Once immersed in water, the hygromorph biocomposite starts
swelling and begins to actuate. Maximal curvature is reached after approximately
2500 min for a biocomposite with 100% printing width,
while samples prepared with 200 and 300% printing width require
less than 1000 min. This trend matches the difference observed for
water uptake of biocomposite according to their printing width
(Fig. 4a). The microstructure associated with varying porosity, induced
by FDM process, could therefore alter the actuation response, since it
controls thewater uptake mechanismand swelling ratio, i.e. the driving
principles. By comparing the values of tmax with maximal sorption time
(around 40,000 min) (Fig. 4a), we can see that actuation is faster than
water absorption when considering a stationary/steady state regime.
This implies that samples cease to bend but continue to absorb water.
Similar behaviours have been reported with flax/PP hygromorph
biocomposites, and are assumed to be linked to mechanosorptive damage
mechanisms [21]. The variation of maximal curvature Kmax as a
function of actuation speed (dK/dt) (Fig. 8a) shows that the experimentally
determined maximal curvature follows a similar trend to that calculated
by bimetallic theory (Fig. 7a). The printing width, the induced
hygro-elastic properties and the microstructure influence Kmax as well
as the actuation speed, but with an opposite trend. Increasing printing
width (from 100 to 300%) decreases the maximal curvature obtained,
while it improves the actuation rate (Fig. 8a)