Effect of Molecular Weight on Gelling and Viscoelastic Properties of Poly(caprolactone)-b-Poly(ethylene glycol)-b-Poly(caprolactone) (PCL-PEG-PCL) Hydrogels

Abstract

Hydrogels based on poly(caprolactone)-b-poly(ethylene glycol)-b-poly(caprolactone) (PCL-PEG-PCL) have been evaluated extensively as potential injectable fillers or depots for controlled release of drugs. Common drawbacks of these copolymer systems include instability of aqueous solutions and low mechanical strength of gels, issues which are commonly overcome by adding pendant groups to the end of the copolymer chains. Here, a systematic study of the effects of increasing polymer molecular weight (MW) is presented, utilizing PEG blocks of MW 2, 4 or 8 kDa. Triblock copolymers were prepared by the ring-opening polymerization of Ɛ-caprolactone by PEG. Copolymers prepared with PEG MW 2 kDa did not form hydrogels at any copolymer molecular weight. Copolymers prepared with PEG MW 4 kDa formed gels at MW between 11 and 13.5 kDa, and copolymers prepared with PEG MW 8 kDa formed gels at MW between 16 and 18 kDa. Copolymers with PEG block 8 kDa formed hydrogels with high viscosity (17,000 Pa·s) and mechanical strength (G' = 14,000 Pa). The increased gel strength afforded by increased molecular weight represents a simple modification of the reactants used in the reaction feed without added synthetic or purification steps. Shear-thinning of PCL-PEG-PCL triblock copolymer hydrogels allowed for injection through a standard 23G syringe, allowing for potential use as dermal fillers or drug delivery depots.

Keywords: PEG–PCL; injectable hydrogels; pseudoplastic.

Figure 1

Structure of PCL–PEG–PCL triblock copolymers prepared by ring-opening polymerization of Ɛ-caprolactone by poly(ethylene glycol) diol with stannous octoate catalyst. (a) FT-IR spectrum of entry X showing characteristic ester stretch at 1721 cm⁻¹; (b) ¹H NMR spectrum of entry X with peak assignments.

Figure 2

(a) PCL–PEG–PCL hydrogels were formed by increasing PEG block MW to 4 or 8 kDa; (b) scanning electron microscope (SEM) micrograph of a dehydrated polymer sample displays the 3D structure of the polymer without heating to form a gel; (c) the gel network resulting from heating at 50 °C for one minute.

Figure 3

Pseudoplastic behavior of PCL–PEG–PCL hydrogels was displayed by the dramatic reduction in viscosity upon increased shear rate. High zero-shear viscosities are expected to provide immobility post-injection.

Figure 4

(a) PCL–PEG–PCL hydrogel was easily loaded into a syringe; (b) the gel was easily injected through a 23G syringe.

Figure 5

(a) Viscoelastic behavior of PCL–PEG–PCL hydrogels. All samples were viscoelastic solids at zero strain (G′ > G″); (b) storage moduli of hydrogels at low strain displayed increased strength of hydrogels based on PEG 8 kDa (VIII, IX) compared to those based on PEG 4 kDa (IV, V).