Highly flexible and porous silk fibroin microneedle wraps for perivascular drug delivery

Abstract

Various perivascular drug delivery techniques have been demonstrated for localized post-treatment of intimal hyperplasia: a vascular inflammatory response caused by endothelial damages. Although most perivascular devices have focused on controlling the delivery duration of anti-proliferation drug, the confined and unidirectional delivery of the drug to the target tissue has become increasingly important. In addition, careful attention should also be paid to the luminal stability and the adequate exchange of vascular protein or cell between the blood vessel and extravascular tissue to avoid any side effect from the long-term application of any perivascular device. Here, a highly flexible and porous silk fibroin microneedle wrap (Silk MN wrap) is proposed to directly inject antiproliferative drug to the anastomosis sites while ensuring sufficient vascular exchanges. Drug-embedded silk MNs were transfer-molded on a highly flexible and porous silk wrap. The enhanced cell compatibility, molecular permeability, and flexibility of silk MN wrap guaranteed the structural integrity of blood vessels. Silk wrap successfully supported the silk MNs and induced multiple MN penetration to the target tissue. Over 28 days, silk MN wrap significantly inhibited intimal hyperplasia with a 62.1% reduction in neointimal formation.

Introduction

Intimal hyperplasia (IH) is a vascular inflammatory response caused by acute endothelial damage to the anastomosis sites, vascular grafts, or blood vessels after bypass graft surgery [[1], [2], [3]] or endarterectomy [4], angioplasty, and stent application [5,6]. IH thickens the vessel layer by the excessive proliferation of vascular smooth muscle cells (SMCs) in tunica media, narrowing the luminal area of the blood vessel [7]. Since vascular failure due to IH is typically induced between the acute phase and a few years into the chronic remodeling phase [8], immediate medical procedure is required. After this time, chronic structural changes similar to atherosclerosis occur with severe risks including stenosis, plaque calcification, and thrombosis due to the vessel rupture. Since IH can be reduced by delivering anti-proliferative drugs such as sirolimus (Sir) [9,10] or paclitaxel [11,12], various studies have investigated the treatment for IH after stent implantation or vascular surgery.

Microneedle (MN) is a promising drug delivery technology that can penetrate the tissue barrier in a minimally invasive way and deliver drugs with high efficiency without causing pain [13]. MNs have been widely applied to various tissues of the body including transdermal [[14], [15], [16], [17]], neural [18], and ocular [[19], [20], [21], [22]] tissues showing improved therapeutic effects. Among others, several cardiovascular MNs have been developed because access to the target vascular tissue is relatively difficult, thus efficient drug delivery using MNs is desired. For example, endovascular MN drug delivery techniques have been reported by penetrating luminal layers for enhanced efficacy [23,24]. However, these techniques are for only one-time delivery and unsuitable for open vascular surgeries. As another approach, perivascular MN devices including an MN cuff and a wrappable PLGA MN mesh have been developed [[25], [26], [27], [28], [29]]. These devices were mounted on the outer surface of a blood vessel by overcoming the tunica adventitia layer and may effectively deliver drug compounds to the tunica media layer. They significantly reduced IH by improving local drug delivery efficiency to the tunica media. However, the blood vessels were often deformed or damaged when relatively stiff MN cuff and PLGA MN mesh devices were applied to vascular tissue tightly [27,29] (unpublished data for PLGA MN mesh). Therefore, the tensile properties of perivascular MN devices needed to be modified to minimize the mechanical rigidity.

Perivascular MN drug delivery should overcome several challenges because the devices remain in the body for an extended period of time. Most of all, implantable devices should minimize any inflammatory reaction in the body after degradation. In addition, when blood vessels are constantly subjected to unwanted mechanical stress by the installed perivascular devices, arterial stiffening can be induced consequently, followed by adverse effects such as hemodynamic turbulent, intimal hyperplasia, and atherosclerotic symptoms [[30], [31], [32]]. While the arterial wall is continuously exposed to cyclic deformation by blood pressure, the viscoelastic property of arterial tissue relieves stress caused by the blood pulsation [33]. Since the cushioning of the arteries transforms pulsatile blood flows into steady-state flows, it is critical to maintain the distensibility of vascular tissue [34]. Due to such delicacy of the mechanical properties of the blood vessels, any possible mechanical restriction by perivascular devices should be avoided. Besides, it was previously shown that the molecular exchange between vascular tissue and the surroundings is critical for vascular integrity [[35], [36], [37]]. Blood vessels can maintain the appropriate structure by exchanging vascular proteins or progenitor cells with the surrounding environment.

Silk fibroin has shown both excellent processability and adjustability for fabricating from one- to three-dimensional structures [[38], [39], [40]]. With the versatile processibility of an aqueous silk solution, a highly porous and stretchable silk micro/nanostructure [[41], [42], [43]] can be developed by both freeze-drying and undergoing treatment in an alcoholic solution or a water vapor environment, to control the crystallinity [44,45]. Additionally, silk showed excellent cell compatibility [46,47] as well as the potential for sustained drug delivery, with controlled release of macromolecular drugs [48,49].

In this study, we propose a highly flexible and porous silk fibroin microneedle wrap (Silk MN wrap) with superior biocompatibility to deliver anti-proliferative drugs into the tunica media layer (Fig. 1a). A vascular-friendly silk MN device was developed for localized inhibition of IH with minimum alteration of the mechanical properties of the vascular tissue. An array of silk MNs loaded with anti-proliferative drug was fabricated on a flexible and porous silk wrap structure using a transfer molding method. Prior to the implantation of silk MN wrap to the vascular tissue, we evaluated its characteristics including cytotoxicity, controlled drug release, molecular permeability, vascular penetrability, and tensile stretchability. In animal studies using a rabbit injury model, silk MN wrap was conformably wrapped around the external surface of injured blood vessels to release the anti-proliferation drug (Sir) directly from the MNs into the injured vascular tissue. The animal studies confirmed the superior pharmaceutical efficacy of silk MN wrap compared to drug loaded silk wrap without use of MNs, with improved vessel patency and the maintenance of healthy tunica media structures.