3D-printed NIR-responsive shape memory polyurethane/magnesium scaffolds with tight-contact for robust bone regeneration

Abstract

Patients with bone defects suffer from a high rate of disability and deformity. Poor contact of grafts with defective bones and insufficient osteogenic activities lead to increased loose risks and unsatisfied repair efficacy. Although self-expanding scaffolds were developed to enhance bone integration, the limitations on the high transition temperature and the unsatisfied bioactivity hindered greatly their clinical application. Herein, we report a near-infrared-responsive and tight-contacting scaffold that comprises of shape memory polyurethane (SMPU) as the thermal-responsive matrix and magnesium (Mg) as the photothermal and bioactive component, which fabricated by the low temperature rapid prototyping (LT-RP) 3D printing technology. As designed, due to synergistic effects of the components and the fabrication approach, the composite scaffold possesses a homogeneously porous structure, significantly improved mechanical properties and stable photothermal effects. The programmed scaffold can be heated to recover under near infrared irradiation in 60s. With 4 wt% Mg, the scaffold has the balanced shape fixity ratio of 93.6% and shape recovery ratio of 95.4%. The compressed composite scaffold could lift a 100 g weight under NIR light, which was more than 1700 times of its own weight. The results of the push-out tests and the finite element analysis (FEA) confirmed the tight-contacting ability of the SMPU/4 wt%Mg scaffold, which had a signficant enhancement compared to the scaffold without shape memory effects. Furthermore, The osteopromotive function of the scaffold has been demonstrated through a series of in vitro and in vivo studies. We envision this scaffold can be a clinically effective strategy for robust bone regeneration.

Keywords: 3D printing; Magnesium; Robust bone regeneration; Shape memory polyurethane; Tight-contact.

Graphical abstract

Fig. 1

Schematic illustration of: (a) 3D printed SMPU/Mg scaffold and its Mg²⁺ releasing: the scaffold could be compressed at T > T_trans and then fixed at T < T_trans; upon T > T_trans, the scaffold could be recovered to its original shape and then Mg²⁺ could be released with degradation of the scaffold; (b) “3R” process: after being implanted, the compressed scaffold will be recovered in NIR light, resulting in the supporting stimulus at interface between the scaffold and tissue; then the Mg²⁺ will be released slowly to improve the regeneration of defective site; at last, the defective bone will be repaired; red arrow: supporting stimulus from the shape recovery process. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2

Characterizations on 3D printed bone scaffolds: (a) Structural observation of SMPU/4 wt%Mg scaffold by micro-CT. (b1) Macro image and micro morphology observation with different magnification of SMPU/4 wt%Mg scaffold by SEM. From left to right at magnification of: 40x, 500x, 800x. (b2) The element composition distributed in the scaffold by SEM-EDS: Element carbon (C, green); Element Oxygen (O, blue); Element Magnesium (Mg, red). (c) XRD patterns.; （d) DSC results. （e-f) Mechanical properties: compress strength (e) and compress modulus (f) derived from the stress-stain curve. (g) Accumulated concentration of released Mg²⁺. n = 3. *, significant difference compared to the SMPU group, p < 0.05. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

Photothermal performances and shape memory effect of 3D printed scaffolds under NIR light. (a1-a2) Photothermal heating curves of the SMPU/Mg scaffolds with different Mg concentrations at dry state (a1) and wet state (a2) irradiated by the NIR laser. (a3) Temperature elevation of the SMPU/4 wt%Mg scaffold for six laser on/off cycles. (b–c) Photographs of shape recovery process in air of the regular (b) and irregular (c) SMPU/4 wt% Mg scaffold irradiated by the 808 nm laser (1 W cm⁻²). OS: original shape; TS-1: Temporary shape-1; TS-2: Temporary shape-2; RS: recovered shape. (d) hotographs of shape recovery process in water of the SMPU/4 wt%Mg irradiated by the 808 nm laser (2 W cm⁻²). (e) Infrared thermal images of the recovery process in water. (f) R_f and R_r of the SMPU/Mg scaffolds with different Mg concentrations. (g) Recovery stress of SMPU and SMPU/4 wt% Mg scaffold irradiated by the 808 nm laser (1 W cm⁻²). (h) Programmed SMPU/4 wt% Mg scaffold compressed with a 100 g weight before and after being irradiated by the 808 nm laser (1 W cm⁻²). n = 3.

Fig. 4

In vitro cell studies of the 3D printed scaffolds. (a1) Fluorescence images of MC3T3-E1 cells after 3 days incubation, where the live cells are in green and dead cells are in red. (a2) Cell proliferation of MC3T3-E1 cells. Osteogenic differentiation: (b1) ALP staining of BMSCs at 3, 7 and 14 days. (b2) ARS staining of BMSCs at 18 days. (b3) ALP activity. (b4) Quantitative analysis of calcium nodules. (c1-c2) Expression levels of Runx2 (c1) and OST (c2) of rat BMSCs at 3, 7 and 14 days. n = 3. *, significant difference compared to the normal, control and SMPU groups, p < 0.05. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5

Implantation of the 3D printed scaffolds at the defective sites. (a) The shape recovery process of the scaffold during implanting process. (b1) Schematic illustration of a push-out jig. (b2) Pull-out test results after implantation immediately. (b3) Pull-out test results after implantation for 12 weeks. (c1-c2) Equivalent stress distribution by FEA during the recovery process of the pristine SMPU (c1) and SMPU/4 wt% Mg scaffolds (c2) in the defect site. Red arrow: the highest von Mises stresses. n = 3. *, compared to the blank and SMPU scaffold groups, significant difference, p < 0.05. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 6

In vivo animal studies. (a1) Micro-CT 3D reconstruction of defective bones and their sagittal images at 4, 8, 12 weeks. Red circle: defective area. Yellow frame: new bone tissues. The scale bars are 2 mm. (a2) BMD and (a3) BV/TV varied in each group at 4, 8, 12 weeks. (b1-b3) Quantitative analysis of the trabecular number (b1), trabecular separation (b2), and trabecular thickness (b3). (c1) Histological sections and magnified views of the defective site with or without the scaffolds after implantation for 4, 8 and 12 weeks stained with HE; OB: old bone tissues, NB: new bone tissues; S: scaffold samples. The scale bar = 500 μm. (c2) Area percent of the new bone in defective sites. n = 3. *, compared to the blank and SMPU scaffold groups, significant difference, p < 0.05. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)