Injectable Biocompatible and Biodegradable pH-Responsive Hollow Particle Gels Containing Poly(acrylic acid):
The Effect of Copolymer Composition on Gel Properties
S. Halacheva1*, D. Adlam2, T. Freemont2, J. Hoyland2 and B. Saunders3
1*
University of Bolton, Institute for Materials Research and Innovation, Deane Road, Bolton, Greater Manchester,
BL3 5AB, U.K, S.Halacheva@bolton.ac.uk
2
Centre for Tissue Injury and Repair, Institute of Inflammation and Repair, Faculty of Medical and Human Sciences,
University of Manchester, Oxford Road, Manchester, M13 9PT, U.K.
3
University of Manchester, School of Materials, Grosvenor Street, Manchester, M13 9PL, U.K.
EXPERIMENTAL METHODS
All copolymers were characterised by 1H NMR and
GPC. Potentiometric titration was conducted using
standardized NaOH solutions. Dispersions were
deposited on SEM stubs by evaporation at room
temperature. Rheology measurements were performed
at 25 °C using a TA AR-G2 rheometer and a 250 µm
gap with a 20 mm diameter steel plate at 25 °C. DLS
measurements were performed on a Malvern Zetasizer
Nano ZS90 autocorrelator, at pH values ranging from
6.0 – 10.0. The hydrodynamic diameter (Dh) of the
particles was measured at 90°. The particle’ volume
swelling ratio, Q, was estimated using Q =
(Dh/Dh(collapse))3, where Dh and Dh(collapse) are the
hydrodynamic diameters of the particles at a given pH
and in the collapsed non-swollen state, respectively.
RESULTS AND DISCUSSION
The physical gels were formed at physiological pH
range only from concentrated dispersions of swollen,
hollow,
polymer-based
PMMA-AA
particles
crosslinked with either cystamine (CYS) or 3,3′dithiodipropionic acid dihydrazide (DTP). A linear
relationship between particle swelling ratios, gel
elasticity and ductility was observed. The PMMA-AA
gels with lower AA contents feature lower swelling
ratios, mechanical strengths and ductilities. The
mechanical properties and performance of the gels were
tuneable upon varying the copolymers’ compositions
and the structure of the crosslinker. Compared to
PMMA-AA/CYS, the PMMA-AA/DTP gels were more
elastic and ductile. The SEM images of PMMAAA/DTP gels show highly porous and interconnected
structures (Figure 1). Biodegradability of the new gels
was tested using glutathione. The rate of network
disassembly was found to be strongly dependent upon
the gel’s structure. The responses of immortalised
human chondrocyte cells to contact with the gels were
measured by Live/Dead and MTT assay (Figure 1). The
gels showed very good biocompatibility, with at least
80% cell viability after 48 hours. Increases in either the
crosslinking density or AA content appear to decrease
the cell viability.
SEM image of the gel
MTT assay
Cell viability (% Control)
INTRODUCTION
Injectable polymer/cell dispersions have emerged as a
superior,
non-surgical
alternative
for
tissue
regeneration.1 We have previously explored novel types
of pH and redox-responsive poly(methyl methacrylateco-methacrylic acid)- (PMMA-MAA) and poly(ethyl
acrylate-co-methacrylic
acid)
(PEA-MAA)-based
hollow particle gels. The gels had micrometer-scale
interconnected porosity, high elasticity and ductility
values and are potentially suitable for future use in
minimally-invasive tissue repair.2 Herein we extend
considerably our earlier studies with the aim of
identifying a system which meets all necessary
requirements for soft tissue repair – i.e., high porosity,
biocompatibility, biodegradability and adequate
mechanical strength. To investigate the effect of tuning
hydrophobicity of the copolymer, the glass transition
temperature and the pKa of the particles upon the gel
morphology, mechanical properties and performance,
we studied the structurally related poly(n-butyl acrylateco-methacrylic acid) (PBA-MAA), poly(methyl
acrylate-co-methacrylic acid) (PMA-MAA) and
poly(methyl methacrylate-co-acrylic acid) (PMMAAA) copolymers for gel formation. These systems have
been chosen to allow a wide range of structural
variation for the constituent pH-responsive copolymers.
We have established that MMA is the optimal
hydrophobic monomer, whereas the use of various
COOH-containing monomers, e.g. MAA and AA, will
always induce a pH-triggered physical gelation.3
100
O
O
MeO
S
80
O
60
40
HO
PMMA-25AA/2DTP
PMMA-33AA/12DTP
PMMA-19AA/15DTP
0
10
20
30
Time (hours)
H
N
O
N
H
O
S
O
H
N
N
H
O
OMe
O
OMe
40
50
OH
O
O
OH
OMe
O
Figure 1. Redox- and pH-responsive hollow PMMAAA/DTP particle gel scaffolds.
CONCLUSION
The new pH and redox-responsive PMMA-AA hollow
particle gels show good biodegradability and
biocompatibility, high interconnected porosity and
adequate mechanical strength. Therefore, they
constitute a promising prototype injectable gel for tissue
repair.
REFERENCES
1. Richardson S. M. et al., Biomaterials 27:4069-4078,
2006
2. Halacheva S. et al., J. Mater. Chem. B 1:4065-4078,
2013
3. S. Halacheva et al., Biomacromolecules, 2014, 15,
1814−1827
ACKNOWLEDGMENTS
The authors would like to thank the EPSRC for funding
this study.