4 Macroscopically Assembled Devices
To fabricate smart flexible devices from responsive and functional hydrogels, it is
keen to integrate hydrogels with different properties or functionalities into single
devices [51]. To date, numerous hydrogel devices based on bilayer structures have
been reported. The classic bilayer architecture, pioneered by Hu et al. [52], is usually
comprised of two hydrogel layers with different moduli or different responsiveness.
Upon external stimulus, each layer swells or deswells differently, leading to different
volume changes and internal stress. As a result, the gel bends to release the internal
stress. Many responsive hydrogels, including thermo-, pH-, ion-, redox-, and lightresponsive hydrogels, have been utilized to fabricate bilayers. This fundamental
principle of hydrogel bilayers has recently been widely extended to fascinating
devices with abundant geometry or architectures that execute delicate movements.
Bilayer flowers, grippers, and others have been demonstrated in literature to reversibly change shapes or grasp small objects.
So far, most bilayer-based hydrogel devices are fabricated by sequential synthesis
of the layers with different components and properties. This procedure is usually
trivial and limited to simple structures. For practical applications, it is desired to
develop novel simple and reliable methods to integrate hydrogel building block into
devices. However, direct assembling [53] or gluing hydrogels together is challenging because most hydrogels are slippery and not adhesive to each other. Harada et al.
utilize supramolecular recognition at the surface and interface of hydrogels to
assemble small hydrogel cubes together [43, 44, 54, 55]. Shi et al. further demonstrate that the success of such macroscopic assembling based on supramolecular
recognition is highly dependent on the rigidity of the gels [56]. It is difficult for gels
with high modulus to assemble together since the macroscopic assembling requires
the readjustment of surface topography to achieve a conformal contact. To tackle this
problem, a soft and flexible surface coating is devised to allow for adaptive and
conformal contact to enable macroscopic assembling [57]. These studies demonstrate the possibility to use hydrogels as building blocks to construct smart devices.
But it remains a challenge to develop versatile strategies applicable for most
hydrogels, particularly strong and tough hydrogels, for the fabrication of smart
devices for practical applications.
In comparison with the short-scale supramolecular recognition, electrostatic
attraction has long-distance effects. In the polyelectrolyte hydrogels, the ionic
functional groups are fixed on the network, acting as immobile charges, while the
free counterions are recognized as mobile ions [58]. The free ions can migrate in
electric field. In fact, as two polyelectrolyte hydrogels with opposite immobile
charges are placed together, a transient electric field forms at the interface
(Fig. 17a). The local electric field will drive migration and redistribution of mobile
ions (Fig. 17b). As a result, a new local electrostatic field and electrostatic equilibrium are established (Fig. 17c).
This concept of macroscopic assembling of hydrogels based on electrostatic
attraction is examined by using F127DA micelle-crosslinked poly(sodium methyl
230
J. Fu
To fabricate smart flexible devices from responsive and functional hydrogels, it is
keen to integrate hydrogels with different properties or functionalities into single
devices [51]. To date, numerous hydrogel devices based on bilayer structures have
been reported. The classic bilayer architecture, pioneered by Hu et al. [52], is usually
comprised of two hydrogel layers with different moduli or different responsiveness.
Upon external stimulus, each layer swells or deswells differently, leading to different
volume changes and internal stress. As a result, the gel bends to release the internal
stress. Many responsive hydrogels, including thermo-, pH-, ion-, redox-, and lightresponsive hydrogels, have been utilized to fabricate bilayers. This fundamental
principle of hydrogel bilayers has recently been widely extended to fascinating
devices with abundant geometry or architectures that execute delicate movements.
Bilayer flowers, grippers, and others have been demonstrated in literature to reversibly change shapes or grasp small objects.
So far, most bilayer-based hydrogel devices are fabricated by sequential synthesis
of the layers with different components and properties. This procedure is usually
trivial and limited to simple structures. For practical applications, it is desired to
develop novel simple and reliable methods to integrate hydrogel building block into
devices. However, direct assembling [53] or gluing hydrogels together is challenging because most hydrogels are slippery and not adhesive to each other. Harada et al.
utilize supramolecular recognition at the surface and interface of hydrogels to
assemble small hydrogel cubes together [43, 44, 54, 55]. Shi et al. further demonstrate that the success of such macroscopic assembling based on supramolecular
recognition is highly dependent on the rigidity of the gels [56]. It is difficult for gels
with high modulus to assemble together since the macroscopic assembling requires
the readjustment of surface topography to achieve a conformal contact. To tackle this
problem, a soft and flexible surface coating is devised to allow for adaptive and
conformal contact to enable macroscopic assembling [57]. These studies demonstrate the possibility to use hydrogels as building blocks to construct smart devices.
But it remains a challenge to develop versatile strategies applicable for most
hydrogels, particularly strong and tough hydrogels, for the fabrication of smart
devices for practical applications.
In comparison with the short-scale supramolecular recognition, electrostatic
attraction has long-distance effects. In the polyelectrolyte hydrogels, the ionic
functional groups are fixed on the network, acting as immobile charges, while the
free counterions are recognized as mobile ions [58]. The free ions can migrate in
electric field. In fact, as two polyelectrolyte hydrogels with opposite immobile
charges are placed together, a transient electric field forms at the interface
(Fig. 17a). The local electric field will drive migration and redistribution of mobile
ions (Fig. 17b). As a result, a new local electrostatic field and electrostatic equilibrium are established (Fig. 17c).
This concept of macroscopic assembling of hydrogels based on electrostatic
attraction is examined by using F127DA micelle-crosslinked poly(sodium methyl
230
J. Fu