Implantable, three-dimensional (3D) polymer scaffolds have been drawing attention from various disciplines because of their unique properties [1–6]. There are several methods of fabricating biocompatible and biodegradable polymer scaffolds [7–12]. These implantable 3D scaffolds, made of various materials, such as lactide and glycolide polymers [1, 5, 6], poly(glycerol sebacate) , silk fibroin , and poly(propylene fumarate) [8, 13, 14], have been adapted for numerous biomedical studies. Efficient drug delivery, with a controlled drug-release manner at the targeted site, is one of the major applications for polymer scaffolds in biomedical research [15–24]. Numerous polymer scaffold candidates have been developed as drug-releasing matrices for in vitro and, potentially, in vivo use. For example, chitosan/hydroxyapatite composite scaffolds have shown a reduced initial burst, followed by a constant release of tetracycline hydrochloride in vitro . Calcium alginate beads coated with silk fibroin showed improved shell stability and controlled release of BSA and FITC-Insulin model drug proteins . Hydroxyapatite scaffolds have been reported to successfully release ceftriaxone drug molecules in a controlled manner due to their well-defined porous surface structure . The drug delivery and release applications of hydrogel scaffolds, further developed by infusion with chitosan, have been recently reviewed . Combining the advantages of each unique scaffold system may develop the optimized attributes that are critical in drug delivery applications: drug-loading capacity, controllable drug release rate; flexibility of drug loading; feasibility of drug delivery and release imaging; and quantitative assessment of loaded and released drug molecules on the scaffold.
The use of 3D poly(propylene fumarate) (PPF) scaffolds as a drug-release matrix is relatively new, but is a promising application in several aspects. First of all, the highly porous structure of PPF scaffolds provides sizable drug-loading spaces. Furthermore, a photo-crosslinked PPF scaffold is able to retain its initial porosity and mechanical properties for 18–32 weeks in vitro , and its degradability can be controlled by adjusting fabrication parameters, such as PPF molecular weight and photoinitiator content . This ability to control PPF degradation may benefit certain applications that require the extended drug release. Although PPF scaffolds are generally recognized for their potential use in bone replacement [27, 28], PPF scaffolds have other advantages such as their biocompatibility, tunable porous surface area, and degradation time, which may promote them as an optimized platform to release drugs in a controlled manner. However, there are also a few innate limitations of the PPF scaffold for drug delivery applications. The direct incorporation of water-soluble drug molecules into the scaffold is challenging due to the hydrophobicity of PPF. Loaded drugs might be degraded by certain fabrication processes, for instance, the salt porogen leaching step . Therefore, an alternative approach for PPF scaffolds to carry over a drug molecule is required.
There have been efforts to utilize nanoparticles as drug carriers and to embed a drug-nanoparticle complex in a scaffold matrix . However, simultaneous detection and quantification of released drug-nanoparticles from these scaffold matrices has not been studied in detail, since most of the detection methodologies rely on only a single method of detection (e.g., fluorescence imaging or absorption spectroscopy), which may be difficult to apply to the detection of discrete nanoparticles and drug molecules.
Rapid improvements in magnetic resonance imaging (MRI) instrumentation and techniques have led to increased spatial resolution (up to 50–100 μm for rodent in vivo imaging). In addition, a variety of novel nanoparticles designed for MRI can enhance the MRI contrast even further, making it possible to image cellular and molecular events non-invasively and co-register these events with 3D anatomical structures. Recently, MRI was also suggested for studying drug release non-invasively from liposomes  using iron oxide-based nanoparticles and gadolinium-based agents [31, 32]. In addition to the more traditional MRI contrast mechanisms, which rely on the longitudinal relaxation (T1) and transverse relaxation (T2) of water protons , a new type of MRI contrast has been developed that relies on direct chemical exchange of protons with bulk water. A variety of organic and organo-metallic compounds have a sufficient number of protons (called “exchangeable protons”) with suitable chemical exchange rates and chemical shifts to be detected sensitively. Once a sample is inside the magnetic field, these exchangeable protons can be “magnetically tagged” using a radiofrequency pulse, called a saturation pulse, which is applied at the exchangeable proton’s resonance frequency. The tagged protons exchange with the protons of surrounding water molecules, and, consequently, reduce the MRI signal from the water protons and enhance the MRI contrast. The exchangeable protons are replaced with fresh protons and the same saturation process is repeated. After several seconds of this process, the effect becomes larger and larger (a so-called “saturation amplification”), and very low concentrations of agents can be detected through the water signal. Hence, these agents are called Chemical Exchange Saturation Transfer (CEST) contrast agents. The main advantage of CEST-MRI is that bio-organic molecules, such as proteins, can be used for increasing the MRI contrast [35–37]. These can be used to image controlled drug release with minimal invasiveness in real time.
Here we report on the use of porous PPF scaffolds loaded with doxorubicin (DOX)-coated iron oxide and manganese oxide nanoparticles as a vehicle for sustained anti-cancer drug release. Using nanoparticles as a drug carrier contributes to more efficient loading of drug molecules onto the PPF scaffold. It also allows monitoring the release of drug-nanoparticle complexes from the PPF scaffold surface via changes in MRI contrast and absorption spectra in a media containing scaffold pieces. In addition, as a proof-of-concept experiment, it was demonstrated that the diamagnetic CEST contrast agent, protamine sulfate (PS), can also be used directly, without nanoparticle carriers, to monitor release from the PPF scaffolds by MRI. This report about drug-delivering 3D PPF scaffolds, with a sustained release rate and bimodal imaging (fluorescence and magnetic resonance) capabilities, suggests this new system may potentially be used in various biomedical applications.