Water swollen polymer networks, generally referred to as
hydrogels, have received a great deal of attention for a wide variety
of biomedical applications such as tissue engineering, cell encapsulation,
wound dressing and drug delivery [1]. The hydrophilic
polymer network of hydrogels allows a high water content similar
to natural tissues. Their swelling properties, as well as their
biocompatibility, makes hydrogels ideal candidate materials for the
repair and regeneration of soft tissues.Application of hydrogels as load-bearing components is often
limited by their mechanical properties. Accordingly, a special
emphasis is given in the literature to the synthesis of hydrogels
with enhanced mechanical performance [2e4]. It iswell recognized
that the stiffness of the polymer networks can be tailored with the
cross-link density. Such an approach however results in an inverse
proportionality between stiffness and toughness as described by
the LakeeThomas model [5].Most methods developed to increase hydrogels' stiffness and
toughness simultaneously require a chemical modification of the
polymer and/or synthesis with several steps. This complexity in
hydrogel synthesis is a drawback in cases where in situ formation of
the hydrogel is required, such as injectable implants and cell
seeding for cell encapsulations applications.
Designing a hybrid hydrogel is an effective approach to address
the aforementioned issue while keeping a simple and fast processing
routes. Yet, such a design clearly involves the choice of
material components, their scale and shape. On this matter, the
concept of composite hydrogels reinforced with an interpenetrated
phase is introduced as a promising solution [6e9].Cellulose nanofibre mats offer a great potential as reinforcement
phase in hydrogel composites due to their large surface-to-volume
ratio, high stiffness and strength. Introduction of a cellulose
nanofibre in a covalently cross-linked polymer network can create
physical entanglements of the cellulose fibres and the polymeric
chains. Hence, the hydrophilicity of the cellulose fibres helps to
keep the hydrogel's high equilibrium water content in the resulted
composite. The integration of the cellulose fibres in the hydrogel is
achieved either through the irreversible physical interactions or
functionalization of the cellulose fibres enabling covalent crosslinking
to the polymeric network.
Although nanofibre cellulose as reinforcement in polymer matrix
composites was investigated [10,11], the use of nanofibre cellulose
in hydrogels [12] and the impact of nanofibre cellulose on the
processing of hydrogels received few attention [13,14]. A detailed
study linking material properties and material processing for such
hybrid materials is still lacking.
In this work, the influence of nanofibrillated cellulose (NFC)
addition to the photopolymerizable poly(ethylene glycol) dimethacrylate
(PEGDM) hydrogel precursors of two different molecular
weights is investigated. PEGDM was chosen as a matrix of the
composite hydrogel due to its biocompatibility, tuneable properties
and rapid photo curing [15]. Characterization of NFC addition to
such a photopolymerizable hydrogel precursor substantially requires
the assessment of the following aspects:
1.1. Processability of the precursor solution
An increase in the concentration of reinforcement in the polymeric
precursor results in a change in the viscosity of the solution.
The latter, as a key processing parameter affects the dispersion of the
ingredients and consequently the homogeneity of the polymerized
hydrogel. Furthermore, potential biomedical applications, such as
the replacement of intervertebral disc tissue, often involve injection
of the precursor and therefore require a lowviscosity of the solution.
1.2. Kinetics of photo-polymerization
The interaction of the precursor solution and the incident radiation
can be altered by the presence of fillers. The additional
filler-phase scatters and absorbs light and also changes diffusion
kinetics of the reacting species.