPU scaffolds was measured via
tensile tests at room temperature and at human body temperature,
respectively. Fig.3(B-G) shows the mechanical behavior and properties of
PTMC/TPU scaffolds at room temperature. When the PTMC:TPU ratio was 2:1,
PTMC/TPU scaffolds had a high ultimate strength of 1.16 ± 0.31 MPa but a
very low strain of 11.52 ± 2.27 %, which was not suitable for mimicking
the functions of human uterus. When the PTMC:TPU ratio was at 1:1, 0.5:1
and 0.25:1, PTMC/TPU scaffolds possessed the desirable mechanical
strength of ~0.6 MPa and an appropriate strain of over
100 %. Particularly, for scaffolds at the PTMC:TPU ratio of 0.25:1, the
strain was about 400 %. Because the glass transition temperatures of
PTMC/TPU scaffolds were below 37 ℃, these scaffolds would become more
elastic at the human body temperature. As shown in Fig.S1 for tests
conducted at 37 ℃, the strain of PTMC/TPU scaffolds dramatically
increased, while the mechanical strength of PTMC/TPU scaffolds
decreased. The highest mechanical strength of PTMC/TPU scaffolds (at the
PTMC:TPU ratio of 0.25:1) was 0.25 ± 0.02 MPa at 37 ℃. Consequently,
owing to the comparable mechanical strength to native uterine tissues,
PTMC/TPU scaffolds at the PTMC:TPU ratio of 0.25:1 were used in
subsequent experiments.
The surface morphology of 3D printed PTMC/TPU scaffolds are displayed in
Fig.3(A), showing that scaffolds at different PTMC:TPU ratios all had
relatively smooth surfaces, especially for scaffolds at the PTMC:TPU
ratio of 0.25:1. The smooth surface makes scaffolds less hydrophilic,
which is unfavorable for cells, including their adhesion, proliferation
and differentiation [28]. Additionally, due to the
high content of TPU, PTMC/TPU (0.25:1) scaffolds had a hydrophobic
surface and the water contact angle was 112.3 ± 6.3° [Fig.S2(A)].
The BSA adsorption experiment also indicated that PTMC/TPU (0.25:1)
scaffolds had the lowest BSA adsorption amount [Fig.S2(B)],
suggesting poor surface properties of PTMC/TPU (0.25:1) scaffolds.