Degradation-Induced Actuation in Oxidation-Responsive Liquid Crystal Elastomers

Abstract

Stimuli-responsive materials that exhibit a mechanical response to specific biological conditions are of considerable interest for responsive, implantable medical devices. Herein, we report the synthesis, processing and characterization of oxidation-responsive liquid crystal elastomers that demonstrate programmable shape changes in response to reactive oxygen species. Direct ink writing (DIW) is used to fabricate Liquid Crystal Elastomers (LCEs) with programmed molecular orientation and anisotropic mechanical properties. LCE structures were immersed in different media (oxidative, basic and saline) at body temperature to measure in vitro degradation. Oxidation-sensitive hydrophobic thioether linkages transition to hydrophilic sulfoxide and sulfone groups. The introduction of these polar moieties brings about anisotropic swelling of the polymer network in an aqueous environment, inducing complex shape changes. 3D-printed uniaxial strips exhibit 8% contraction along the nematic director and 16% orthogonal expansion in oxidative media, while printed LCEs azimuthally deform into cones 19 times their original thickness. Ultimately, these LCEs degrade completely. In contrast, LCEs subjected to basic and saline solutions showed no apparent response. These oxidation-responsive LCEs with programmable shape changes may enable a wide range of applications in target specific drug delivery systems and other diagnostic and therapeutic tools.

Keywords: ROS-responsive polymers; liquid crystal elastomers; stimuli-responsive polymers.

Figure 1.

(a) Chemical structures of the liquid crystal (LC) monomer, (1,4-bis-[4-(6-acryloyloxhexyloxy)benzoyloxy]-2-methylbenzene (RM82)), thiol spacer, (2,2′-(ethylenedioxy) diethanethiol (EDDT)), and the vinyl crosslinker, (1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO)), used to synthesize the oxidation-responsive polymers. (b) Schematic of a reactive oxtgen species (ROS)-sensitive Liquid Crystal Elastomer (LCE) disc with a patterned molecular orientation morphing from a flat configuration to a complex 3D structure in the presence of ROS.

Figure 2.

Shape change and mass change of 3D printed uniaxial strips. (a) Print schematic of single layer uniaxial strips. Print path is set to be along the short axis of the rectangular strips resulting in nematic director parallel to the short axis and perpendicular to the long axis. (b) Mass change (%) as a function of time for uniaxial strips in all four media. Test specimens in oxidative media swell significantly while those in PBS and NaOH solution showed approximately no change. (c) Normalized length (mm/mm) for uniaxial strips, parallel and perpendicular to the nematic director (n), as a function for time for strips in all four media. Films in oxidative media demonstrate anisotropic swelling with contraction along the short axis and elongation along the long axis. (d) Shape change of uniaxial strips before and after oxidation in 20% H₂O₂ and 3% H₂O₂.

Figure 3.

Effects of degradation on the viscoelastic properties of LCEs. (a) Storage Modulus (E’) as a function of temperature, comparing non-degraded films with films oxidized for 1, 2 and 7 days in 20% H₂O₂/CoCl₂ at 37 °C. (b) Respective tan delta curve as a function of temperature. (c) Storage Modulus (E’) as a function of time for LCE films after exposure to 3% H₂O₂ at 37 °C for 1, 2 and 7 days and non-degraded films. (d) Respective curve of tan delta as a function of temperature.

Figure 4.

Material characterization of degraded LCE films after drying. (a) Differential scanning calorimetry (DSC) plot of second heating cycle. Heat flow as a function of temperature for 1 mm thick films after exposure to all media for 7 days and then dried compared to untreated LCEs. (b) DSC plot of first cooling cycle. (c) FTIR ATR results showing the effect of oxidation on 1 mm thick dried LCEs after exposure to oxidative media for 7 days. SEM images compare surface topographies of oxidized LCEs to unoxidized LCEs. (d) Effects of oxidation in 20% H₂O₂. (e) Effects of oxidation in 3% H₂O₂. (f) Unoxidized LCE.

Figure 5.

Shape change of 3D printed nonuniaxial structures. (a) Print path schematic of a disc with an azimuthal alignment pattern. (b) Side view images of 3D printed discs before and after oxidation in 20% H₂O₂ and 3% H₂O₂ . LCEs swell anisotropically and morph from a flat to conical shape. (c) Normalized cone height as a function of time after exposure to all media. (d) Print path schematic of bilayer LCE strips with the print path of opposing layers directed to be ±45° to the long axis of the film. (e) Flat rectangular bilayers morph into spiral ribbons or helicoids after oxidation. (f) Number of twists as a function of time demonstrating effect of media on bilayer.