Design, Modelling and Characterisation of a Miniature Fibre-Reinforced Soft Bending Actuator for Endoluminal Interventions

Xiangyi Tan; Aoife McDonald-Bowyer; Danail Stoyanov; Agostino Stilli

cs.ROMar 25, 2026

Design, Modelling and Characterisation of a Miniature Fibre-Reinforced Soft Bending Actuator for Endoluminal Interventions

Authors: Xiangyi Tan, Aoife McDonald-Bowyer, Danail Stoyanov, Agostino Stilli

Organizations: All authors are with the UCL Hawkes Institute, University College London, WC1E 6BT, UK.

Abstract

Miniaturised soft pneumatic actuators are crucial for robotic intervention within highly constrained anatomical pathways. This work presents the design and validation of a fibre-reinforced soft actuator at the centimetre scale for inte- gration into an endoluminal robotic platform for natural-orifice interventional and diagnostic applications. A single-chamber geometry reinforced with embedded Kevlar fibre was de- signed to maximise curvature while preserving sealing integrity, fabricated using a multi-stage multi-stiffness silicone casting process, and validated against a high-fidelity Abaqus FEM using experimentally parametrised hyperelastic material models and embedded beam reinforcement. The semi-cylindrical actuator has an outer diameter of 18,mm and a length of 37.5,mm. Single and double helix winding configurations, fibre pitch, and fibre density were investigated. The optimal 100 SH configuration achieved a bending angle of 202.9° experimentally and 297.6° in simulation, with structural robustness maintained up to 100,kPa and radial expansion effectively constrained by the fibre reinforcement. Workspace evaluation confirmed suitability for integration into the target device envelope, demonstrating that fibre-reinforcement strategies can be effectively translated to the centimetre regime while retaining actuator performance.

Figures & tables

Fig. 1: Cross-sectional dimensions and structure of the actuator. The green region denotes the cap, the yellow the Kevlar fibre winding, the thin grey sheet the inextensible fibreglass layer, the dark grey the air chamber, and the semi-transparent grey the silicone body. Symbols correspond to Table I . (a) Side view, (b) Front view, (c) Cross-sectional view.

Symbol	Definition	Value
$D_{i}$	Internal air chamber diameter	$14~\mathrm{mm}$
$D_{O}$	Outer geometry diameter	$18~\mathrm{mm}$
$\delta_{f}$	Silicone cover at flat interface	$1.6~\mathrm{mm}$
$\delta_{c}$	Silicone cover at curved side	$0.2~\mathrm{mm}$
$\delta_{t}$	Internal thickness between fibre and chamber	$0.3~\mathrm{mm}$
$\delta_{r}$	Offset from flat surface to datum origin	$2~\mathrm{mm}$

TABLE I: GEOMETRICAL PARAMETERS OF THE ACTUATOR

Fig. 2: Fabrication of actuator with Geometry A: (a) Mould assembly for the inner chamber; Ecoflex 00–50 is injected to form the chamber wall. (b) A Kevlar fabric inextensible layer, pre-soaked in Ecoflex 00–50, is bonded to the flat surface of the half-cured inner chamber, and Kevlar fibre are wound through it. (c) The reinforced inner chamber is placed into the outer chamber mould, and Ecoflex 00–50 is injected. (d) Fully cured chamber structure incorporating inner chamber, inextensible layer, fibre reinforcement, and outer chamber. (e) Smooth-Sil 960 is poured into the cap mould, and the chamber structure is dipped to form the cap, with an air tube inserted. (f) Completed actuator with Geometry A.

Fig. 3: Illustration of the bending angle measurement method used in this study. Representative deformation of 30 DH actuator under FEM increasing internal pressure (0, 30, 60, 100 kPa).

Fig. 4: FEM simulation results for Geometry A actuator configurations. (a)–(e): SH fibre windings with turn counts of 9, 18, 30, 50, and 100, respectively. Pressures: (a) 62 kPa, (b) 93 kPa, (c)–(e) 100 kPa. (f)–(i): DH fibre windings at 100 kPa. (f) 30 turns with fibre radius halved (0.05 mm), modelled following [ 10 ] . (g) 30 turns, (h) 50 turns, (i) 100 turns DH. (j) DH actuator front view; (k) SH actuator front view.

Figure Ref.	Actuator Configuration	Bending Angle $\theta$ ( ∘ )
(a)	9 turns SH at 62 kPa	61.12
(b)	18 turns SH at 93 kPa	86.12
(c)	30 turns SH at 100 kPa	90.00
(d)	50 turns SH at 100 kPa	106.04
(e)	100 turns SH at 100 kPa	148.80
(f)	30 turns DH at 100 kPa (fibre radius halved)	98.72

TABLE II: Bending angle of configurations shown in Fig. 4 .

Fig. 5: Comparison of average radial expansion for selected fibre winding configurations.

Fig. 6: FEM simulation results for the device. (a) 30-turn DH; (b) 100-turn DH. (c) 100-turn DH with modified side grooves

Fig. 7: Representative Experimental bending angle at 100kPa. (a) 30 DH actuator. (b) 100 SH actuator. (c) 30 DH with the Device. (d) 30 DH with the Device embedded camera.

Fig. 8: Hysteresis analysis for the 100-turn SH winding actuator: bending angle–pressure curve (forward and backward) and hysteresis ratio–pressure curve.

Fig. 9: Cross-sectional dimensions and structure of the actuator geometry; Fabrication of actuator with Geometry B: Symbols correspond to Table. (a) Side view (b) Front view (c) Cross-sectional view. (d) Assembly of outer chamber moulds. (e) Kevlar fibre wound around 5 mm diameter metal rods (one clockwise, one counter-clockwise) are inserted into the assembled outer chamber moulds, and Ecoflex 00–50 is injected. (f) Cured outer chamber with fibre embedded. (g) Assembly of inner chamber moulds with 4 mm diameter rods; Ecoflex 00–50 is injected to form the inner chamber, enclosing fibre between outer and inner layers. (h) Cured tubular chamber structure. (i) Formation of a spherical cap to seal one end. (j) Dipping to form the cap and seal the actuator, with air tube inserted. (k) Completed actuator with Geometry B.

Fig. 10: FEM simulation results for Geometry B under various configurations: (a) actuator alone, elongation-dominated deformation at 100 kPa; (b) actuator integrated into device. (c) actuator with embedded fibreglass inextensible layer. (d) fabric-reinforced actuator integrated into device.

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Soft Pneumatic Actuators for Soft Robotics: A Motion-Based Review of Actuation Mechanisms and Performance Trade-offs

Mohammed Abboodi

Soft Pneumatic Actuators Soft Robotics Pneumatic Actuation

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Soft pneumatic actuators are widely used in soft robotics because they can produce large motions while remaining compliant enough to interact safely with objects, environments, and the human body. However, their performance is not solely determined by pressure. Instead, the response depends on the way the actuator is built, including the shape of its chambers, the placement of reinforcements, the use of folds, material stiffness, and the constraints that guide its deformation. As the literature has expanded, it has become more difficult to determine which mechanism is most suitable for a given application and which reported results can be compared across studies. This review examines soft pneumatic actuators according to the design strategies used to generate four motion classes: linear, bending, twisting, and omnidirectional actuation. For each class, it analyzes the structural features that define the deformation path, including braid angle, fold geometry, fiber orientation, chamber arrangement, structural asymmetry, and internal constraint layers. It then discusses how the design choice affect motion output, force generation, air demand, repeatability, durability, fabrication difficulty, and robotic integration. The review further identifies key conditions that must be considered when selecting or comparing actuators, including pressure, loading condition, actuator size, pneumatic supply, and hysteresis This approach helps explain why actuators with similar motion outputs may differ substantially in design requirements, pneumatic demand, and practical suitability. It also highlights the design priorities needed for compact, efficient, repeatable, and deployable soft pneumatic systems in wearable, biomedical, and mobile robotic applications.

Abstract

Soft Pneumatic Actuators

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