Electrospun biodegradable poly(ε-caprolactone) membranes for annulus fibrosus repair: Long-term material stability and mechanical competence

Abstract

Background: Electrospun (ES) poly(ɛ-caprolactone) (PCL) is widely used to provide critical mechanical support in tissue engineering and regenerative medicine applications. Therefore, there is a clear need for understanding the change in the mechanical response of the membranes as the material degrades in physiological conditions.

Study design: ES membranes with fiber diameters from 1.6 to 6.7 μm were exposed to in vitro conditions at 37°C in Dulbecco's modified Eagle's medium (DMEM) or dry for up to 6 months.

Methods: During this period, the mechanical properties were assessed using cyclic mechanical loading, and material properties such as crystallinity and ester bond degradation were measured.

Results: No significant difference was found for any parameters between samples kept dry and in DMEM. The increase in crystallinity was linear with time, while the ester bond degradation showed an inverse logarithmic correlation with time. All samples showed an increase in modulus with exposure time for the first loading cycle. Modulus changes for the consecutive loading cycles showed a nonlinear relationship to the exposure time that depended on membrane type and maximum strain. In addition, the recovered elastic range showed an expected increase with the maximum strain reached. The mechanical response of ES membranes was compared to experimental tensile properties of the human annulus fibrosus tissue and an in silico model of the intervertebral disk. The modulus of the tested membranes was at the lower range of the values found in literature, while the elastically recoverable strain after preconditioning for all membrane types lies within the desired strain range for this application.

Conclusion: The long-term assessment under application-specific conditions allowed to establish the mechanical competence of the electrospun PCL membranes. It can be concluded that with the use of appropriate fixation, the membranes can be used to create a seal on the damaged AF.

Keywords: degradation; electrospinning; intervertebral disk; long‐term; mechanics; poly(ε‐caprolactone); regenerative; repair.

FIGURE 1

Representative micrographs of three different membranes types used in the study. A, Thin. B, Medium. C, Thick

FIGURE 2

Sample preparation from electrospun mat to single samples. A, Three membranes of the same type are produced. B and C, The membranes are cut into smaller pieces. D, The pieces are randomly separated into groups of four for each time point (t_n), which are then split into sample and control group

FIGURE 3

A, Representative dogbone sample. B, Mechanical testing rig and dogbone sample placed in the rig

FIGURE 4

A parameterized finite element (FE) model of a L4‐L5 intervertebral disk. Nine different vertebrae geometries were investigated in the study of Helgason et al. ³⁷ The vertebra body size was varied from −1.5 to +1.5 SD from the average geometry reported by Panjabi et al. ³⁸ The disk height and lordotic angle were varied from −1.0 to +1.5 SD from the average of the values reported by Abuzayed et al ⁷⁶ the study of Rohlmann et al. ⁴⁰

FIGURE 5

The effect of incubation time on Young's modulus of the first loading cycle of three types of samples. n = 9, * P < .05, ** P < .005. n = 18

FIGURE 6

Graph showing change of Young's modulus. Seven consecutive 5% strain steps from 5% strain are shown grouped by membrane type at each time point. n = 18

FIGURE 7

Graph showing change of hysteresis normalized 0 time point. Seven consecutive 5% strain steps are shown grouped by membrane type at each time point. Time showed a significant effect on the change in hysteresis for all samples (P < .05). Fiber diameter shows a strongly significant effect on this change (P < .0005), as well as the strain level for the Medium (P < .05) and thick sample (P < .05). n = 9

FIGURE 8

Graph showing change in elastically recovered strain in % strain calculated at consecutive 5% maximum strain steps grouped by membrane type at each time point. n = 18

FIGURE 9

K (A) and Plateau (B) values for the one phase decay fitted to recovery against maximum strain data grouped by time point and membrane type. n = 5

FIGURE 10

Graph showing normalized frequency of strains predicted by the finite element (FE) model

FIGURE 11

Change of ester bond density against time grouped by sample type, A, membrane made up of thin fiber (1.6 μm), B, medium fibers (3.31 μm) and C, thick fibers (6.73 μm). Crystalline fraction is represented with respect to time and grouped by the same sample types

FIGURE 12

Mean, maximum, and minimum tensile linear modulus of electrospun membranes obtained on the last loading cycle with a maximum strain of 35% from all time points combined with circumferential linear moduli from tissue level AF response found in literature ⁶² , ⁶³ , ⁶⁴ , ⁶⁵ , ⁶⁶ , ⁶⁷ , ⁶⁸ , ⁶⁹ , ⁷⁰ , ⁷¹ , ⁷² , ⁷³ visualized as stress vs strain curves