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3D Printing of hydrogel composite systems: Recent advances in technology for tissue engineering
[19]
precise beam controlling and scanning system . So far, stereolithographic strategy to fabricate nanoresolution
μSLA can be categorized into (i) scanning μSLA system structures without undergoing the layer-by-layer process.
and (ii) mask (or integral) projection μSLA system. In The light source of TPP system is femtosecond infrared
the scanning μSLA, UV beam is fixed with a stationary laser pulses that is focused into the volume of photocurable
spot position onto the liquid hydrogel surface, and liquid materials, and initiates photolytic polymerization
scanning is conducted by moving the work piece stage process without any masks [3,18,25] . In contrast to the UV
including the vat, which can eliminate the unstable mobile light, nonlinear behavior and existence of polymerization
optical elements leading to focusing errors and poor threshold intensity of infrared light allow direct fabrication
resolution [20,21] . In the mask projection system, light beam of complex 3D structure inside the photosensitive liquid
exposure of each liquid hydrogel layers is conducted in with much higher structural resolution as high as 200 nm.
a single radiation using dynamic pattern generator mask. Thus, 3D printed objects by TPP obtain better quality than
The sliced 2D patterns of 3D structure is converted into the parts fabricated by conventional stereolithography
a data file and input to the light beam mask, which can techniques, as shown in (Figure 1C) [3,18] . Because of its
generate the precise patterns corresponding to the each ultra-small focusing spot size, the scanning of tightly
layer of structure [22,23] . The patterned beam is focused by focused beam of ultra-short laser pulse is precisely
the computer-controlled focusing optical components controlled by a computer positioning system combined
to reduce the whole pattern size into micrometers. The with piezoelectric stages and/or optical scanning systems.
highly precise 3D structure containing complex engineered The high intensity of photons from the two-focused beam
shapes can make a unique interface between the nano- source causes excitation of the photoinitiator molecules
scale functional second materials and macro-scale hydrogel resulting in the creation of free radicals. These free radicals
molecules, which provides an engineering platform for break the unstable bond of monomers and initiate the
various industries, such as tissue engineering, photonics, polymerization process. As a result, polymer chains can be
and microelectromechanical system (MEMS) [19,23] . formed and grown by combining monomers and adding to
the chains. Although conventional photosensitive materials
2.1.2 Digital Light Projection and initiators have been developed, such polymer still
Digital light projection (DLP) is developed from the suffers from the insufficient free radical density within
mask projection system of SLA. However, in this 3D the extremely small cross-sectional focusing area for two-
printing system, digital mirror device or liquid crystal photons [26,27] . Thus, enough duration of each scan position,
displays play a role of dynamic pattern generator mask, high density of photoinitiator, and intensity of focused
which consists of several millions of arrayed mirror light beam are most important parameters to terminate the
or LCD pixel units to generate an individual on-off photopolymerization procedure with enough crosslinking
beam signal (Figure 1B). The DLP light source allows density.
fabrication of the 3D structure with high resolution TPP is promising 3D printing technique for biological
between 25 and 150 μm, which can be further enhanced applications, such as drug delivery, implants, biosensors,
with additional multi lens components to focus the light and tissue engineering. Since infrared light does not
beam sources [2,3,18] . In comparison to other 3D printing cause any harmful effect to the living cells or organisms,
systems that have bottom-up construction approach, customized 3D scaffold structure can be directly fabricated
[24]
DLP is based on a top-down working principle . The in the presence of living cells . Furthermore, it allows the
[28]
beam source is placed at the bottom part of system, introduction of pores at any location within the structure,
photocurable liquid hydrogel is exposed by the beam which enables precise control of the cell position, movement,
through the transparent contact window underneath the interaction, and organization inside the scaffold and,
vat. The building plate or carrier is immersed into the consequently, integrity with host tissue inside the body [18,27,28] .
liquid hydrogel and moves vertically upward direction For the practical application in tissue engineering, hybrid
after each layer is polymerized. In this process, fresh materials with either organic/organic or organic/inorganics
liquid hydrogel is automatically supplied to the bottom are consistently introduced for the better cell affinity and
layer through the capillary action, and each repetitive biocompatibility of 3D printed scaffolds.
processing steps can be conducted within 15 seconds [2,3] .
In the DLP system, any planarization or levelling process 2.1.4 Solid Ground Curing
is not required which allows to increase the building Solid ground curing (SGC) is one kind of projection
speeds and thus eliminate the fabricated parts from beam systems developed by Cubital Inc. in 1986. In
damaging during the wiping actions. this system, the fabrication of the each layer patterns
of 3D object is done by a high-powered UV lamp in
2.1.3 Two-Photon Polymerisation the presence of the patterned mask over the surface of
Two-photon polymerisation (TPP) is an entirely new photocurable materials [18,29,30] . The patterned mask is

4 International Journal of Bioprinting (2018)–Volume 4, Issue 1