McKibben artificial muscles are one of the most pragmatic contractile actuators, offering performances similar to skeletal muscles. The McKibben muscles operate by pumping pressurized fluid into a bladder constrained by a stiff braid so that tensile force generated is amplified in comparison to a conventional hydraulic ram. The need for heavy and bulky compressors/ pumps makes pneumatic or hydraulic McKibben muscles unsuitable for microactuators, where a highly compact design is required. In an alternative approach, this article describes a new type of McKibben muscle using an expandable guest fill material, such as temperature-sensitive paraffin, to achieve a more compact and lightweight actuation system. Two different types of paraffin-filled McKibben muscles are introduced and compared. In the first system, the paraffinfilled McKibben muscle is simply immersed in a hot water bath and generates isometric forces up to 850 mN and a free contraction strain of 8.3% at 95C. In the second system, paraffin is heated directly by embedded heating elements and exhibits the maximum isometric force of 2 N and 9% contraction strain. A quantitative model is also developed to predict the actuation performance of these temperature sensitive McKibben muscles as a function of temperature.