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Silicon Nanoparticles and Cells Can Now Be Printed with Lasers

IMAGE: 3D printed cell structures (left) include stem cells printed in grid patterns (a) immediately after printing, and later differentiated toward (b) bone, (c) cartilage, and (d) adipose tissue. Histologic sections of skin tissue (right) consist of printed fibroblasts and keratinocytes (a) immediately after printing and (b) 10 days after being implanted in mice, as well as (c) implanted collagen-elastin matrix without printed cells, used as a control, and (d) native mouse skin. (Image credit: Laser Zentrum Hannover)

An article in SPIE Newsroom (Boris Chichkov et al., DOI: 10.1117/2.1201503.005750, March 30, 2015) describes how Laser Zentrum Hannover (Hannover, Germany) researchers are printing biological cells and silicon nanoparticles using lasers. The technique prints spherical silicon (Si) nanoparticles onto a specific position of a receiver glass substrate. The printed nanoparticles have a predefined size and are characterized by unique optical properties. With sizes of 100-200 nm in diameter, they exhibit pronounced electric and magnetic dipole resonances within the visible spectral range. Due to these resonances, they appear colorful in a dark-field microscopic image.

For the printing process, we focused single 50fs laser pulses at a wavelength of 780 nm through a receiver glass substrate and onto a silicon-on-insulator (SOI) substrate. This substrate consists of a 50 nm crystalline silicon layer on a 200 nm silicon dioxide layer with an underlying Si wafer substrate. Each laser pulse induces strongly localized melting in the Si layer. Due to surface tension, the melted volume contracts into a sphere, which is ejected back toward the receiver glass substrate.

Figure 2. Representation of laser printing of silicon nanoparticles from silicon-on-insulator substrate. The transferred silicon nanoparticles are shown in scanning electron microscopic images on the right.
Figure 3. Laser-induced crystallization of amorphous silicon nanoparticles on a glass substrate. The dark-field microscopic image (right) shows a visible color change due to crystallization. a-Si: Amorphous silicon. c-Si: Crystalline silicon.

The generated Si nanoparticles are initially in an amorphous phase and can be recrystallized by a second femtosecond laser pulse with a square-shaped flat top intensity distribution to get a homogenous intensity distribution, and thus a homogenous crystallization. After this step, the optical response of the Si nanoparticles is much stronger and their color is different, indicating they are crystallized.

The forward transfer setup used for cell printing consists of a glass slide or transparent ribbon that is coated with a layer of a laser-absorbing material and a second layer of a biomaterial to be printed. Typically, this biomaterial layer is a hydrogel with embedded cells. The coated glass slide is mounted upside-down and focused laser pulses pass through the slide into the absorption layer, which evaporates in the focal spot. The subjacent biomaterial is then propelled down (in the forward direction) by the vapor pressure and deposited as a droplet onto a surface located under the glass slide. By moving the glass slide and the laser beam focus, any desired 2D and 3D patterns can be printed layer-by-layer. In this printing setup, the group applied a laser with a 10 ns pulse duration at a wavelength of 1064 nm.

In a series of publications, the researchers proved that cells are not harmed by this printing process. Future research will focus on printing bacteria and microorganisms.

Source: SPIE Newsroom



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