1. Introduction
The polyesters are some of the most used polymeric materials for biomedical applications such as sutures, implants, artificial skin and controlled drug release [
1,
2,
3,
4,
5]. Such is the case of the use of polylactide (PLA), polyglycolide (PGA), polycaprolactone (PCL), poly(lactide-
co-glycolide) (PLGA), polydioxanone (PDO) or polyhydroxybutyrate (PHB). These polymeric materials degrade by hydrolytic processes and result in low molar mass species, that can be bioabsorbed and metabolised by the human body.
The irruption of electrospinning in tissue engineering has boosted the technology of production of biomaterials based on architectures of fibres, in which diameters can vary from the micrometric scale to hundreds of nanometres [
6,
7,
8], mimicking the native extracellular matrix and allowing enough porosity to facilitate cellular growth [
9]. Precisely, these scaffolds are required to ensure an appropriate balance between enough time of structural endurance to permit angiogenesis, and suitable degradation profiles to be decomposed avoiding an inflammatory response and eluding the delivery of toxic low molar mass compounds. These degradation by-products may affect healthy cells as well as interact with sensitive substances such as proteins and peptides or drugs [
10].
Monitoring and understanding the physico-chemical processes underwent by biopolymeric scaffolds along their exposure to physiological conditions is essential to ensure their performance during application. In this sense, the study of their degradation profiles is necessary to guarantee the desired behaviour, according to the biomedical purpose, such as skin reparation [
11,
12] or cardiac surgeries [
13,
14,
15], among others, which will need different times of endurance [
16]. For instance, dissimilar regeneration periods may be considered according to the renewal rate on different tissues of the human body: 2–9 days for stomach cells, 10–30 days for skin epidermis cells, 30–60 days for trachea cells or 180 days for bone osteoblasts [
17].
In the balance of the performance-to-degradation ratio of the scaffolds, the polymer composition plays an important role. Polymers from different origins and physico-chemical features have been proposed for tissue engineering [
17]. Among them, natural polymers produced by microbes such as the polyhydroxybutyrate (PHB), as well as synthetic polymers such as the poly(lactide-co-glycolide) (PLGA), the polycaprolactone (PCL) or the polydioxanone (PDO) have been widely used for scaffold development. All of them are known to be resorbable, absorbable or bioabsorbable when implanted in the living body. However, their differences in the crystalline morphology may determine its hydrolytic degradation behaviour.
In vitro experiments are relevant to test and compare the performance of biopolymeric materials and assess their durability [
18,
19]. When the aqueous solution penetrates into the polymer, it swells incrementing the dimensions of the interface of contact, which promotes further degradation. The mechanism of degradation of polyesters under abiotic aqueous environments takes place through hydrolysis of the ester bonds, auto-catalysed by carboxylic groups, exponentially increasing along the exposure time [
20]. These microstructural changes induce the formation of macroscopic pores or cracks, the loss of monomeric and oligomeric species, thus reducing the mass of the polymers and finally decomposing their architecture until bioassimilation or excretion [
21]. Accordingly, after a successful growing and spreading of the cells into the scaffold, polyester-based scaffolds are expected to degrade, disintegrate and be assimilated by the patient.
Characterisation studies of the in vitro degradation of the most used polyesters for biomedical applications in bulk have been reported, with respect to its water-uptake rate, degradation kinetics and mechanisms, morphology and physico-chemical properties [
22]. As electrospun scaffolds, some characterisation studies of in vitro degradation generally consider the monitoring of mass-loss and molar-mass along with hydrolytic exposure. The in vitro behaviour under physiological conditions for PLGA [
14,
23,
24], PDO [
25], PCL [
26] and PHB [
27] scaffolds has been individually assessed. These studies are usually carried out in the first stages of immersion and complete degradation or disintegration is not commonly addressed. Therefore, a comparison of the disintegration behaviour of these polymers under identic simulated conditions may offer a broader vision of their applicability, having in mind the expected lifetime for the future specific application [
10].
The aim of this study was, therefore, to compare the in vitro degradation behaviour of PLGA, PCL, PDO and PHB scaffolds from an experimental and comprehensive perspective, under simulated physiologic conditions until complete disintegration. For this purpose, electrospun scaffolds were subjected to ultra-pure water and phosphate buffer solution (PBS) at 37 °C. Accordingly, a set of analytical techniques such as pH measurement, gravimetry, size-exclusion chromatography (SEC), differential scanning calorimetry (DSC) and field-emission scanning electron microscopy (FE-SEM) was proposed to point out the appropriate indicators to monitor and compare the in vitro hydrolytic degradation profiles of these polyester-based scaffolds for biomedical purposes.
4. Discussion
Given the polyester nature of the scaffolds, the feasible ester bond scission and the generation of shorter polymer segments with new carboxyl groups and acidification of the media during immersion may take place [
10]. In this line, due to the different polymers analysed, the changes in the molar mass are expected to depend on the chemical structure, but also on the scaffold crystalline morphology. As well, the variation of the fibrous structure of the scaffolds may be correlated to the above-cited behaviour. For a proper comparison, the overview of the degradation features of the different analysed scaffolds is gathered in
Table 6.
The PLGA fibres swelled as a function of the immersion time, reducing the inter-fibre distance and tending to close the pores required for satisfactory cell adhesion and proliferation. The mainly amorphous morphology of this polymer may have contributed to rapid diffusion of the aqueous solvent into the fibres. Meanwhile, the hydrolytic degradation of the polymer molecules occurred from the very beginning of immersion, following a random scission pattern, as suggested by the increase in the polydispersity index. Consequently, the molar mass dramatically decreased and low molar mass compounds were released from the scaffold, promoting acidification of the surrounding media. Finally, the scaffold collapsed and released glycolic and lactic acid monomers and oligomers that were completely dissolved in the hydrolytic media.
The PDO scaffolds kept the fibrous morphology along the degradation until disintegration. The fibre diameter slightly diminished during immersion and small cracks perpendicular to the axis of the fibres were perceived. The presence of the ether bond in the backbone may have promoted the more progressive degradation behaviour of this material. The typical cold-crystallisation of this polymer disappeared along the immersion and an imperfect crystalline population was developed, which induced higher resistance than PLGA to coalesce and collapse. Nonetheless, the fibre erosion and breakage resulted in the progressive molar mass loss, mass-loss and the reduction of the pH due to the release of low molar mass species, as produced from the preferential bond scission of the ester linkages.
The morphology of the PCL fibres remained mainly unaffected along the immersion. The average fibre diameter slightly varied and the fibrous morphology persisted until disintegration. Crystallinity was developed in this scaffold due to the increase of mobility of short hydrolysed segments, aided by the plasticizing effect of water along with the temperature above the glass transition. As a consequence of having more crystalline domains with higher lamellar thickness, PCL was more capable of preventing the structure of its fibres from degradation. The molar mass followed an almost linear diminishing trend until complete disintegration. The polydispersity increased, suggesting a random chain scission phenomenon that resulted in dissimilar segment sizes during degradation.
The hydrolytic behaviour of the PHB fibres revealed that the mass, the pH, the fibre diameter and the fibrous morphology remained constant until disintegration. Moreover, the crystallinity degree did not significantly varied during immersion. The highly stable crystalline structure of the PHB stimulated a slight and progressive chain scission process along the immersion, as suggested by the evolution of the molar mass distributions. The broad multi-modal behaviour of PHB remained almost unchanged as a function of the immersion time. However, the peak correlated to high molar mass segments decreased, while that of low molar mass polymer chains increased. This behaviour suggested preferential degradation of high molar mass domains. Overall, the PHB scaffolds were found to be the most stable of the evaluated materials, which revealed the slowest degradation behaviour when subjected to physiologic conditions.
5. Conclusions
The hydrolytic degradation patterns of scaffolds based on polyesters such as poly(lactide-co-glycolide) (PLGA), polydioxanone (PDO), polycaprolactone (PCL), and polyhydroxybutyrate (PHB) were monitored under in vitro simulated physiologic conditions. The PLGA and PDO scaffolds exhibited a short-term degradation performance, while those of PCL and PHB revealed a long-term and progressive degradation profile. Further evaluation of the behaviour of the scaffolds during immersion revealed that they possess different mechanisms of degradation, in which the decrease of the molar mass is strictly correlated to the polymer composition, but also to the scaffold crystalline structure, which will determine its subsequent biomedical application.
The results of this study may serve as a reference point in the design and selection of polyester-based electrospun scaffolds for biomedical applications, from the perspective of an adequate balance between the durability and degradation pattern under physiologic conditions.