Page 26 - MSAM-2-2

Materials Science in Additive Manufacturing Water repellence via pinecone structures

membranes arranged in three dimensions, is increasingly good mechanical properties and biological compatibility.
used for biomedical research. During the device fabrication The 2D structures were the text “CALM”, and the 3D
process, TPP offers potentials in the development phase as structures were cone and pinecone structures, as shown
it enables rapid prototyping with design modification . in Figure 1. The dimensions of the cone and pinecone
[14]
TPP can be used to fabricate microfluidic device for structures are indicated in Figures 1B and C. The diameter
cell culture practices to provide dynamic conditions for the trunk cone was 10 μm, and the height was 15
and improve in vivo biological conditions and μm. The nanostructures on the pinecone surface had a
[15]
multifunctional biochips with filtration and mixing diameter of 0.5 μm and a height of 2.4 μm. The scanning
functions . A promising type of biochips is one, in which parameters for printing the 3D structures are listed
[16]
fluid flow must be tuned to ensure the desired reactions. in Table 1. Unidirectional hatching/contour hatching
This function is mainly achieved by adapting some bionic (CHT/UHT)-m-n represents unidirectional or contour
micro/nanoscale structures. For example, many researchers scanning patterns with hatch spacing, m (nm), and layer
have fabricated structures with superhydrophobic thickness, n (nm), respectively. The TPP fabrication was
properties by mimicking the surface structure of lotus leaf. conducted in a femtosecond laser processing system
The lotus leaf surface structures are characterized by the (FemtoLAB-MPP, Workshop of Photonics, Lithuania),
micro/nano hierarchical morphology of papillae, of which which was equipped with a laser (514 nm wavelength), a
the diameter ranges from 2 ~ 8 μm and the height ranges high-resolution stage (Aerotech), and 63× oil immersed
from 6 ~ 12 μm. These micro/nano hierarchical structures objective lens with numerical aperture (NA) of 1.4. The
are mainly notable for their hydrophobicity. However, they stage is equipped with a back lamp. For preparing the
are rarely used in biochips due to the complex fabrication. fabrication, the photoresist was deposited on a cover-
glass slide ultrasonically cleaned in alcohol and acetone
TPP leverages two-photon absorption of laser radiation for 30 min, respectively. The cover-glass slide was then
with sub-diffraction limit resolution within the bulk of a placed upside down on the stage. The immersion oil was
photosensitive material [17-21] . TPP fabrication is a process deposited on the top surface of the cover-glass slide, right
of the spatial stack-up of the fundamental element voxel, above the photoresist drop, as shown in Figure 2.
which resembles a spinning ellipsoid [22-24] . Therefore, the
voxel size and arrangement are important for the resolution After the TPP fabrication, the “CALM” text and the
and accuracy. Scholars have attempted to improve the 3D structures were developed in ethanol for 2 h and
resolution of TPP by decreasing the voxel size and dried. The samples were then prepared with gold coating
increasing the overlap between them . The parametric for observation and characterized with a field emission-
[25]
study, including laser power and scanning speed, has been scanning electron microscope (FE-SEM, Quanta 250, FEI
well performed. Lim and Yang found that the surface Company, USA) at an accelerating voltage of 15 kV.
roughness of the square plates fabricated through TPP Water flow tests of single and double rows of pinecone
increases significantly when the laser dose is insufficient. structures were performed to evaluate the water-repellent
The surface turned out to be almost flat with laser power performance with deionized water. Forty individual
over 30 mW due to the accumulated polymerization pinecone structures were designed in each row, and the
between the voxels . Scanning parameters, including center-to-center distance between the adjacent structures
[26]
scanning strategies, hatch spacing, and layer thickness, was 20 μm. The water droplets were deposited adjacent
are also important but less reported. Meanwhile, the to the corresponding single and double rows, as shown in
parametric study was usually performed to improve Figure 3. A 0.5-μL water droplet was added every 10 s. The
fabrication efficiency. water-repellent performance was compared by timing the

The present work investigated the influence of laser liquid flow on the substrate. Optical microscopy images
processing parameters on the feature size of 2D structures, and videos were taken using a microscope (Axio Vert.A1,
surface roughness, and the dimensional accuracy of 3D Carl Zeiss Microscopy, Germany) equipped with a charge-
cone structures. Optimal direct-writing parameters were coupled device camera.
obtained to prepare pinecone structures designed based on 3. Results
the surface structures of a lotus leaf. The water repellence
of the pinecone structures was evaluated and discussed. 3.1. “CALM” structures
2. Materials and methods “CALM” structures fabrication through TPP was
performed to investigate the influence of laser power on
The photoresist OrmoComp for the TPP fabrication was the width, that is, the feature size, of nanowires. The linear
®
purchased from Micro Resist Technology, Germany. It has trace of the “CALM” structures could not be distinguished

Volume 2 Issue 2 (2023) 2 https://doi.org/10.36922/msam.0879