Metal Ion-Loaded Nanofibre Matrices for Calcification Inhibition in Polyurethane Implants

Abstract

Pathologic calcification leads to structural deterioration of implant materials via stiffening, stress cracking, and other structural disintegration mechanisms, and the effect can be critical for implants intended for long-term or permanent implantation. This study demonstrates the potential of using specific metal ions (MI)s for inhibiting pathological calcification in polyurethane (PU) implants. The hypothesis of using MIs as anti-calcification agents was based on the natural calcium-antagonist role of Mg²⁺ ions in human body, and the anti-calcification effect of Fe³⁺ ions in bio-prosthetic heart valves has previously been confirmed. In vitro calcification results indicated that a protective covering mesh of MI-doped PU can prevent calcification by preventing hydroxyapatite crystal growth. However, microstructure and mechanical characterisation revealed oxidative degradation effects from Fe³⁺ ions on the mechanical properties of the PU matrix. Therefore, from both a mechanical and anti-calcification effects point of view, Mg²⁺ ions are more promising candidates than Fe³⁺ ions. The in vitro MI release experiments demonstrated that PU microphase separation and the structural design of PU-MI matrices were important determinants of release kinetics. Increased phase separation in doped PU assisted in consistent long-term release of dissolved MIs from both hard and soft segments of the PU. The use of a composite-sandwich mesh design prevented an initial burst release which improved the late (>20 days) release rate of MIs from the matrix.

Keywords: Alizarin red S staining; Von Kossa method; anti-calcification; calcification; hydroxyapatite; magnesium; metal ion; nanofibre matrix.

Figure 1

Scanning electron microscopy (SEM) micrographs of electrospun (a) Polyurethane (PU); (b) MgSO₄-loaded sample (PU-MS); (c) MgCl₂-loaded sample (PU-MC); and (d) FeCl₃-loaded sample (PU-FC) films.

Figure 2

Fourier transform infra red (FTIR) spectrum of electrospun polyurethane film.

Figure 3

FTIR spectrum of electrospun control PU and metal ion (MI) loaded PU films in the 2500–4000 cm⁻¹ frequency range.

Figure 4

FTIR spectrum of electrospun control and MI-loaded PU films in the 1500–2300 cm⁻¹ frequency range.

Figure 5

FTIR spectrum of electrospun control and MI-loaded PU films in the 900–1500 cm⁻¹ frequency range.

Figure 6

Stress-strain relation of control and MI loaded PU films.

Figure 7

Release profile of MIs from PU-MI matrices in (A) solid-sandwich; and (B) composite-sandwich configurations.

Figure 8

SEM micrographs of (a) PU; (b) PU-MS; (c) PU-MC; and (d) PU-FC films after calcium solution incubation for 60 days. Calcium deposits are visible as randomly segregated crystals adhering to fibres.

Figure 9

SEM micrographs of large calcium deposits on the surface of Control PU film after calcium solution incubation for 60 days.

Figure 10

Light microscopy of (a) PU; (b) PU-MS; (c) PU-MC; and (d) PU-FC films after Von Kossa staining and calcium solution incubation for 60 days. Dark black-brown spots indicate aggregated calcium deposits.

Figure 10

Light microscopy of (a) PU; (b) PU-MS; (c) PU-MC; and (d) PU-FC films after Von Kossa staining and calcium solution incubation for 60 days. Dark black-brown spots indicate aggregated calcium deposits.

Figure 11

Light microscopy of (a) PU; (b) PU-MS; (c) PU-MC; and (d) PU-FC films after Alizarin Red staining and calcium solution incubation for 60 days. Dark red-orange spots on the surface indicate aggregated calcium deposits.

Figure 12

FTIR spectrum of control PU and MI loaded PU films after 90 days of incubation in calcification solution.

Figure 13

Schematic of electrospinning setup for developing metal salt-loaded PU films.

Figure 14

Representation showing cross-section of (A) solid-sandwich; and (B) composite-sandwich electrospun films.