br Photothermal performance and imaging of scaffold in vivo
3.3. Photothermal performance and imaging of scaffold in vivo
The photothermal effects of the fabricated scaffolds in vivo were evaluated by implanting them in tumor tissues of mice, followed
by irradiation with an 808 nm laser (0.5 W cm 2). The data showed that the temperature of the Alg-PDA scaffold was increased to 58.3 and 67.7 LC after NIR laser irradiation for 1 and 5 min, respectively. However, the temperature increased only to 42.2 and 43.5 LC after irradiation for 5 min in laser only and alginate scaffold groups, respectively (Fig. S3). The prepared Alg-PDA scaffold presented excellent photothermal effect in vivo, which was also confirmed by the NIR thermal images that showed a white color area in the Alg-PDA scaffold (Fig. 4a).
In addition, it is important to monitor the performance of the scaffolds after implantation in vivo . MRI with excellent tempo-ral and spatial resolutions has been broadly applied in clinical practice. Mn cations with high-spin quantum numbers and long longitudinal electronic relaxation times have been studied as MRI contrast agents [36,46]. In this study, Mn2+ ions were used for crosslinking of Alg-DA hydrogels, as well as coordinate with PDA moieties. Moreover, Mn2+ ions also worked as contrast agents for MRI of the scaffold. From Fig. 4b, T1 images showed that the Alg-PDA scaffold highly contrasted (the bright areas) in the tumor area after implantation in mice for 1 and 14 days.
PAI as a newly imaging technique has attracted tremendous interest in the field of biomedical engineering [47,48]. Compared to other optical imaging methods (such as fluorescence), PAI has high spatial resolution and deep imaging depth [49,50]. PDA nanoparticles with strong NIR absorbance have been used as a
Fig. 3. Heating curves of 3D-printed Alg, Alg-DA, and Alg-PDA scaffolds with different concentrations of DA in PBS with 0.5 W cm 2 808-nm laser irradiation (a). Photothermal conversion cycling test of Alg-PDA scaffold (5 mg/mL DA) (b). Live/dead staining of tumor BAY-598 (4T1 cells) (d) and cell viability (c) without treatment (control), with 0.5 W cm 2 808 nm laser irradiation (laser only) for 5 min, with laser through the alginate scaffold, with and without 0.5 W cm 2 808 nm laser through Alg-PDA scaffolds for 5 min.
PAI contrast agent [27,33]. As expected, strong PA signals were observed in the Alg-PDA scaffold before (in vitro) and after implan-tation in vivo. Moreover, the PA signals decreased with time (Fig. 4c, d), which indicates that PAI could monitor the in vivo performance of the Alg-PDA scaffold.
Because of the flexible mechanical behavior and excellent pho-tothermal effect, the printed Alg-PDA scaffold could kill the local recurrent cancer cells, as well as support new tissue growth after
Fig. 4. IR thermal images of 4T1 tumor-bearing mice implanted with Alg and Alg-PDA scaffolds exposed to 0.5 W cm 2 808 nm laser for different time points, respectively (a). MRI images of the breast cancer region implanted with Alg-PDA scaffold for 1 and 14 days. Yellow circles indicate the location of the scaffold. Photoacoustic imaging (c) and photoacoustic intensity (d) of Alg scaffolds and Alg-PDA scaffolds before (in vitro) and after implantation at tumor sites of mice for 2 and 9 days. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
primary tumor resection. The residual or recurrent cancer cells were able to be selectively killed by local PTT. As shown in Fig. 4a, the Alg-PDA scaffold exhibited excellent photothermal effect in vivo. Upon NIR laser irradiation (0.5 W cm 2), the volume of tumors in mice implanted with the Alg-PDA scaffold was signif-icantly reduced, while tumor volume in laser only, alginate with NIR, and Alg-PDA without NIR treatment groups increased rapidly (Fig. 5a, d). Body weights of all mice were maintained in the nor-mal level (Fig. 5b). The H&E-stained images revealed that the organs heart, liver, spleen, lung, and kidney harvested from healthy mice and mice treated with the Alg-PDA scaffold plus laser showed no significant difference (Fig. 5e). These results indicated that the PTT of the printed hydrogel scaffolds showed no significant dam-age to the major organs of mice because the heat was generated only in the local area. Additionally, the negative effect of PTT on normal cells on scaffolds and tissues could be minimized by adjust-ing the laser power and the irradiation time, as cancer cells were generally more sensitive to hyperthermia than normal cells .