  We describe a fluidic actuator design that replaces the sealed chamber of a
hydraulic cylinder using a soft actuator to provide compliant linear
compression with a large force ($\geq$100 N) at a low operation pressure
($\leq$50 kPa) for a lower-limb wearable. The external shells constrain the
deformation of the soft actuator under fluidic pressurization. This enables us
to use latex party balloons as a quick and cheap alternative for initial design
investigation. We found that the forces exerted by the soft material
deformation are well-captured by the rigid shells, removing the necessity of
explicitly describing the mechanics of the soft material deformation and its
interaction with the rigid structure. One can use the classical Force, Pressure
and Area formula factored with an efficiency parameter to characterize the
actuator performance. Furthermore, we proposed an engineering design of the
hybrid actuator using a customized soft actuator placed inside a single shell
cavity with an open end for the compression force. Our results show that the
proposed design can generate a very high force within a short stroke distance.
At a low input pressure of 50 kPa, the exerted block force is approaching only
about 3\% less than the classical equation predicted. The actuator is fitted to
a new gait augmentation design for correcting knee alignment, which is usually
challenging for actuators made from the purely soft material.
