Hybrid Actuation in Wearable Systems

• Hybrid actuation is a system that integrates a soft, fluid-filled bladder within a rigid shell to convert pressurization into controlled, cylinder-like force.
• The design simplifies modeling by localizing deformation and achieving efficiency up to 77% in experimental tests using both latex and custom elastomer bladders.
• Optimized for short-stroke, high-force applications, this approach underpins wearable knee alignment devices by balancing compliant interfaces with robust force delivery.

Hybrid actuation, in the sense developed for a gait augmentation wearable, denotes an architecture in which the sealed metal chamber of a conventional hydraulic or pneumatic cylinder is replaced by a soft, fluid-filled bladder housed inside a rigid shell. When the bladder is pressurized, the shell constrains its deformation so that force is delivered primarily through a well-defined open-end interaction area, allowing the actuator to behave in a cylinder-like manner while retaining a compliant output interface. In the reported implementation, the concept was first validated with latex party balloons and then realized as a customized elastomer actuator integrated into a lower-limb wearable for knee alignment correction (Wan et al., 2020).

1. Core architectural concept

The core idea is to place a soft bladder inside a rigid shell cavity with an open end. Under fluidic pressurization, the bladder expands until it fills the cavity and then pushes against the open end to produce compressive force. In the initial design studies, the bladder was a latex party balloon; in the wearable-oriented implementation, it was replaced by a custom-molded elastomer bladder (Wan et al., 2020).

The rigid shell is not a secondary packaging element but the defining mechanical component of the architecture. Once the bladder has expanded to fill the cavity, the shell constrains further deformation and localizes the effective pressure transmission to the open-end interface. As a result, the output force is governed primarily by internal pressure and effective interaction area rather than by the full distributed stress field in the soft material. This distinguishes the design from many purely soft actuators in which the soft body simultaneously acts as pressure vessel and transmission path.

A central implication of this arrangement is that detailed modeling of soft-body deformation can be avoided at the gross actuator level. The shell effectively “captures” the deformation of the bladder and converts it into a fixed interaction geometry. The nonlinearities of the soft material, leakage, friction, shell compliance, and hysteresis are then lumped into an efficiency term rather than modeled explicitly.

2. Governing relations and actuator behavior

Once the cavity is filled, the actuator is well approximated by the classical pressure–area relation with an efficiency factor,

$F = \eta P A,$

where $P$ is the internal fluid pressure, $A$ is the effective interaction area at the shell’s open end, and $\eta \in (0,1]$ lumps losses due to bladder compliance, leakage, friction between bladder and shell, shell deformation, and material hysteresis (Wan et al., 2020).

The same efficiency can be written as

$\eta = \frac{F_{\text{out}}}{PA},$

with complementary loss fraction

$L = 1 - \eta.$

In balloon-in-shell experiments, efficiency was low before the cavity was fully taken up, but after pre-pressurization it increased with pressure. Over the range $30$–$60$ kPa, the fitted loss fraction was approximately

$L(P) = -0.005P + 0.522,$

with $P$ in kPa, yielding

$P$0

At $P$1 kPa this gives $P$2, consistent with the measured value of about $P$3.

The architecture is explicitly designed for short stroke. In benchtop block-force validation, shell travel was limited to about $P$4 mm, and the wearable also targeted short-stroke compressive loading. Effective stiffness around block can be interpreted as

$P$5

and because shell motion is small and the cavity is essentially filled in the operating region, the apparent stiffness is high. Volumetric compliance,

$P$6

is dominated at low pressure by remaining clearance volume, but drops once the cavity is filled, at which point force tracking approaches cylinder-like behavior. The energy per compression stroke is

$P$7

and with a few millimeters of stroke and forces around $P$8 N, the energy remains modest, on the order of $P$9–$A$0 J. This suits alignment correction rather than large-displacement assistance.

3. Materials, geometry, and construction

For rapid prototyping, the internal bladder was implemented with common thin latex party balloons. These were preconditioned by ten inflate/deflate cycles to stabilize elasticity, since the first inflations permanently alter the stress–strain response. The rigid shells were 3D-printed plastic cylinders or shell structures, enabling quick and low-cost iteration (Wan et al., 2020).

The engineered actuator used a customized elastomer bladder fabricated by two-stage molding with approximately $A$1 mm wall thickness to improve durability and safely withstand pressure. In the wearable version, the top surface included a “pop-up” feature that contacted the leg directly, removing the need for a sliding shell at the output and reducing overall stack thickness. The mounting shell formed the base, while the compliant elastomer surface served as the force-delivery interface.

The benchtop test cavities explored multiple cross-sectional shapes—circle, triangle, square, and rectangle—all with the same area as a circle of radius $A$2 mm, giving

$A$3

Square and rounded-rectangle cavities slightly outperformed the triangle and circle, which guided the wearable design toward rounded rectangles in order to reduce stress concentrations and improve efficiency. Pneumatic actuation with compressed air was used throughout, with standard tubing connected to the bladder. Balloons were reported as extremely low-cost and suitable for concept screening but not durable, whereas the molded elastomer significantly increased robustness.

4. Experimental characterization and measured performance

The block-force test rig fixed the lower shell to a frame and connected the upper sliding shell to a load cell. The bladder was loosely inserted and then pressurized in $A$4 kPa increments up to $A$5 kPa. At each pressure level, force was measured three times, with stroke limited to $A$6 mm to emulate block-force conditions (Wan et al., 2020).

In the balloon-in-shell proof-of-concept, measured forces ranged from about $A$7 N to $A$8 N across $A$9–$\eta \in (0,1]$0 kPa. Efficiency was initially low, about $\eta \in (0,1]$1–$\eta \in (0,1]$2 before $\eta \in (0,1]$3 kPa, because part of the input pressure was spent filling the cavity. Above roughly $\eta \in (0,1]$4 kPa, efficiency increased approximately linearly with pressure and reached about $\eta \in (0,1]$5 at $\eta \in (0,1]$6 kPa. For the common-area test geometry, the ideal force at $\eta \in (0,1]$7 kPa was

$\eta \in (0,1]$8

while the measured value of about $\eta \in (0,1]$9 N corresponded to $\eta = \frac{F_{\text{out}}}{PA},$0, consistent with the fitted trend.

The engineered hybrid actuator showed markedly higher performance. At only $\eta = \frac{F_{\text{out}}}{PA},$1 kPa, the measured block force exceeded $\eta = \frac{F_{\text{out}}}{PA},$2 N, and the force loss was about $\eta = \frac{F_{\text{out}}}{PA},$3 relative to the classical prediction, implying $\eta = \frac{F_{\text{out}}}{PA},$4. Geometry variants with identical effective area produced nearly identical force–pressure curves, confirming that once the cavity is filled, the effective area governs force and corner rounding mainly affects stress concentration and efficiency rather than the fundamental scaling law. The force–pressure relation in the operating range was essentially linear.

Detailed dynamic bandwidth was not reported. However, repeatability was supported by triplicate measurements at each pressure step, and the engineered elastomer’s low loss at $\eta = \frac{F_{\text{out}}}{PA},$5 kPa suggests low hysteresis in the intended operating range. This suggests that the architecture is better interpreted as a compact, short-stroke force generator than as a long-stroke soft continuum actuator.

5. Integration into a gait augmentation wearable

The actuator was incorporated into a knee brace architecture composed of two lightweight “X”-shaped structures, one wrapping the thigh and one the shank. Each leg carried six actuators: one pair at the knee sides, one pair at the distal thigh, and one pair at the proximal shank. These actuators compressed laterally against the limb to generate corrective moments that influence knee alignment (Wan et al., 2020).

Actuation was pneumatic, and the intended coordination strategy was selective pressurization of different actuators during different gait phases. The compliant elastomer output interface provided direct soft-to-skin contact, improving comfort and distributing pressure. Low operating pressure, at or below $\eta = \frac{F_{\text{out}}}{PA},$6 kPa, was presented as a safety feature, while the short-stroke, high-force behavior matched the requirements of bracing and alignment correction rather than large-amplitude limb assistance.

For knee alignment correction, selective pressurization of lateral actuators at the thigh, knee, and shank was described as producing a 3–4 force system to generate corrective moments about the knee during gait or at rest. The engineered actuator’s block force of more than $\eta = \frac{F_{\text{out}}}{PA},$7 N at no more than $\eta = \frac{F_{\text{out}}}{PA},$8 kPa was reported as more than sufficient for the intended wearable task, which in turn suggested that the interaction area could potentially be reduced to improve comfort and power economy. User trials and closed-loop gait control were not reported; portable pneumatics and closed-loop control with valves and sensing were stated to be under development.

6. Comparative position, trade-offs, and design guidelines

Relative to purely soft pneumatic actuators, the hybrid shell–bladder concept targets a different operating regime. Many soft pneumatic actuators were described as delivering less than $\eta = \frac{F_{\text{out}}}{PA},$9 N at $L = 1 - \eta.$0–$L = 1 - \eta.$1 kPa, while notable reinforced designs could reach about $L = 1 - \eta.$2 N at $L = 1 - \eta.$3 kPa. By contrast, the balloon-based hybrid actuator produced about $L = 1 - \eta.$4–$L = 1 - \eta.$5 N at $L = 1 - \eta.$6–$L = 1 - \eta.$7 kPa, and the engineered version exceeded $L = 1 - \eta.$8 N at $L = 1 - \eta.$9 kPa (Wan et al., 2020). The important point is not only higher force, but the recovery of cylinder-like pressure utilization in a compliant package.

Relative to traditional cylinders, the actuator avoids precision sliding seals and rods because the soft bladder itself provides sealing and compliance. The trade-off is that the design is intentionally short-stroke. Force scales with effective area at fixed pressure, so the stated sizing guideline is

$30$0

An explicit example given was that with $30$1, $30$2 kPa, and $30$3, the predicted force is approximately $30$4 N. Increasing force without increasing pressure therefore requires increasing area, whereas reducing bulk requires decreasing area while preserving sufficient force for the clinical task.

Several design constraints and failure modes were identified. Bladder rupture can occur from overpressure or stress concentration at sharp corners, motivating rounded rectangles and wall thickness of about $30$5 mm in the wearable implementation. Shell deformation reduces efficiency and therefore requires sufficiently stiff shells. Friction and wrinkling between bladder and shell can increase pre-fill losses, so smooth internal finishes and limited clearance are desirable. Efficiency at very low pressure is inherently reduced until the cavity is taken up by the bladder.

A common misconception is that the actuator is simply a soft pneumatic chamber placed in a rigid case. The reported results indicate a more specific principle: the rigid shell defines the pressure–force interface, and the soft bladder supplies sealing and compliance. This suggests that the innovation lies less in combining soft and rigid materials per se than in partitioning their mechanical roles so that soft nonlinearities are absorbed into an efficiency term while the overall force law remains close to that of classical fluid power.