Four-Dimensional Printing Hierarchy Scaffolds with Highly Biocompatible Smart Polymers for Tissue Engineering Applications

Abstract

The objective of this study was to four-dimensional (4D) print novel biomimetic gradient tissue scaffolds with highly biocompatible naturally derived smart polymers. The term "4D printing" refers to the inherent smart shape transformation of fabricated constructs when implanted minimally invasively for seamless and dynamic integration. For this purpose, a series of novel shape memory polymers with excellent biocompatibility and tunable shape changing effects were synthesized and cured in the presence of three-dimensional printed sacrificial molds, which were subsequently dissolved to create controllable and graded porosity within the scaffold. Surface morphology, thermal, mechanical, and biocompatible properties as well as shape memory effects of the synthesized smart polymers and resultant porous scaffolds were characterized. Fourier transform infrared spectroscopy and gel content analysis confirmed the formation of chemical crosslinking by reacting polycaprolactone triol and castor oil with multi-isocyanate groups. Differential scanning calorimetry revealed an adjustable glass transition temperature in a range from -8°C to 35°C. Uniaxial compression testing indicated that the obtained polymers, possessing a highly crosslinked interpenetrating polymeric networks, have similar compressive modulus to polycaprolactone. Shape memory tests revealed that the smart polymers display finely tunable recovery speed and exhibit greater than 92% shape fixing at -18°C or 0°C and full shape recovery at physiological temperature. Scanning electron microscopy analysis of fabricated scaffolds revealed a graded microporous structure, which mimics the nonuniform distribution of porosity found within natural tissues. With polycaprolactone serving as a control, human bone marrow-derived mesenchymal stem cell adhesion, proliferation, and differentiation greatly increased on our novel smart polymers. The current work will significantly advance the future design and development of novel and functional biomedical scaffolds with advanced 4D printing technology and highly biocompatible smart biomaterials.

Keywords: 4D printing; mesenchymal stem cell; polycaprolactone; shape memory; smart polymer.

FIG. 1.

The illustration of the process for preparing biomedical scaffold according to a specific traumatic defect. Color images available online at www.liebertpub.com/tec

FIG. 2.

The reaction mechanism for synthesis of smart polymers. Color images available online at www.liebertpub.com/tec

FIG. 3.

FTIR spectra of five smart polymers when compared to PH, castor oil, and Ptriol300. (A) The FTIR spectra from 4000 to 600 cm⁻¹; (B) The range from 3800 to 3000 cm⁻¹; (C) The range from 1350 to 1150 cm⁻¹. Color images available online at www.liebertpub.com/tec

FIG. 4.

Gel content (A), water contact angle (B), SEM (C), and photo images (D) the synthesized smart polymers. SEM, scanning electron microscopy. Color images available online at www.liebertpub.com/tec

FIG. 5.

(A) DSC curves of the synthesized smart polymers; DSC, differential scanning calorimeter. (B) The effect of castor oil content on the Tgc. (C) The effect of castor oil content on the Tgc breadth. Color images available online at www.liebertpub.com/tec

FIG. 6.

Compression modulus of the synthesized smart polymers. Data are mean ± standard deviation, n = 5. *p < 0.05, **p < 0.01, and ***p < 0.001. Color images available online at www.liebertpub.com/tec

FIG. 7.

The demonstration of the shape memory effects of the synthesized smart polymers: (A) sample C40P300PH was fixed at 0°C and recovered at 37°C; (B) samples C80P300PH, C40P300HD, and C40P300PH were fixed as “GWU” at −18°C and recovered at 37°C with different recovery speed. Color images available online at www.liebertpub.com/tec

FIG. 8.

Recovery curves of the synthesized smart polymers that were fixed at −18°C for a temporary shape and recovered at 37°C to their permanent shape. Color images available online at www.liebertpub.com/tec

FIG. 9.

The fabricated scaffold. (A) A 5-mm-diameter and 3-mm-thickness scaffold compared to a cent. (B) The SEM image of the pore distribution in the scaffold. (C) Varied pore diameters in different directions. (D) The potential for minimally invasive application; a, sample original shape; b, temporary shape at −18°C; c, 0 s at 37°C; d, 10 s at 37°C; e, 3 min at 37°C; from left to right, the samples are C80P300PH, C60P300PH, C40P300PH, C20P300PH, and C40P300HD, respectively. Color images available online at www.liebertpub.com/tec

FIG. 10.

Four-hour adhesion of MSCs on the synthesized smart polymers. Data are mean ± standard deviation, n = 6. *p < 0.05, **p < 0.01, and ***p < 0.001. MSCs, mesenchymal stem cells. Color images available online at www.liebertpub.com/tec

FIG. 11.

One-, 3-, and 5-day proliferation of MSCs on the synthesized smart polymers. Data are mean ± standard deviation, n = 6. *p < 0.05, **p < 0.01, and ***p < 0.001. Color images available online at www.liebertpub.com/tec

FIG. 12.

Confocal microscopy images of MSC growth and spreading morphology on C40P300PH and C20P300PH when compared with PCL control after 1-, 3-, and 5-day culture. The color red represents cell cytoskeleton and the color blue represents cell nuclei. Color images available online at www.liebertpub.com/tec

FIG. 13.

ALP activity on different synthesized smart polymers compared to PCL control. Data are mean ± standard deviation, n = 6. *p < 0.05, **p < 0.01, and ***p < 0.001. ALP, alkaline phosphatase. Color images available online at www.liebertpub.com/tec

FIG. 14.

Enhanced total calcium deposition on smart polymers compared to PCL control. Data are mean ± standard deviation, n = 6. *p < 0.05. Color images available online at www.liebertpub.com/tec