We report a bilayer
of sodium alginate/polyvinylidene fluoride (SA/PVDF) that is chemically
bonded through a series of interfacial coupling reactions. The SA
layer is hydrophilic in structure and is capable of strong interaction
with water molecules, thus presenting high sensitivity to humidity,
whereas the PVDF layer is hydrophobic, inert to humidity. This structural
feature results in the bilayer having asymmetric humidity-responsive
performances that can thus make its shape change with directionality,
which cannot be achieved in an SA single layer. The responsive process
to humidity can be adjusted by exposure of the bilayer to sunlight
by means of a photothermal effect that accelerates dehydration of
the bilayer to cause more rapid shape deformations. When the sunlight
is removed, the bilayer adsorbs humidity again and returns to its
original shape, indicating good reversibility. To exactly regulate
the shape deformations of the bilayer with external stimuli, we employ
Ca2+-treated filter paper to customize crosslinking reactions
in the SA layer as desired patterns which are capable of causing
different mechanical tensors and swellabilities in the bilayer so
as to regulate and control the actuations for self-folding, curling,
twisting, and coiling in response to sunlight and humidity.On the
other hand, the chemically bonded bilayer has stronger interfacial
toughness and is capable of reaching 300 J m–2,
which is around 12 times the interfacial toughness of the physically
combined bilayer; as a result, the chemically bonded bilayer is capable
of sustaining continuous shape deformations without interfacial failure.
The directionally mechanical actuations can be utilized in designing
an indicator to roughly indicate the range of intensity of sunlight
by coupling the chemically bonded bilayer into a typical electric
circuit, in which the range of intensity of sunlight can be easily
estimated by visual observation of the light-emitting diodes.