Despite the huge interest that magnetic nanoparticles (MNPs) have gained in multidisciplinary fields, their application in the medical field has become a subject of attraction in the last decade because of their unique magnetic properties (superparamagnetism and high susceptibility) and physical properties (biocompatibility and stability) [1], [2], [3]. Magnetic nanoparticles are easily engineered nanoparticles formed with the support of a magnetic field. The introduction of external magnetic fields to MNPs causes the domain to align with the field, which is responsible for the nanoparticles gathering around the field (magnetism). However, when MNPs are dissociated from the magnetic field, the particles lose their magnetic properties. This important feature of MNPs (superparamagnetism) prevents the agglomeration or cluster formation of nanoparticles in the biological system [4]. Although, the application of MNPs biomedical applications have eased the diagnosis and treatment of life-threatening diseases such as cancer due to their ability to remotely control the distribution of therapeutic agents through the transfer of particles to the target site, MNPs still exert some toxicity effects on cells. These effects can be prevented by coating the surface of MNPs with organic materials such as polymers and surfactants, to further prevent agglomeration and oxidation of the particles [5], [6]. Most MNPs are made up of two components; magnetic component (iron and cobalt) and a chemical component for functionality [7]. The functionalized surface of MNPs with suitable organic chemical components such as surfactants, enhance the biocompatibility, physical and chemical stability of MNPs [8].

The development of nanocomposites has assisted greatly in overcoming the limitations associated with the control of elemental composition of microlithicand monolithic composites [9]. Nanocomposites are classified into ceramic matrix nanocomposites, metal matrix nanocomposites and polymer matrix nanocomposites, based on their mode of processing and the matrix or host material used [9]. Polymer nanocomposites are nanosized materials made from the dispersion of organic or inorganic materials inside a polymer matrix (nanofillers) with the intent of combining the properties of the nanofillers and the polymers to enhance the macroscopic properties (strength, hardness and toughness) of the nanocomposites [10]. The shapes and forms of nanoparticles play a major role in the formation of polymer nanocomposites with nanorods and nanoplates having a better effect than spherical nanoparticles[10]. Caprolactone monomer is a seven-membered ring cyclic ester which is usually obtained by Baeyer-Villiger oxidation of cyclohexanone using caproic acid as the starting material (Fig. 1) [11], [12]. There are isomers of caprolactones, namely α-caprolactones, β-caprolactones, γ-caprolactones and δ-caprolactones, which are all stereo-chemically chiral. Apart from these isomers, there is an ε-caprolactones which is achiral and is easy to polymerize than the chiral isomers [13], [14]. ε-caprolactones can be mechanically controlled and this property made it to be suitable for several applications than the other caprolactones isomers [11].

Poly (ε-caprolactone) (PCL) is a hydrophobic, semi-crystalline, biodegradable polyester formed from crude oil [15]. It is an important nanocarrier because of its special multifaceted features such as structural stability, non-toxicity, crystallinity and biodegradability [16]. The solubility of PCL in organic solvents, its low melting point, remarkable blending ability with co-polymers and longer biological degradation when compared with other polyesters (PGA, PLGA or PLA) makes it a preferred polymer of choice in the biomedical field [15], [17], [18]. PCL is made by ring-opening reactions of caprolactones in the presence of initiators or catalysts (Fig. 2). One of the catalysts that has been used is the combination of supercritical carbon dioxide and Candida antarctica lipase B (Fig. 2) [19]. Imidazolium-based ionic liquid 1-n-butyl-3-methylimidazolium heptachlorodiferrate has also been reported to be effective in the ring opening reaction of caprolactone to form its polymer [20].

Under biological conditions, PCL is degraded by hydrolysis or with the help of microbes; however, its degradation in aqueous environment is facilitated by free hydroxyl radicals [21]. The surface area to volume ratio, crystallinity, structural and morphological formations of PCL are crucial factors influencing the degree of degradation of PCL [22], [23]. During PCL hydrolysis, there is chain scission of the ester groups in the material bulk. Moreover, when PCL absorbs water molecules into their material bulk, the molecular chains become hydrated and the ester bonds are sliced, shortening the polymer chain lengths. The reduction in chain length results in the formation of carboxylic acid (capronic). The rearrangement of the sliced chains is ensued with the removal of the carboxylic acid. However, if capronic is not removed, the polymer becomes acidic causing autocatalysis of the polymer which further enhances PCL degradation [23].

Since PCL is hydrophobic in nature, there is a limited rate of water absorption, which slows down the rate of PCL degradation. It is believed that the hydrolytic degradation of PCL takes about 2–3 years for degradation to be complete [23], [24]. Thus, PCL degradation with microbial enzymes is considered as an alternate route as it is faster and energy efficient [25]. This route of polymer degradation is initiated through the adsorption of enzymes onto the material surface resulting in the loss of material from the surface bulk or reduction in the molecular weight [23], [26]. In most cases, enzymatic degradation of PCL occurs at the chain ends (where chain mobility is higher) through the rupturing of the ester bonds into monomers or water-insoluble oligomers [24], [26]. Enzymes from bacterial and fungi have shown potential for polymer degradation. The two most widely used enzymes (lipases) that accelerates the degradation of PCL are Rhizopus delemer and Rhizopus arrhizus [24].

Magnetic nanocomposites are complex nanomaterials formed from the incorporation of magnetic nanoparticles into a matrix (magnetic or non-magnetic) with a wide range of functional properties. Magnetic nanocomposites possess the properties (mechanical, chemical, electrical or magnetic) of the matrix materials as well as the nanofillers [3], [27]. In the biomedical field, it is apparent that strengthening polymers with nanofillers enhances the properties of the polymer and improves the properties of the newly formulated nanocomposites by combining the strength of the individual components [28]. For instance, in an investigation by Hedayatnasab and colleagues, the surface coating of superparamagnetic iron oxide nanoparticles (SPIONS) with PCL improved the magnetic behavior of SPIONS owing to the systematic arrangement of the magnetic moment present in the crystal [16]. In addition, entrapping MNPs in acceptable polymers such as PCL prevents nanoparticles from oxidization, enhance their chemical stability and decrease vulnerability to leakage, due to surface coating of the nanoparticles, by generating repulsive forces that will equilibrate the van der Waals attractive forces and the magnetic force [29].

Based on the properties of magnetic nanoparticles (particularly, iron oxides), and the unique nature of PCL, more research have been carried out on the assessment of biomedical application of polymer nanocomposites and magnetic nanoparticles [10], [30], however, little attention has been given to the role of PCL in the enhancement of magnetic nanoparticles in biomedical applications. Hence, the current work comprehensively reviews the various biomedical applications and their composites. The research to carry out this literature review was based on electronic resources. To achieve this aim, the Pubmed database was used as the main source of information, complemented with official sources from the European Medicines Agency (EMA) and the United States Food and Drug Administration (FDA). Apart from these sites, other information sources such as Research Gate and Google Scholar were used.