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The shape memory of starch

Evolution of starch samples
Shape memory polymers are “intelligent” materials capable of changing shape in response to an environmental stimulus such as temperature variation. We have shown that starch, a natural biopolymer, has just these properties, which can be modulated by temperature or humidity. It is therefore possible to develop shape memory objects from starchy raw materials such as cereal flours.

Studying the properties of glassy-state starchy materials


“Classic” shape memory polymers are molecules with a complex structure (often copolymers) whose chemical synthesis is difficult and expensive and whose impact may be harmful for the environment.  Their use is therefore generally reserved for high added-value applications such as biomedical materials, temperature tracers or micromechanics.  
Study of the transformation and the properties of glassy-state starchy materials, particularly their ability to store residual stress, made it possible to develop a process to produce shape memory material using starch.  


...the shape recovery

Shape memory starch is made using a precise thermomechanical cycle, illustrated in Fig. 1.  It is obtained by extrusion under classic conditions (T=120°C; water content: 25%). It is first given an F1 shape (a twisted cylinder in the example) under heat, as soon as it leaves the extruder, and frozen into shape by rapid cooling.  
It is then given a new, temporary F2 shape (unfolded cylinder in the example), when it is subsequently heated at a temperature greater that that of its glassy transition (Tg).  This shape is set by cooling at room temperature, under mechanical stress.
It returns to its initial shape (F2 to F1) spontaneously if the temperature of the object becomes greater than its Tg, for example, almost instantaneously when heated in a microwave, and slower in hot water or when cooked in an oil bath.  



Figure 1. Thermomechanical cycle for extruded potato starch
 

Shape recovery can also be obtained by hydration when the starchy material is placed in a humid environment, as illustrated in Fig. 2.  The initial form, F1 (the initials “INRA” printed in relief on the extruded samples), is totally erased when the sample is thermomoulded into a 3.5-cm bar (2 minutes at 300 bars and 120°C).
The initials (INRA) reappear after 72 h when the samples are hydrated under high relative humidity (RH=97% at 20°C).



Figure 2. From top to bottom: potato starch samples with green and red food colouring, and corn flour sample


The mechanisms involved (macromolecular orientations) can be revealed by the study of residual stress stored in samples in their temporary F2 shape.  Therefore, the initials, “INRA”, not visible under normal light, can be seen under polarised light (Fig. 3). 
 




Figure 3. The initials, “INRA”, not visible in the F2 shape, are visible under polarised light.

Uses in the agri-food and biomedical sectors...

Starch shape memory can by used for classic applications of shape memory polymers, such as humidity tracers integrated into food product packaging.  Since the products are edible, applications can also be developed in the agri-food sector, for example, cereal products with varying shapes. 
A partnership with INSERM should also make it possible to validate the applications of these materials in the biomedical sector, including the development of implants that could be reabsorbed by the body or delayed release systems for active ingredients.  
Other works are under devopment with ADEME to study the mechanism of residual stress in starchy raw materials

Protected by european patent
(04/2008) : EP 08300188.3

Reference
Chaunier, L. and Lourdin, D. 2009. The shape memory of starch. Starch/Stärke. Vol. 61 : 116-118




Writing: Laurent Chaunier, Denis Lourdin
Creation date: 07 July 2010
Contact: Biopolymers, interactions & assemblies Unit
INRA Nantes

Denis Lourdin,  lourdin@nantes.inra.fr
Laurent Chaunier,  chaunier@nantes.inra.fr