Hydro Muscle:

WPI Popovic Labs Hydro Muscle ICRA2016 Slides Movie , November 11, 2016
WPI Popovic Labs Hydro Muscle video material for ICRA 2016 , November 10, 2016
Hydro Muscle Test 5 2014, October 17, 2014

Hydro Muscle, invented in Popovic Labs, is a soft compliant actuator that is powered by pressurized fluids. The rubber core stores energy when pressurized, while the outer sheathing confines radial expansion and allows actuation in axial direction. In contrast to the PAM McKibben artificial muscles, Hydro Muscles elongate axially, stiffen radially, and do not bulge when pressurized. When depressurized, they contract axially, and soften radially. Hydro Muscles exhibit better efficiency than McKibben’s muscles. Substantial radial expansion and deflation of McKibbens elastomer introduces substantial energy loss in transfer from the source to the actuated load. Furthermore, Hydro Muscles can easily reach strains of 3.0, while McKibben muscles typically have a maximum strain of about 0.4. Biological muscles have an average maximum strain of 1.0 .

More efficient and more biologically realistic in terms of contractile behavior than well-known McKibben Muscle, Hydro Muscle is now top contender for the best artificial muscle in the world. According to WPI patent office, it is also the best compensated invention in WPI history based on received IP license fees.


Hydro Muscles are linear actuators resembling ordinary biological muscles in terms of active dynamic output, passive material properties and appearance. The passive and dynamic characteristics of the latex based Hydro Muscle are addressed. The control tests of modular muscles are presented together with a muscle model relating sensed quantities with net force. Hydro Muscles are discussed in the context of conventional actuators. The hypothesis that Hydro Muscles have greater efficiency than McKibben Muscles is experimentally verified. Hydro Muscle peak efficiency with (without) back flow consideration was 88% (27%). Possible uses of Hydro Muscles are illustrated by relevant robotics projects at WPI. It is proposed that Hydro Muscles can also be an excellent educational tool for moderate-budget robotics classrooms and labs; the muscles are inexpensive (in the order of standard latex tubes of comparable size), made of off-the-shelf elements in less than 10 minutes, easily customizable, lightweight, biologically inspired, efficient, compliant soft linear actuators that are adept for power-augmentation. Moreover, a single source can actuate many muscles by utilizing control of flow and/or pressure. Still further, these muscles can utilize ordinary tap water and successfully operate within a safe range of pressures not overly exceeding standard water household pressure of about 0.59 MPa (85 psi).

Publications and Presentations:

1. Matthew Bowers, Chinmay Harmalkar, Ankur Agrawal, Abhishek Kashyap, Jonathan Tai, and Marko Popovic (2017) “Design and test of biologically inspired multi-fiber Hydro Muscle actuated ankle,” Proceedings of 2017 IEEE International Workshop on Advanced Robotics and its Social Impacts, March 8-10, 2017, University of Texas at Austin, Austin, TX, USA

2. Saivimal Sridar, Corey J Majeika, Phillip Schaffer, Matthew Bowers, Seiichiro Ueda, Andrew J Barth, Jon L Sorrells, Jon T Wu, Thane R Hunt, and Marko Popovic (2016) “Hydro Muscle - a novel soft fluidic actuator,” 2016 IEEE International Conference on Robotics and Automation (ICRA), pp 4014-4021

3. M.B. Popovic, C. Onal, G. McCarthy, N. Corso, D. Effraimidis, and B. Jennings (Publication date 2015/12/17) “Actuators and Methods of Use” (Hydro Artificial Muscles). United States Patent and Trademark Office, Assignee Worcester Polytechnic Institute. Serial No.: 62/011,830. Filed: June 13, 2014. US Patent 20,150,359,698, 2015

4. G. McCarthy, D. Effraimidis, B. Jennings, N. Corso, C. Onal and M. B. Popovic (2014) "Hydraulically Actuated Muscle (HAM) Exo-Musculature" in "Robot Makers: The future of digital rapid design and fabrication of robots" (RoMa) Workshop, the 2014 Robotics: Science and Systems Conference, Berkeley, CA, July 12, 2014.

MQP Reports:

Hydro-Muscle Actuated Exo-Legs for Therapy and Video Gaming
Nicholas Earle Benson, Steven Dean Ruotolo Primary Advisor: Marko Popovic (PH/RBE/BME), Co-advisor: Eben Cobb

Fluidic Muscle Ornithopter
Alphan Canga, Michael Kurt Delia, Alexander M Hyman, Angela Marie Nagelin
Primary Advisor: Marko Popovic (PH/RBE/BME), Co-advisor: Cadgas Onal (ME, RBE)

Legged Robot Using Hydro-Muscles
Daniel Patrick Coffey, Peter Starek
Primary Advisor: Marko Popovic (PH/RBE/BME), Co-advisor: Cagdas Onal (ME, RBE)

HydroDog: A Quadruped Robot Actuated by Soft Fluidic Muscles
Daniel Fitzgerald, Thane Hunt, Andres Leiro
Primary Advisor: Marko Popovic (PH/RBE/BME), Co-advisors: Cagdas Onal and Michael Gennert

Hydro Muscle Control System
Andrew Barth, Jonathan Sorrells, Seiichiro Ueda, Jonathan Wu
Primary Advisor: Marko Popovic (PH/RBE/BME), Co-advisors: Fred J. Looft and Dmitry Berenson

Hydro Artificial Muscle (HAM) Exo-Musculature
Daniil Effraimidis, Brian Jennings, Gregory McCarthy, Nicholas Corso
Primary Advisor: Marko Popovic (PH/RBE/BME), Co-advisor: Cagdas Onal (ME/RBE)
2013/14 Robotics Engineering (RBE) 2nd place, Provost's MQP Award Runner up

Current Researchers:

Marko B. Popovic

Past Researchers:

Ankur Agrawal, Andrew Barth, Nicholas Earle Benson, Dmitry Berenson, Matthew Bowers, Alphan Canga, Eben Cobb, Daniel Patrick Coffey, Nicholas Corso, Michael Kurt Delia, Daniil Effraimidis, Daniel Fitzgerald, Michael Gennert, Chinmay Harmalkar, Thane Hunt, Alexander M Hyman, Brian Jennings, Abhishek Kashyap, Andres Leiro, Fred J. Looft, Corey J Majeika, Gregory McCarthy, Angela Marie Nagelin, Cagdas Onal, Steven Dean Ruotolo, Phillip Schaffer, Jonathan Sorrells, Saivimal Sridar, Peter Starek, Jonathan Tai, Seiichiro Ueda, Jonathan Wu