Free-standing microchamber arrays as a biodegradable drug depot system for implant coatings

Highlights

• A PLLA microchamber arrays films are proposed to deliver water-soluble substances.

• Rhodamine B was completely released from PLLA microchambers in vitro within 13 days.

• Low frequency ultrasound damages and detaches PLLA microchambers.

• Free-standing microchamber arrays can be applied as endovascular stent cover.

Abstract

The combination of an efficient encapsulation method of small water-soluble substances with a stimuli-responsive release of defined quantity remains a challenging task. A novel drug delivery system (DDS) representing a free-standing PLLA microchamber arrays film and its application as the cover for implantable endovascular stent are reported in this work. The proposed DDS preparation method is consisting of a patterned polydimethylsiloxane (PDMS)-stamp dip-coated into a polymer solution followed by drug loading and sealing it by polymer pre-coated substrate. It was shown that using 1 wt% PLLA solution is optimal for obtaining microchamber arrays with an individual cargo capacity of 2.88 × 10⁻⁹ µg, which was successfully loaded by model drug substance Rhodamine B. Rhodamine B was completely released in vitro during 13 days in PBS at 37 °C by diffusion. It was demonstrated that low-frequency ultrasound (LFUS, 20 kHz) allows triggering RhB release due to microchamber damage and detachment of individual PLLA microchambers over time. LFUS exposure time up to 25 s led to RhB release of up to 8.4 × 10⁻⁴ µg (approximately 55%) from microchambers located on the flat substrate; up to 5.2 × 10⁻⁴ µg from microchambers located on the stent with using a simplified vessel model. Furthermore, the free-standing printed PLLA microchamber arrays were demonstrated to be applied as endovascular stent cover that can be used for complementary pharmacological effect, for example, triggered local delivery of anticoagulants.

Introduction

During the past few decades variety of micro- and nano drug delivery systems (DDS) (i.e. capsules [1], [2], [3], particles [4], [5], micelles [6], [7], [8], liposomes [9], [10], hydrogels [11], [12], [13] or fibers [14]) are being intensively studied in order to improve therapeutic effectiveness of encapsulated drugs and substances for personalized drug therapy and regenerative medicine. However, the combination of an efficient encapsulation of small water-soluble substances by a relatively easy and fast method, it releasing during a long time together with a stimuli-responsive releasing of its defined quantity when it necessary remains a challenging task.

The encapsulation of water-soluble substances is possible within the DDS which have a “core-shell” structure. Emulsification and electrospray methods have proved to be successful for encapsulation of water-soluble substances, although, the concentration of chemicals, encapsulated substance, surface-active substance and its quantities and time of emulsification or electrospray process have to be determined for every new encapsulated substance [5], [15], [16], [17], [18].

Preparation of identical microchambers for encapsulation of small water-soluble substances based on microcontact printing of reusable micropatterned stamp was recently purposed for overcoming such disadvantages and provide a possibility of site-specific release of defined low quantity substance amount [15]. The previously approach of microchambers preparation based on layer-by-layer self-assembly method [21], [22] was found ineffective for long-term small water-soluble molecules encapsulation due to its high permeability and time-consuming [16], [17]. Thus, hydrophobic biodegradable polymers such as polylactic acid can be used instead of non-biodegradable polyelectrolyte multilayers. Polylactic acid (PLA) is one of the most extensively used polymer for biomedical application due to its biodegrade ability, biocompatibility and non-toxic nature [18], [19].

Apart from encapsulation of small hydrophilic molecules, a stimuli-responsive release of encapsulated substances in predetermined areas is another challenge. External stimulus for activation and controlled release of substances such as ultrasound [3], [20], [21], laser [2], [22], [23], light [24], [25], magnetic field [26], [27], pH [6], [7], [28] and redox reactions [8], [29], [30] have been previously reported. Ultrasound is one of the most suitable mechanical stimulus to activate substance release from microchambers due to its thermal and non-thermal effects [31], [32], [33], [34]. It could be even considered as a biologically benign stimulus for extracorporeal cargo system activation, since it is already widely employed as a diagnostic and therapeutic method in biomedical field, due to it is long penetration depth and noninvasive nature. For that purpose, high-intensity focused ultrasound (HIFU, >1 MHz) [35], [36] and low-frequency ultrasound (LFUS, 20–100 kHz) [37], [38], [39] might be used as release stimuli. HIFU can precisely expose and heat predetermined small areas, while LFUS can penetrate deeper into tissues and initiate drug release more effectively, due to lower acoustic impedance at lower frequencies [31], [32], [40]. The amount of drug released increases with LFUS intensity, however intensity higher than 3 W/cm² could lead to cell and tissue damage [34], [41].

Previously was showed, that microchambers can be used as a drug delivery system enabling prolonged subaqueous storage of small hydrophilic salts and molecules [20], [27], [48]. These microchambers allow triggered release by ultrasound exposure and can be used for delivery of the necessary amount of hydrophilic model drugs. This system can be surgically introduced into the body as a micron-sized small coating of implants and vascular stents or at the site of tumor resection.

Metal endovascular stents can be implanted within coronary and peripheral arteries; however, its metal-based nature can cause thrombosis [42]. Combined stent functions with complementary pharmacological effect might in the future prevent failure of vascular repair [43]. For example, heparin is a hydrophilic anticoagulant drug widely used for treatment of peripheral vascular disease (i.e. acute coronary syndrome, arterial fibrillation, deep-vein thrombosis and pulmonary embolism) and has profound effects on anti-cancer activity [44]. After oral ingestion heparin is digested and becomes ineffective, therefore an immediate need for controlled heparin release from devices that can be implanted in and around the desired arterial structures [45] is required. Previously the methods of coating an implantable device having a plurality of depots formed in a surface thereof for delivery of therapeutic substances was reported [46]; however, this method does not allow triggered on demand delivery of therapeutic substances and only allows unpredictable release from the bulk of polymer composites.

In this work, we aim to prepare a free-standing biodegradable microchamber arrays film loaded with dye model of drug and to deposit it as a cover for implantable cardiac stent for local drug delivery.