2026-08-28
Stanford and MIT grow a vine robot open-loop around an object, then fasten the tip into a tensile loop. They lift a 74.1 kg person; peak growth pressure is 16.95 kPa.
A grasp has two jobs. First the mechanism has to wind itself into a stable configuration around the object. Then it has to hold that configuration under load without crushing something fragile. Almost every gripper keeps one morphology for both. Open-loop designs (fingers, continuum arms, soft jaws) have a free tip and can thread through clutter, but holding force comes from bending stiffness, so strong and gentle fight each other. Closed-loop designs (slings, straps) can carry load in pure tension with near-zero bending stiffness, which is what heavy-yet-fragile objects need. Their tip is pinned, so they cannot thread under an object sitting on a table. A person usually has to wrap the strap by hand.
Mechanical Engineering groups at Stanford and MIT treat this as topology. Relative to the object, the mechanism is open-loop or closed-loop. Use each topology for the stage it is good at, and close the loop in between. They call the method loop closure grasping.
The mechanism is a base, a linkage, and a tip. Open-loop: the tip is free, and with enough bending degrees of freedom it can follow any path wider than its cross-section. Closed-loop: the tip is grounded to the same rigid body as the base, and the object sits inside the loop. An appendix argument says an open-loop holder needs bending stiffness to resist pull-out; a closed loop can put the entire load into tension. More bending stiffness concentrates contact pressure at the two touchdown points.
Three steps: grow around the object while open, fasten the tip to the base without changing shape, then deflate and hold in tension. The hardware is a vine robot, a pneumatically grown soft beam that everts from its tip. Internal material peels out at the tip, so the contact surface barely slides and the body can squeeze through a gap narrower than its own diameter. Deflated, the membrane is a sheet: high tensile strength, almost no bending stiffness. The base is a pressurized box with a motorized winch. The tip fastener is a corrugated clamp-winch whose circular segments exponentiate holding force through capstan friction.
Two module sets. Large-scale vines are 70-denier nylon with a TPU coat; small-scale vines are LDPE at about 10 MPa tensile strength. Theoretical single-loop limits are 623 kg (tip-winch retraction) and 8.43 kg (membrane yield). Steering is preforming plus obstacle-aided growth. There is no active bending actuator.
The paper is demonstration-led. The numbers that matter are gap width, payload, and contact pressure.
A 6.8 kg kettlebell comes out of a cluttered bin through a gap of about 17 mm; the vine is 26 mm across. Four vines weave a net around a 178 mm ball. A single vine threads a ring or a bucket handle into a Hopf link that cannot be pulled apart without breaking a loop; the authors present this as a first in robotic grasping. From 3.0 m away the vine closes on a box and hauls it back, having grown past 6.7 m, about nine times the 0.335 m base.
Two large-scale vines lift a 74.1 kg, 170.2 cm volunteer about 25 cm off a bed. Growth runs at 24.1 kPa, then 13.8 kPa after the far side, then full depressurization and winding from both ends. Under a 79.4 kg manikin the peak pressure during growth is 16.95 kPa, below the >26.7 kPa reported for medical transfer slings. Two small-scale vines lift a 5.9 kg watermelon and a 1.0 kg glass vase without damage. Tightening the closed loop also cinches a pile of pipes; feeding material from one winch to the other rotates a cylinder in mid-air.
| Demo | Number |
| Large-scale theoretical load | 623 kg (tip-winch limit) |
| Small-scale theoretical load | 8.43 kg (membrane) |
| Human lift | 74.1 kg, 25 cm |
| Peak growth pressure | 16.95 kPa (79.4 kg manikin) |
| Long-range pull | 3.0 m, vine >6.7 m |
Soft grippers have been stuck on a slogan: compliant means weak. This design splits the two properties across topology. Grow like a tendril, hold like a sling. Vine robots already know both tricks, because eversion wants an open tip and a deflated membrane is a tensile strap. For care work, rescue, and heavy industry, that split is more useful than one more underactuated finger. Clamps, winches, and preforming are parts a shop can build.
This is mechanism design, not learned grasping. No vision loop, no policy network. Anyone hunting a foundation grasping model will not find the weights here.
The authors say the tip reaches the fastener along a preformed path, with no active steering. Robust 3D bending at this scale is still open. Contact mechanics under load are almost unanalyzed. Comfort used a Likert scale; the paper never reports the scores. 623 kg is a theoretical bottleneck; the heaviest real lifts are a 74 kg person and a 6.8 kg kettlebell. Manikin pressure covers growth, not the lift. Growth trajectories are easy to knock off course, and vines are hard to instrument for closed-loop position control. Two vines under a person, the authors note, may not be the comfortable arrangement.