Page 35 - IJB-4-1

3D Printing of hydrogel composite systems: Recent advances in technology for tissue engineering

materials for 3D printing are hydrogels because they can understood and evaluated conventional manufacturing
be easily functionalized or modified, without complex processes, hydrogel composite 3D printing systems
synthesis steps, to replicate the physicochemical properties remain a relatively new area of research and much more
of most biological tissues [8,9] . They possess a highly can be studied with regards to their physicochemical
hydrated polymeric structure, exhibiting up to 40-fold properties such as viscosity, dispersion, reinforcement,
change in volume as they swell or shrink in the presence and its size and shape [16,17] . In recent years, significant
or absence of water, respectively, and can be modified progress has been made in the development of 3D
to respond to various physical and biological stimuli printing systems for hydrogel composites with improved
such as temperature, light, pH, ions, and biochemical mechanical performance and biofunctionality [1,16,17] .
signals [9,10] . These unique features make hydrogels excellent Herein, we first provide a brief introduction of hydrogel
environments for cell attachment and proliferation within composite 3D printing techniques and their application
their hydrated hydrogel networks, which offer abundant in the field of tissue engineering. We shall categorize 3D
space for cell growth while facilitating the transportation printing into (a) laser based-3D printing, (b) nozzle based-
of essential metabolites and nutrients to the encapsulated 3D printing, and (c) inkjet printer based-3D printing
cells [8,11] . However, most hydrogels suffer from a lack of systems, and discuss their working principles and recent
mechanical strength and unsuitable degradation behavior trends. In particular, we will discuss four different hydrogel
compared with native tissues such as ligament, tendon, composite systems: i) polymer- or hydrogel-hydrogel
muscle, or cartilage. Therefore, augmenting the mechanical composite, ii) particle-reinforced hydrogel composite iii)
properties and bioactivity of hydrogel have been a fiber-reinforced hydrogel composite, and iv) anisotropic
[8]
challenging task for material scientists . filler-reinforced hydrogel composite, and highlight tailored
Hydrogel composite system is one of the most suitable physical properties and their functionality. Additionally,
strategy for incorporating and combining various hydrogel several emerging potential applications of hydrogel
functions and properties, not attainable by any single composites in the field of tissue engineering and their
[11]
hydrogel alone . Over the past few decades, a diverse accompanying challenges are discussed in parallel.
range of reinforcements have been proposed utilizing 2. 3D Printing Technology for Hydrogels
various composite designs such as particle-, anisotropic Composite
filler-, and fiber-hydrogel composite systems in which
reinforcements are stabilized and immobilized via physical 2.1 Laser-based Hydrogel 3D Printing Systems
or chemical interactions in the hydrogel matrix [8,9,11,12] .
In the case of hydrogel-hydrogel composite system, the Most laser-based 3D printing systems are applicable for
interpenetration between the two polymer networks forms the hydrogel composite fabrications, which builds a 3D
a mechanical anchoring behavior, and these complexes structures in a vat of photocurable hydrogels under the
strongly affect the hydrogel rheology, degradation rate, deposition of laser energy, usually UV range, in specific
[3]
[13]
permeability, and mechanical properties . Conventional designed patterns . The exposure of UV laser on to the
inorganic reinforcements are based on physical interactions surface of photocurable liquid causes gel-formation of a
with the hydrogel matrix in which secondary or van der thin single layer, and it is sequentially moved upward or
Waals forces including London dispersion forces, dipolar downward with the sample stage to allow the next layer
[14]
interactions and hydrogen bonding are involved . These formation on top of preformed structure. During this
physical interactions generate strong adhesion between the process, designed 3D structures are directly materialized
reinforcements and hydrogel matrix, and the enhancement in the liquid vat that means the hydrogel composites
of hydrogel properties are dependent on the amount can be built within the photocurable organic-inorganic
[3,18,19]
of reinforcements and the volume ratio of physically solution .
interacted- and non-interacted-polymer networks [15] . In Since the development of laser-based 3D printing in
the case of chemical modifications, the introduction of 1980s, several other commercially available techniques
chemical groups and the covalent bonding formations at have emerged, and they are widely used for biomedical
the interface induce superior interfacial bonding strength applications such as scaffolds, drug delivery, implants,
[2,18]
of which energy is generally in between the 40 to 400 kJ/ and devices . Laser-based 3D printing system can be
mol i.e. much higher than physical interaction (8–16 kJ/ divided into several categories based on the type of laser
[14]
mol) . Thus, it is possible to provide substantial increase of source, beam delivering system, method of scanning or
mechanical strength to the hydrogel composite system. exposure, and type of stage movement system. However,
While the hydrogel composite has attracted a lot of most of these techniques require post-processing such
attention due to its superior properties, most review as support removal and other unwanted materials.
articles report on conventional fabrication techniques In addition, a post-curing procedure is necessary to
such as molding or casting [5,7] . Compared with well- completely cure the built structure for intact mechanical

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