pH-responsive drug delivery system based on hollow silicon dioxide micropillars coated with polyelectrolyte multilayers
© Alba et al.; licensee Springer. 2014
Received: 9 May 2014
Accepted: 8 July 2014
Published: 21 August 2014
We report on the fabrication of polyelectrolyte multilayer-coated hollow silicon dioxide micropillars as pH-responsive drug delivery systems. Silicon dioxide micropillars are based on macroporous silicon formed by electrochemical etching. Due to their hollow core capable of being loaded with chemically active agents, silicon dioxide micropillars provide additional function such as drug delivery system. The polyelectrolyte multilayer was assembled by the layer-by-layer technique based on the alternative deposition of cationic and anionic polyelectrolytes. The polyelectrolyte pair poly(allylamine hydrochloride) and sodium poly(styrene sulfonate) exhibited pH-responsive properties for the loading and release of a positively charged drug doxorubicin. The drug release rate was observed to be higher at pH 5.2 compared to that at pH 7.4. Furthermore, we assessed the effect of the number of polyelectrolyte bilayers on the drug release loading and release rate. Thus, this hybrid composite could be potentially applicable as a pH-controlled system for localized drug release.
KeywordsHollow micropillars Porous silicon Drug delivery pH-responsive Controlled release Doxorubicin
Micro- and nanoporous structures based on the electrochemical etching of porous silicon have attracted much attention in medical and biotechnological applications owing to their biodegradability, nontoxicity and versatile physico-chemical properties, including surface functionality, size and porosity [1–5]. The combination of electrochemical etching and microfabricaton techniques have also enabled the fabrication of neatly defined and monodispersed structures with a precise control on particle dimensions and shape, which can be critical for eliminating variability, improving pharmacokinetics and adapting microscale features in several bioapplications [6–9]. Particularly, hollow silicon dioxide (SiO2) micropillars exhibit remarkable advantages such as high chemical and mechanical stability, tunable size and functional modifiable surface [10, 11]. These 3D structures are obtained from silicon macropores produced on lithographically pre-patterned silicon wafers . The conformal growth of thermal SiO2 opens the way for the formation of inverted structures [10, 13]. The hollow volume of micropillars can be loaded with active species, such as drugs, bioactive agents, enzymes and antibiotics. Furthermore, a differential inner/outer functionalization can activate the external surface in order to facilitate the interaction with species grafted on the external side .
Compared to conventional form of dosage, micro- and nanomaterial-based drug delivery systems have many advantages, such as reduced release rate, minimized harmful side effects and improved therapeutic efficiency [7, 14, 15]. However, the premature release of active species from the cargo-loaded micropillars can represent a drawback. Hence, a triggered and prolonged release of guest molecules upon specific stimuli may be desired. This stimulus for the drug delivery system can be induced by physical , chemical  or biogenic signals . In this context, polyelectrolyte multilayer (PEM) has been widely explored to create coatings on the surface of a number of inorganic structures for the controlled delivery of drugs [19–23]. The PEM assembly is based on the layer-by-layer (LbL) approach which involves alternative adsorption of oppositely charged polyelectrolytes to create multilayer architectures in a conformal manner [24–26]. By the incorporation of appropriate responsive polyelectrolytes, the PEM can allow the controlled release of active agents on the basis of stimuli such as pH , temperature  or ionic strength . Particularly, pH-sensitive systems are of great interest in drug delivery due to the variations in pH that the human body exhibits. For instance, the gastrointestinal tract exhibits pH ranging from acidic in the stomach (pH 2) to basic in the intestine (pH 5 to 8). And compared to healthy tissues and the bloodstream (pH 7.4), most cancer and wound tissues constitute an acidic environment (pH 7.2 to 5.4) . pH-responsive PEM films contain ionizable groups which exhibit volume changes in response to variations in pH and facilitate drug delivery control .
The polyelectrolyte pair comprising poly(allylamine hydrochloride) (PAH) and sodium poly(styrene sulfonate) (PSS) has been extensively investigated for drug delivery applications due to their remarkable sensitivity to pH and improved biocompatibility [20, 32]. The deposition of the first layer of cationic polyelectrolyte PAH on the internal sidewalls of hollow micropillars is favoured by the negative charge of the SiO2 surface above the isoelectric point (pH 2 to 3) . Then, the anionic PSS is deposited onto PAH by electrostatic attraction. Furthermore, to facilitate the infiltration of the polyelectrolytes inside the pores and obtain a uniform surface coating without pore blockage, a multivalent salt such as CaCl2 can be added to the aqueous polyelectrolyte solution. The presence of multivalent salts causes a much stronger shrinking of the polyelectrolyte chain owing to a higher attraction between charged monomers along the chain [34, 35]. The infiltration of the drug inside the polyelectrolyte multilayer can also be assisted by electrostatic attractive forces. The negative charge of the most external PSS layer gives extra electrostatic attraction to positively charged drugs, such as doxorubicin hydrochloride (DOX). DOX is a chemotherapeutic agent widely used in the treatment of a number of tumours, such as breast, lung or ovarian cancers [36, 37]. Its inherent fluorescence gives DOX an additional imaging capability which makes it a remarkable theranostic agent [14, 38–40].
Herein, we present the combination of SiO2 micropillars with PEM coating as an approach to develop new functional materials for sustained release of drug molecules. The hollow micropillars are used as reservoirs for doxorubicin and the PAH/PSS coating as a pH-responsive switch. The polyelectrolyte multilayer on the interior surface prevents the premature release of the drug and enables an enhanced use of the hollow volume by increasing the loading capacity. The effect of the number of PAH/PSS layers in the drug loading and release is also investigated.
Hydrofluoric acid (HF, 40%), N,N-dymethylformamide (DMF), buffered hydrofluoric acid (BHF) and tetramethylammonium hydroxide (TMAH, 25%), PAH (Mw 58,000) and PSS (Mw 70,000) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Acetate buffer (ABS) pH 5.2 and phosphate buffer (PBS) pH 7.4 solutions were also obtained from Sigma-Aldrich. Doxorubicin hydrochloride was obtained from the European Pharmacopoeia (Strasbourg, France). All other chemicals used in the experiments were obtained from commercial sources as analytical reagents without further purification. Milli-Q water (Millipore, Billerica, MA, USA) with a resistivity of 18.2 MΩ cm was used throughout the study. Boron-doped (p-type) silicon wafers (1 0 0) and resistivity 10 to 20 Ω cm were supplied by Si-Mat (Kaufering, Germany).
Fabrication of SiO2 micropillars
Polyelectrolyte multilayer coating
PAH/PSS multilayer coating was deposited by alternately exposing the internal side of the micropillar sample to solutions of PAH and PSS (1 mg mL−1 in CaCl2 0.5 M) for 20 min each in an ultrasonic bath (E in Figure 1). After the deposition of each polyelectrolyte, the sample was thoroughly washed twice in Milli-Q water for 5 min each. This sequence was repeated until obtaining the desired number (4, 8 or 12) of PAH/PSS bilayers.
The morphology and structure of the macroporous silicon and subsequent silicon dioxide micropillars were characterized by scanning electron microscopy (SEM) using a FEI Quanta 600 environmental scanning electron microscope (FEI, Hillsboro, OR, USA) operating at an accelerating voltage between 15 and 25 kV. The micropillars were also morphologically characterized by transmission electron microscopy (TEM) using a JEOL 1011 (JEOL Ltd., Akishima-shi, Japan) operating in dark-field mode at 80 kV. Confocal laser scanning microscopy images were taken using a Nikon Eclipse TE2000-E inverted microscope, equipped with a C1 laser confocal system (EZ-C1 software, Nikon, Tokyo, Japan). A 488-nm helium-neon laser was used as excitation source for DOX-loaded micropillars. The emission was collected through a 590 ± 30 bandpass emission filter (red channel). All fluorescence images were captured using a 5-megapixel CCD. The concentrations of DOX were determined using a spectrofluorometer (PTI Quantamaster 40, Photon Technologies International, Edison, NJ, USA) at an exciting wavelength of 480 nm.
DOX loading and pH-responsive drug release
Doxorubicin was loaded inside the PEM-coated micropillar, as well as in bare SiO2 samples. To perform the drug loading, the micropillar samples were exposed to a solution of DOX 1 mg mL−1, adjusted to pH 2.0 with HCl 1 M, for 20 h in the dark (F in Figure 1). Then, DOX solution was adjusted to pH 8.0 with NaOH 0.1 M and further stirred for 2 h (G in Figure 1). The drug-loaded samples were washed three times in water at pH 8 for 10 min each. The amount of released DOX in solutions of pH 7.4 (phosphate buffer) and 5.2 (acetate buffer) was monitored over time (up to 24 h) at an exciting wavelength of 480 nm (H in Figure 1).
Results and discussion
In summary, an organic/inorganic hybrid drug delivery system was developed based on SiO2 hollow micropillars internally coated with multilayers of PAH/PSS by the LbL technique. Confocal fluorescence microscopy showed a uniform PEM coating and a successful loading of the model drug doxorubicin into the polyelectrolyte matrix. The interaction between polyelectrolyte multilayers and DOX molecules is significantly dependent on the pH for the loading and release of active agents. Thus, the release rate of DOX at pH 5.2 was found to be higher than that at pH 7.4. The effect of the number of PAH/PSS bilayers should be also considered in the drug loading. The DOX loaded was significantly higher in the PEM-coated micropillars than in those without polyelectrolytes. This system has great potential in applications of localized and targeted drug delivery.
sodium poly(styrene sulfonate)
transmission electron microscopy
scanning electron microscopy
buffered hydrofluoric acid
This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under grant No. TEC2012-34397 and by the Catalan authority - AGAUR 2014 SGR 1344.
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