Tuning the sphere-to-rod transition in the self-assembly of thermoresponsive polymer hybrids

Highlights

• Natural–synthetic polymer hybrids form micelles near body temperature.

• A transition from spheres to rods depends on the molecular parameters.

• Controlling the micelle structure may be critical in biomedical applications.

Abstract

Nano-scale drug delivery systems have undergone extensive development, and control of size and structure is critical for regulation of their biological responses and therapeutic efficacy. Amphiphilic polymers that form self-assembled structures in aqueous media have been investigated and used for the diagnosis and therapy of various diseases, including cancer. Here, we report the design and fabrication of thermoresponsive polymeric micelles from alginate conjugated with poly(N-isopropylacrylamide) (PNIPAAm). Alginate–PNIPAAm hybrids formed self-aggregated structures in response to temperature changes near body temperature. A structural transition from micellar spheres to rods of alginate–PNIPAAm hybrids was observed depending on the molecular weight of PNIPAAm and the polymer concentration. Additionally, hydrogels with nanofibrous structures were formed by simply increasing the polymer concentration. This approach to controlling the structure of polymer micelles from nanoparticles to fibrous hydrogels may be useful in applications in drug delivery and tissue engineering.

Introduction

The design and fabrication of nano-scale biomaterials is important for applications in medicine. In particular, multifunctional nanoparticles have potential applications in medical diagnoses and therapies. Nanoparticles, classified as organic, inorganic, and organic/inorganic hybrids, have been developed for diagnostic and/or therapeutic purposes [1], [2], [3]. Self-assembly, phase separation, emulsification, and spray drying are typical methods for the fabrication of organic nanoparticles [4], [5], [6]. Among these, self-assembled nanoparticles have recently attracted much attention as potential delivery vehicles, because they enable the formation of nano-structures without the use of an excipient solvent or detergent. Moreover, their core–shell structures can readily encapsulate bioactive molecules, including anticancer drugs, into the core.

Various self-assembled systems based on amphiphilic molecules – including small molecules, peptides, and polymers – have been reported [7], [8], [9]. Small amphiphilic molecules can form vesicles, lamellae, bicontinuous phases, spheres, and cylinders depending on pH, ionic strength, and type and concentration of surfactant [10]. As materials science and technology develops, several types of amphiphilic polymers – such as star-, graft-, and block-type copolymers – have been designed and synthesized, leading to the production of self-assembled nano-structures with various sizes and shapes [11]. Recently, physical cross-linking has been used to generate self-assembled structures useful in many biomedical science and engineering areas without using ‘conventional’ chemical cross-linking methods, such as radical polymerization, chemical reactions, and irradiation [12]. Additionally, manufacturing process controls and the resulting characteristics of inorganic materials with various shapes and sizes, and their morphological transitions, have been reported [13], [14], [15], [16], [17]. However, there have been few studies on the structural control of self-assembled polymeric systems.

While formation of self-assembled structures by various natural and synthetic polymers has been examined [5], [18], few are promising for clinical application, mainly due to the toxicity of the polymers. We thus, chose alginate as a base polymer due to its excellent biocompatibility, low toxicity, and low immunogenicity, which have led to its use in many biomedical applications to date [19]. Nanoparticles and hydrogels based on alginate can be fabricated simply by the addition of divalent cations, such as Ca²⁺, which can be used as an injectable vehicle or matrix for cell and drug delivery in a minimally invasive manner [20], [21]. However, alginate does not form self-assembled structures unless hydrophobic moieties are introduced to the alginate backbone. We thus introduced poly(N-isopropylacrylamide) (PNIPAAm) as a hydrophobic domain to alginate, and synthesized hybrid-type polymers that can undergo structural transitions depending on temperature change. PNIPAAm has a lower critical solution temperature (LCST) of ∼32 °C and shows a hydrophilic-to-hydrophobic transition above its LCST [22]. PNIPAAm demonstrates distinctive, reversible functionality simply by changing the temperature, and has been used to prepare stimuli-responsive systems [23], [24], [25].

We hypothesized that hybrid polymers composed of alginate and PNIPAAm could form self-assembled structures near body temperature in a reversible manner and could undergo structural transitions from spheres to rods, depending on molecular parameters (Fig. 1). We investigated the effects of design parameters – such as concentration, molecular weight, and hydrophobicity of the hybrid polymers – on self-assembled nanostructures. Amine-terminated PNIPAAms of various molecular weights were synthesized and coupled to the alginate backbone by carbodiimide chemistry, and various physiochemical properties of the hybrid polymers – such as critical aggregation concentration, size, and morphology – were investigated. Amphiphilic polymers consisting of PNIPAAm as a hydrophobic segment can load hydrophobic drugs into the core of the micelles [26]. Alginate–PNIPAAm hybrids are thus expected to be useful for the delivery of drug molecules as an injectable for diagnostic and therapeutic purposes.