Kiri-Spoon: A Soft Shape-Changing Utensil for Robot-Assisted Feeding

TL;DR

Kiri-Spoon introduces a kirigami-based, morphable spoon end-effector designed for robot-assisted feeding, blending traditional utensil form with soft-gripper function. The authors develop a spring-loaded four-bar deformation model coupled with a catenary approximation to capture geometry and actuation forces, and validate it across multiple kirigami sheets. A user study with 12 participants demonstrates that Kiri-Spoon substantially reduces spills and improves control compared to traditional utensils, while maintaining comparable transfer performance. The work offers a practical path toward robust, human-friendly robotic feeding by uniting familiar utensil shapes with soft-gripping capabilities, with future work addressing surface gaps and liquid foods.

Abstract

Assistive robot arms have the potential to help disabled or elderly adults eat everyday meals without relying on a caregiver. To provide meaningful assistance, these robots must reach for food items, pick them up, and then carry them to the human's mouth. Current work equips robot arms with standard utensils (e.g., forks and spoons). But -- although these utensils are intuitive for humans -- they are not easy for robots to control. If the robot arm does not carefully and precisely orchestrate its motion, food items may fall out of a spoon or slide off of the fork. Accordingly, in this paper we design, model, and test Kiri-Spoon, a novel utensil specifically intended for robot-assisted feeding. Kiri-Spoon combines the familiar shape of traditional utensils with the capabilities of soft grippers. By actuating a kirigami structure the robot can rapidly adjust the curvature of Kiri-Spoon: at one extreme the utensil wraps around food items to make them easier for the robot to pick up and carry, and at the other extreme the utensil returns to a typical spoon shape so that human users can easily take a bite of food. Our studies with able-bodied human operators suggest that robot arms equipped with Kiri-Spoon carry foods more robustly than when leveraging traditional utensils. See videos here: https://youtu.be/nddAniZLFPk

Figures (6)

• Figure 1: Human controlling the robot arm and Kiri-Spoon during robot-assisted feeding. The operator relies on the robot arm to pick up and carry food items. Our proposed Kiri-Spoon makes this easier by encapsulating and releasing foods within a soft kirigami structure with adjustable curvature. Using Kiri-Spoon the robot can scoop food from a bowl or pick food off a plate while wrapping around that food firmly to prevent spills.
• Figure 2: Kiri-Spoon is composed of a $2$D kirigami sheet actuated by a pulley. (Left) The food-safe sheet is cut into an ellipse with one boundary ribbon and multiple discrete ribbons. (Top) When forces are applied to the ends of the boundary ribbon, the discrete ribbons buckle and the $2$D sheet morphs into a $3$D bowl with adjustable curvature. (Right) The flexible joint enables the entire Kiri-Spoon to bend when colliding with objects (e.g., a plate or bowl).
• Figure 3: (a) Spring-loaded four-bar linkage. The dimensions $l_{x}$ and $l_{y}$ represent the principal axes of the elliptical kirigami sheet. The tensile force $F_{t}$ displaces the slider, increasing the curvature of the kirigami structure. This displacement is opposed by two springs with constants, $k_{x}$ and $k_{y}$, that model the bending stiffness of the boundary and the reaction force of the discrete ribbons, respectively. (b) Testing setup for validating our proposed deformation model. The kirigami sheets A, B, and C have a circular boundary with a radius of $23.5$ mm, while sheets D and E have an elliptical boundary with the same dimensions described in Section \ref{['sec:design']}. All sheets are made from PET, except sheet C which is made from thermoplastic polyurethane (TPU). The thickness of each sheet and the width of their discrete ribbons are specified in Table \ref{['tab:stiffness']}.
• Figure 4: Model validation results. (a)-(c) The predicted width and depth of the kirigami sheet E and the corresponding tensile force for each displacement along the x-axis. These results show that our model predictions closely approximate the actual measurements of the dimensions and tensile forces for sheet E. (d) Leave-one-out cross-validation (LOOCV) results for each sheet. The mean absolute error in the estimated tensile force was at most $0.86$ N.
• Figure 5: Objective results from the user study. In this study participants controlled a robot arm to scoop food from a bowl and pick food from a plate using feeding utensils attached to the robot's end-effector. Participants then controlled the robot to carry the food across the table and finally drop the food in another container. We compared traditional utensils (e.g., forks and spoons) to our Kiri-Spoon. We found that while users transferred similar amounts of food (Transferred Weight) and spent a similar amount of time picking the food (Picking Time) with both types of feeding utensils, they spent significantly less time orienting the end effector (Orient), had no Timeouts, and spilled the least amount of food (Spills) when using Kiri-Spoon.