Coded Skeleton: Programmable bodies for shape changing user interfaces
Miyu Iwafune1
1

Taisuke Ohshima2∗

Shibaura Institute of Technology2

Yoichi Ochiai3

The University of Tokyo3

University of Tsukuba

(b)

(a)

(c)

(d)

Figure 1: Application example of Coded Skeleton. (a) Shape memory application. (b) Deformable skeleton. (c) Analyzed of a positive
Gaussian curvature. (d) Actuated button.

Abstract(a)(b)

Silicon Tube

Local beam
Ruling line

Shape Memory Alloy

We propose novel design method to fabricate user interfaces withmechanical metamaterial called Coded Skeleton. The Coded Skeleton is combination of shape memory alloys (SMA) and 3-D printed
bodies, and it has computationally designed structure that is flexible
in one deformation mode but is stiff in the other modes. This property helps to realize materials that automatically deform by a small
and lightweight actuator such as SMA. Also it enables to sense user
inputs with the resistance value of SMA. In this paper, we propose
shape-changing user interfaces by integrating sensors and actuators
as Coded Skeleton. The deformation and stiffness of this structure
is computationally designed and also controllable. Further, we propose interactions and applications with user interfaces fabricatedusing our design method.
Keywords: Shape changing user interfaces, Metamaterials, Digital fabrication
Concepts: •Human-centered computing → Interaction design
process and methods; User interface design;1

Overview

1.1Structures

A Coded Skeleton interface is a 3-D printed bodies with embedded
actuator and sensor. The bodies parameters of the Coded Skeleton
define flexibility and stiffness in a manner that it does not requirecomplex composite construction. The Coded Skeleton consists of
only a 3-D printed bodies, microcontroller, and SMAs. The SMAs
functions as not only an actuator but also a sensor. We embedded∗ Joint first authorship
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the owner/author(s). ⃝SIGGRAPH ’16 Posters, July 24-28, 2016, Anaheim, CA,
ISBN: 978-1-4503-4371-8/16/07
DOI: http://dx.doi.org/10.1145/2945078.2945096

Inserted shell(c)Stretching

Bendingin plane

Bendingout of plane

Twisting

Figure 2: (a) Coded Skeleton with shape memory alloys. (b) Coded
skeleton. (c) Deformation modes in repetitive slit pattern.

the SMAs into the structure and used silicon tubes not to melt by
SMA s heat (Figure 2 (a)).

1.2 Printed bodies
The deformation shape of the Coded Skeleton is a developable strip,and its shape can be predefined with the required stiffness and flexibility. The proposed structure is an The deformation shape of theCoded Skeleton is a developable strip, and its shape can be predefined with the required stiffness and flexibility. The proposed
structure is an improvement of repetitive slit pattern and is flexible
in the designed deformation but is stiff in other deformations. We
achieved this improvement by inserting a thin shell into the middleof this structure shown in Figure 2 (b), which prevents the structure from stretching, bending in plane, and twisting, thus preserving
flexibility in out-of-plane bending (Figure 2 (c)). In addition, we
propose Coded Skeletons on a surface, which deforms into nonzero
Gaussian curvature surfaces such as dome or saddle surfaces (Figure 3 (b)(c)).

(a)

(b)

Hinge parts

Compliant joints(d)Natural Plane

(e)

(f)
Bend in plane
Bend out of plane
StretchTwisting(c)Transform the strip

Previous pattern

Repetitive slit pattern

Proposed pattern

Coded skeleton

Figure 3: (a) Applying compliant joint into hinge parts. (b) A Structure deforms negative Gaussian curvature. (c) A Structure deforms
positive Gaussian curvature. (d) Actuation of Coded Skeleton. (e) Eigenmode and eigenvalue of coded skeleton. (f) Strain and resistance ofCoded Skeleton.

1.3

2 Actuator

Deformation

Zero Gaussian curvature surface: To evaluate deformation property, we use eigenvalue analysis proposed by [Filipov et al. 2015].
Each eigenvector (eigenmode) represents a structural deformationmode, and its corresponding eigenvalue represents an excitation
frequency that would result in the given deformation. The result
demonstrated through Figure 3 (e) show the lowest eigenmode asa developable surface and the following eigenmodes are the Nthmodes of the previous eigenmodes. The next eigenmode is twisting, however, its eigenvalue is relatively high, and thus twisting
hardly occurs. Therefore, we considered the deformation shape of
the Coded Skeleton as a developable strip (Figure 3 (e)). Furthermore, we can define the ruling line of the strip using directions oflocal beams (Figure 2 (b)). By using the pattern parameter proposed by [Taisuke Ohshima and Yamaguchi 2015], we can control
the flexibility of the Coded Skeleton.
Nonzero Gaussian curvature surface: We constructed the structure with a Coded Skeleton to realize non developable deformations
and to evaluate these structures through eigenvalue analysis. The
key idea of this structure is convex and concave hexagon (Figure3 (a)). These two structures have hinge in its vertices. A convex
hexagon has a positive Poisson s ratio representing negative Gaussian curvature such as a saddle shape, and a convex hexagon has anegative Poisson s ratio representing positive Gaussian curvature.
We applied compliant joints into hinge parts to make them function
as hinges in Figure 3 (a) and its joints enable materials to have wide
range of flexibility. Figure 3 (b)(c) shows out-of-plane deformation
for the two structures. We analyzed these two structures by using
Inventor provided by AUTODESK and used ABS resin for thisanalysis.

1.4

Analytical model

We propose an analytical model of deformation for a Coded Skeleton. We assume that it has only one deformation mode and its
global deformation is large with an infinitesimal strain. The analytical model of the repetitive slit pattern and lamina emergent
joints were proposed by [Taisuke Ohshima and Yamaguchi 2015]
and [Delimont et al. 2015]. We modified this model considering
our improvements. In order to enhance the stiffness preserving theflexibility in one mode, we inserted a thin shell into the repetitive
slit pattern. Thus, we added stiffness term of the thin shell.
(M=Ed324∵ J is torsion constant
ϕ is bending curnature
G is shear modulus(a + b)GJ(a, b) +)ϕThe main idea for actuating a Coded Skeleton is to demonstrategeometric distance changes. The Coded Skeleton s strip has afinite thickness, and the length geometrically varies according tothe deformation and distance from the neutral plane. We used thisproperty to transform the strip with SMAs as shown in Figure 3 (d).

3 Sensor
SMAs can be used not only for actuators but also for sensors. Their
resistance varies according to the strain and is almost proportional
to strain (Figure 3 (f)).

4 Application
We propose 3 applications with user interfaces fabricated using aCoded Skeleton. First, we propose a shape memory application(Figure 1 (a)). A Coded Skeleton sense user inputs and changes
stiffness and shape. Deformed shape keeps its state when it ispowered on. This application indicates the Coded Skeleton has atleast two shape state, initial state and deformed state. Programming
shape memory between these two states are next challenge of ourresearch.
Secondly, we propose a deformable skeleton (Figure 1 (b)). We embed the Coded Skeleton into plush doll. This structure has stiffness
and predefined flexibility so that it works as bone and muscle both.
Finally, we propose an Actuated button (Figure 1 (d)). The Actuated button interface with Coded Skeletons on surface. Advantagesof this application is its simplicity, flat shape embedded SMA deforms negative and positive surface so that it has inner space indeformed state and enable user to interact with.

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