The
development of a hydrogel-based synthetic cartilage has the
potential to overcome many limitations of current chondral defect
treatments. Many attempts have been made to replicate the unique characteristics
of cartilage in hydrogels, but none have simultaneously achieved high
modulus, strength, and toughness while maintaining the necessary hydration
required for lubricity. Herein, double network (DN) hydrogels, composed
of a poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) first
network and a poly(N-isopropylacrylamide-co-acrylamide) [P(NIPAAm-co-AAm)] second
network, are evaluated as a potential off-the-shelf material for cartilage
replacement. While predominantly used for its thermosensitivity, PNIPAAm
is employed to achieve superior mechanical properties with its thermal
transition temperature tuned above the physiological range. These
PNIPAAm-based DNs demonstrate a 50-fold increase in compressive strength
(∼25 MPa, similar to cartilage) compared to traditional single
network hydrogels while also achieving cartilage-like modulus (∼1
MPa) and hydration (∼80%). In direct comparison to healthy
cartilage (porcine), these hydrogels were confirmed to not only parallel
the strength, modulus, and hydration of native articular cartilage
but also exhibit a 50% lower coefficient of friction (COF). The exceptional
cartilage-like properties of the PAMPS/P(NIPAAm-co-AAm) DN hydrogels makes them candidates for synthetic cartilage
grafts for chondral defect repair, even in load-bearing regions of
the body.
The utility of thermoresponsive hydrogels, such as those based on poly(N-isopropylacrylamide) (PNIPAAm), is severely limited by their deficient mechanical properties. In particular, the simultaneous achievement of high strength and stiffness remains unreported. In this work, a thermoresponsive hydrogel is prepared having the unique combination of ultra-high compressive strength (~23 MPa) and excellent compressive modulus (~1.5 MPa). This is accomplished by employing a double network (DN) design comprised of a tightly crosslinked, highly negatively charged 1st network based on poly(2-acrylamido-2-methylpropane sulfonic acid (PAMPS) and a loosely crosslinked, zwitterionic 2nd network based on a copolymer of thermoresponsive NIPAAm and zwitterionic 2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (MEDSAH). Comparison to other DN designs reveals that this PAMPS/P(NIPAAm-co-MEDSAH) DN hydrogel’s remarkable properties stem from the intra- and inter-network ionic interactions of the two networks. Finally, this mechanically robust hydrogel retains the desirable thermosensitivity of PNIPAAm hydrogels, exhibiting a volume phase transition temperature (VPTT) of ~35 °C.
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