Viscoelastic and biological performance of low-modulus, reactive calcium phosphate-filled, degradable, polymeric bone adhesives

Abstract

The aim of this study was to investigate the effect of reactive mono- and tricalcium phosphate addition on the mechanical, surface free energy, degradation and cell compatibility properties of poly(lactide-co-propylene glycol-co-lactide) dimethacrylate (PPGLDMA) thin films. Dry composites containing up to 70 wt.% filler were in a flexible rubber state at body temperature. Filler addition increased the initial strength and Young's modulus and reduced the elastic and permanent deformation under load. The polymer had high polar surface free energy, which might enable greater spread upon bone. This was significantly reduced by filler addition but not by water immersion for 7 days. The samples exhibited reduced water sorption and associated bulk degradation when compared with previous work with thicker samples. Their cell compatibility was also improved. Filler raised water sorption and degradation but improved cell proliferation. The materials are promising bone adhesive candidates for low-load-bearing areas.

Fig. 1

Percentage mass changes, in standard growth medium at 37 °C, of composites filled with 50–70 wt.% calcium phosphate filler and unfilled polymer. n = 3; error bars are ±SD.

Fig. 2

Example plots of storage modulus ($E^{\prime }$), loss modulus ($E^{″}$) and tan δ vs. temperature for PPGLDMA filled with 0 or 70 wt.% MCPM/β-TCP.

Fig. 3

Average ultimate tensile strength (a) and Young’s modulus (b) of PPGLDMA filled with 0 and 70 wt.% MCPM/β-TCP. Specimens had either been kept dry or stored for 24 h in water. Studies were performed at 22 °C. Additionally, with dry samples, measurements were made at 37 °C. n = 5; error bars are ±SD.

Fig. 4

Examples of single specimen deformation behaviour on application of 500 mN load and recovery for the dry polymer and 70 wt.% filled composite.

Fig. 5

SEM images showing early attachment of human mesenchymal stem cells seeded on the surface of 300 μm thick sheet of the polymer (a and d), composites filled with 60 wt.% MCPM/β-TCP filler (b and e), and Thermanox® (cf. positive control surface) after 4 h (a–c) and 24 h (d–f) of culture.

Fig. 6

Background-subtracted Alamar® blue fluorescence due to active human mesenchymal stem cells growing on the surface of composites filled with 50–70 wt.% MCPM/β-TCP, polymer or tissue culture plastic control at 1, 3 and 7 days. Bars represent average values (n = 3); error bars are ±SD.