Metal Nanoparticles in Laser Bioprinting

Abstract

Laser bioprinting is a promising method for applications in biotechnology, tissue engineering, and regenerative medicine. It is based on a microdroplet transfer from a donor slide induced by laser pulse heating of a thin metal absorption film covered with a layer of hydrogel containing living cells (bioink). Due to the presence of the metal absorption layer, some debris in the form of metal nanoparticles is printed together with bioink microdroplets. In this article, experimental investigations of the amount of metal nanoparticles formed during the laser bioprinting process and transported in bioink microdroplets are performed. As metal absorption layers, Ti films with the thickness in the range of 25-400 nm, produced by magnetron spattering, were applied. Dependences of the volume of bioink microdroplets and the amount of Ti nanoparticles within them on the laser pulse fluence were obtained. It has been experimentally found that practically all nanoparticles remain in the hydrogel layer on the donor slide during bioprinting, with only a small fraction of them transferred within the microdroplet (0.5% to 2.5%). These results are very important for applications of laser bioprinting since the transferred metal nanoparticles can potentially affect living systems. The good news is that the amount of such nanoparticles is very low to produce any negative effect on the printed cells.

Keywords: Ti nanoparticles; gel microdroplets; laser bioprinting.

Figure 1

Schematic diagram of bioprinting setup. A donor slide is shown with a titanium layer on which a relatively thick layer of hydrogel with living microorganisms is deposited. Under the action of a short laser pulse, a gel microdroplet is transferred to the acceptor slide with a nutrient. This microdroplet contains both microorganisms and nanoparticles of the destroyed metal film.

Figure 2

Optical micrographs of dried gel microdroplets on the acceptor slide produced with the same laser irradiation parameters (τ = 8 ns, E = 20 μJ, and F = 2.8 J/cm²) and different thicknesses of the Ti film on the donor slide. The numbers show the thickness of the Ti film in nanometers. In the last two images, Ti particles both inside and outside microdroplets can be seen.

Figure 3

Images of the Ti film with a thickness of 50 nm after laser exposure. (A) SEM image. The dotted line shows a circle with a diameter of 30 μm, E = 7 μJ (F = 1 J/cm²). (B) AFM images of the ablated areas, E = 24 μJ (F = 3.4 J/cm²).

Figure 4

Dependences of the square of the hole diameter in Ti film (A), the volume of the transferred gel microdroplets (B), and the percentage of TiNP in microdroplets on the laser fluence (C). Figure 4A shows values for the Ti film with and without a gel layer (empty red circles). Examples of SEM images of holes for F = 540 mJ/cm² are also shown.

Figure 5

Results of the action of single laser pulses with the energy E = 19.5 mJ and laser fluence F = 2.8 J/cm² on the Ti film located in close contact with the sapphire acceptor plate. (A) SEM image (SEM) and (B) the surface distribution map of titanium particles (EDX). The dotted circles denote an area with the diameter of 54 μm. (C) X-ray diffraction patterns of the Ti film in the region near the dotted circle.

Figure 6

Results of the action of single laser pulses with the energy E = 19.5 mJ and laser fluence F = 2.8 J/cm² on the sapphire acceptor plate placed in close contact with the Ti film on the donor slide. (A) SEM images (SEM) and surface distribution maps of Ti particles (EDX). The dotted circles denote an area with a diameter of 54 μm. (B) The surface distribution map of Ti particles. (C) X-ray diffraction patterns of the sapphire surface inside the dotted circle before and after the laser pulse exposure.

Figure 7

AFM images of the sapphire plate surface after the laser pulse exposure at the same laser parameters as in previous figures. (A) General view of the surface. (B) The enlarged part at the border area (marked by the square in (A)). (C) Cross-sectional profile along the dotted green line M in (A,D). The size distribution of TiNP. On the inset—a surface area with nanoparticles.

Figure 8

Dependence of the ratio (in percentages) of TiNP mass transferred in microdroplets to the mass of the ablated Ti film on the laser pulse fluence. The inset shows a titanium film, a jet of gel, and a microdroplet. The numbers on the Figure provide the percentage of material removed from the Ti film (~75%), as well as the percentage of the removed material located in the jet (99.5–97.5%) and in gel microdroplet (0.5–2.5%).