Nanodiamonds/poly(vinylidene fluoride) composites for tissue engineering applications
Graphical abstract
Introduction
The inclusion of nanomaterials in polymer matrices is an area of increasing interest, since it allows to introduce or tune specific characteristics of the materials such as mechanical, electrical, optical and thermal properties to reach specific application demands [1], [2], [3]. Polymer nanocomposites have been prepared with different nanofiller structures, such as nanoparticles, nanotubes and nanofibers, among others. Carbon fillers have demonstrated large interest due to their positive impact on the physical and chemical properties of the composites, such as electrical conductivity and mechanical properties, low density and ease of recycling [4], [5].
Nanoscale diamond particles, known as nanodiamonds (NDs), were first produced in the 1960s but only by the end of the 1990s became widely interesting for various purposes [6], [7]. NDs show excellent mechanical properties, high surface area and tunable surface structures, high thermal conductivity (up to 2000 W/m.K) and electrical resistivity, optical properties and fluorescence, chemical stability and resistance to harsh environments [7], [8], [9]. Hence, NDs have been proposed as fillers for high performance thermal interface materials and are promising to solve the challenge of increasing heat dissipation for electronic devices [2], [8]. They also show large grain boundary density and low to negative electron affinity, being therefore suitable for electronic applications. NDs can enhance the performance of electrochemically active polymers, improving their cycle stability, increasing their specific capacitance and capacitance retention at fast sweep rate [10], [11]. These nanoparticles present excellent optical properties due to their transparency in the visible region of the spectrum and high refraction index [12]. The presence of a nitrogen atom next to a vacancy, nitrogen–vacancy (NV) centers in nanodiamonds, leads to fluorescence properties. Fluorescent NDs combine the advantages of semiconductor quantum dots – small size, high photostability, bright multicolour fluorescence, with non-toxicity and biocompatibility, showing therefore large potential to develop in vitro and in vivo imaging applications. Furthermore, photoluminescent NDs present quantum yields, non-blinking, non-photobleaching characteristics and long luminescence lifetimes, making them advantageous among nanoparticles and dyes [7], [9].
The use of NDs as fillers in polymeric matrices leads to nanocomposites with superior mechanical, thermal and optical properties, which together with their biocompatibility and chemical stability make these materials excellent choices for the development of biomedical applications [12], [13]. Thus, NDs have an increasing impact in drug delivery, targeted cancer therapies, biosensor, anti-viral and antibacterial treatments, surgical implants and tissue engineering [7], [12]. The drug delivery feature for ND systems is accomplished by covalent attachment or physisorption of therapeutic substances. These mechanisms together with biocompatibility and the possibility of combination with imaging techniques confers NDs high potential for therapeutic treatment [9], [12], including breast cancer and liver cancer treatments [9], [13].
Furthermore, the ND properties in combination with the ability to deliver drugs and biologically active molecules are advantageous for the reinforcement of biodegradable polymers to create multifunctional tissue engineering scaffolds [12], [14]. NDs depict surface functionalization of scaffolds to promote tissue regeneration [15]. Octadecylamine (ODA) modified nanodiamond, poly (l-lactic acid) (ND-ODA-PLLA) composites were investigated for bone tissue engineering: PLLA is biocompatible and bioresorbable, but it is not mechanically robust for load bearing implants; the addition of ND-ODA leads to hardness and Young's modulus values that are close to those of human cortical bone [16], [17].
Electroactive polymer scaffolds based on piezoelectric polymers have been proposed for active tissue engineering approaches. These materials are characterized by providing an electrical response to a mechanical excitation or vice versa. Piezoelectricity can be found in living tissues, playing a significant role in several physiological phenomena. Piezoelectric polymers are often used to bone, neural and muscle regeneration [18]. Poly (vinylidene fluoride) (PVDF) is the most used piezoelectric polymer, due to its larger piezoelectric response, and provides an ideal material platform for proving the concept of mechano-electrical transductions for tissue engineering [18], [19]. Furthermore, PVDF is characterized by chemical resistance, suitable mechanical properties and hydrophobicity [20]. Thus, the incorporation of NDs within a PVDF matrix is a promising approach to produce composites with superior thermal, electrical, optical and biological properties.
The properties of NDs are influenced by the post-synthesis treatments to chemically modify the surface. The functionalization processes provide the incorporation of functional groups that are an intrinsic part of the NDs structure, influencing their thermodynamic stability compared to bulk diamond, controlling the agglomeration and removing impurities [7], [21], [22]. The main functional groups chemically bonded to the NDs surface are carboxyls (COOH), consisting of a carbon double bonded to oxygen and single bonded to a hydroxyl group (OH). Oxygen containing groups are already partially oxidized carbon, which produces less heat upon oxidation to CO2 compared to unoxidized carbon, thereby stabilizing NDs and enabling their formation and persistence [23].
Other functional groups, such as amino, thiol and halogens can also functionalize the NDs surface. However, they require more complex procedures for the direct attachment on the diamond lattice. In these cases, the functional groups enable the immobilization of polymer chains, drug molecules, biomolecules or catalyst by simple adsorption of the functional unit or by directed covalent grafting [7]. NDs can be applied in biomedical or high performance materials, but demand a deep understanding and control of the surface properties of nanodiamond [21]. Furthermore, bacterial population proliferation can be controlled according to the reactive oxygen-containing surface groups on surfaces [24].
In this view, this work reports on the production of polymer composites based on PVDF using two types of ND nanoparticles and their morphological, structural, optical, thermal and electrical characterization. Furthermore, cell culture viability was studied with the use of pre-osteoblast cells.
Section snippets
Materials
Poly (vinylidene fluoride) (PVDF) (Solef® 1010) was supplied by Solvay. The solvent N,N-dimethylformamide (DMF, 99.5%) was purchased from Fluka. The nanodiamonds (NDs, <10 nm, 200–450 m2/g) from Sigma Aldrich, were produced by detonating carbon-containing explosives in a closed chamber [12]. ND particles were oxidized (NDox) by heating in an open-air oven at 703 K for several hours [25]. Both pristine and oxidized ND were used for the preparation of the composites.
Sample preparation
Firstly, the ND particles were
Morphology, surface charge, polymer phase, optical and thermal properties
The morphology of the composite samples and the distribution of the ND fillers within the polymer matrix were evaluated by SEM (Fig. 1). Due to the similarity of the SEM images obtained, just representative results are shown for the samples with 0.5 wt% filler content.
Fig. 1a shows the surface of the composite film with 0.5 wt% of ND, which shows the typical spherulitic structure of PVDF with small pores between spherulites as a consequence of the solidification process from high temperature
Conclusions
PVDF based composites with different types (ND and NDox) and concentration (0.1, 0.5 and 1 wt%) of NDs as fillers were produced by solvent casting in two morphologies: compact film and porous membrane.
The ND composites showed good nanofillers dispersion and no significant ND agglomerates were found, whereas NDox composites showed some nanoparticle clusters.
All composite films presented higher absorbance than pristine PVDF with maximum recorded for the sample with 1 wt% ND. All films
Acknowledgments
The authors thank FEDER funds through the COMPETE 2020 Programme and National Funds through FCT - Portuguese Foundation for Science and Technology under Strategic Funding UID/FIS/04650/2013 and projects PTDC/EEI-SII/5582/2014 and PTDC/CTM-ENE/5387/2014. The authors acknowledge funding by the Spanish Ministry of Economy and Competitiveness (MINECO) through the project MAT2016-76039-C4-3-R. SACC thanks FCT for financial support, including Investigador FCT program (IF/01381/2013/CP1160/CT0007),
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