Alginate Sphere-Based Soft Actuators

Abstract

Alginate hydrogels offer distinct advantages as ionically crosslinked, biocompatible networks that can be shaped into spherical beads with high compositional flexibility. These spherical architectures provide isotropic geometry, modularity and the capacity for encapsulation, making them ideal platforms for scalable, stimuli-responsive actuation. Their ability to respond to thermal, magnetic, electrical, optical and chemical stimuli has enabled applications in targeted delivery, artificial muscles, microrobotics and environmental interfaces. This review examines recent advances in alginate sphere-based actuators, focusing on fabrication methods such as droplet microfluidics, coaxial flow and functional surface patterning, and strategies for introducing multi-stimuli responsiveness using smart polymers, nanoparticles and biologically active components. Actuation behaviours are understood and correlated with physical mechanisms including swelling kinetics, photothermal effects and the field-induced torque, supported by analytical and multiphysics models. Their demonstrated functionalities include shape transformation, locomotion and mechano-optical feedback. The review concludes with an outlook on the existing limitations, such as the material stability, cyclic durability and integration complexity, and proposes future directions toward the development of autonomous, multifunctional soft systems.

Keywords: alginate; hydrogel sphere; smart materials; soft actuators; stimuli-responsive polymers.

Figure 1

Fabrication strategies and evolution pathways for alginate sphere-based actuators. Arrows indicate logical transitions and/or relationships between methods and their role in realising programmable, multifunctional soft actuators.

Figure 2

Functionalisation strategies for and integrated capabilities of alginate sphere-based soft actuators.

Figure 3

Modelling relationships and design strategies for alginate sphere-based actuators. (a) Swelling kinetics model: sphere radius increases over time based on diffusion coefficient $D$ and initial size $R_0$, informing design of systems with fast responses. (b) Axial force generation in braided architectures: force output varies with internal swelling pressure, braid angle $\theta$ and cross-sectional area $A$, supporting mechanical optimisation of McKibben-type actuators. (c) Photothermal actuation: temperature rise in response to increased light intensity ($I$) and filler absorption coefficient ($\alpha$) enables programmable, wireless activation in optically triggered systems. (d) Bilayer curvature: mismatch in strain and modulus across layers governs bending deformation, useful for shape-morphing and folding actuators.

Figure 4

Representative application demonstrations of alginate sphere-based actuators. (a) NIR-triggered self-folding Janus microrobots for encapsulation and delivery (scale bar: 200 µm) and microrobot manipulation along a selected route (scale bar: 50 µm). Robot follows preplanned trajectory. Red circles denote target destinations and circles turn blue as robot passes over them [94]. (b) Biomorph soft actuators mimicking tardigrade tun formation with microwave-driven contraction (scale bar: 50 µm) [134].

Figure 5

Schematic illustration of a magnetic microsphere scaffold (MMS)-based microrobot designed for targeted stem cell delivery [135].

Figure 6

Strategic landscape of alginate actuator development.