Thermoresponsive Brushes Facilitate Effective Reinforcement of Calcium Phosphate Cements

Abstract

Calcium phosphate ceramics are frequently applied to stimulate regeneration of bone in view of their excellent biological compatibility with bone tissue. Unfortunately, these bioceramics are also highly brittle. To improve their toughness, fibers can be incorporated as the reinforcing component for the calcium phosphate cements. Herein, we functionalize the surface of poly(vinyl alcohol) fibers with thermoresponsive poly(N-isopropylacrylamide) brushes of tunable thickness to improve simultaneously fiber dispersion and fiber-matrix affinity. These brushes shift from hydrophilic to hydrophobic behavior at temperatures above their lower critical solution temperature of 32 °C. This dual thermoresponsive shift favors fiber dispersion throughout the hydrophilic calcium phosphate cements (at 21 °C) and toughens these cements when reaching their hydrophobic state (at 37 °C). The reinforcement efficacy of these surface-modified fibers was almost double at 37 versus 21 °C, which confirms the strong potential of thermoresponsive fibers for reinforcement of calcium phosphate cements.

Keywords: calcium phosphate cements; interface; poly(-isopropylacrylamide); poly(vinyl alcohol) fibers; reinforcement.

Figure 1

Synthetic pathway to grow PNIPAM brushes onto the surface of PVA fibers.

Figure 2

(a) Density of carboxyl groups measured on PVA fibers, (b) EDX spectrum of the Br-treated PVA fiber, (c) bromine content measured after attachment of the BrIbB initiator to PVA fibers and the remaining bromine content after the ATRP reaction.

Figure 3

Scanning electron micrographs of pristine and modified fibers at two different magnifications: (a, b) pristine PVA fibers, (c, d) PVA-PNIPAM 25-modified fibers, (e, f) PVA-PNIPAM 50-modified fibers, and (g, h) PVA-PNIPAM 100-modified fibers.

Figure 4

Scanning electron micrographs of cross sections of pristine and PNIPAM-modified fibers at low and high magnifications as well as the histogram of the thickness of dry PNIPAM brushes. (a, b) pristine PVA fibers, (c–e) PVA-PNIPAM 25 fibers, (f–h) PVA-PNIPAM 50 fibers, and (i–k) PVA-PNIPAM 100 fibers; the separation between PNIPAM brushes and the fiber is marked by the purple line.

Figure 5

Characterization of PNIPAM-modified PVA fibers using DSC and contact angle measurements: (a, b) DSC analysis of PNIPAM-modified PVA fibers in H₂O, PBS, and NaH₂PO₄; (c) meniscus contact angle measurements performed on pristine and PNIPAM-modified PVA fibers at 21 and 37 °C in H₂O and NaH₂PO₄. Statistically significant differences are indicated with * (p < 0.05).

Figure 6

Mechanical properties of fiber-free and fiber-reinforced CPCs containing PNIPAM-modified PVA fibers: (a) flexural strength, (b) flexural modulus, and (c) work of fracture; statistically significant differences are indicated with * (vs CPC controls), % (vs CPC-PVA), and & (among the same group) (p < 0.05).

Figure 7

Force vs displacement curves for single-fiber pullout of PVA fibers from the CPC matrix: (a) PVA fibers pulled out at 21 and 37 °C, (b) PVA-THF fibers pulled out at 21 and 37 °C, (c) PVA-PNIPAM 100 fibers pulled out at 37 °C or (d) at 21 °C.

Figure 8

(a) Interfacial shear strength between CPC and pristine or surface-modified PVA fibers. (b) Work of pullout values resulting from the pullout test performed at 21 and 37 °C; statistically significant differences are indicated with * (vs CPC-PVA), $ (vs CPC-PVA-PNIPAM 50), # (vs CPC-PVA-THF), and & (among the same group) (p < 0.05).

Figure 9

Scanning electron micrographs of PVA fibers after the pullout test: (a) as-received PVA fiber, (b) THF-treated PVA fiber, (c) PVA-PNIPAM 100 fiber after a pullout test performed at 21 °C, and (d) the corresponding EDX spectrum, (e) PVA-PNIPAM 100 fiber after a pullout test performed at 37 °C, and (f) the corresponding EDX spectrum.