  Programmable materials are desirable for a variety of functional applications
that range from biomedical devices, actuators and soft robots to adaptive
surfaces and deployable structures. However, current smart materials are often
designed to respond to single stimuli (like temperature, humidity, or light).
Here, a novel multi-stimuli-responsive composite is fabricated using direct ink
writing (DIW) to enable programmability in both space and time and computation
of logic operations. The composite hydrogels consist of double-network matrices
of poly(N-isopropylacrylamide) (PNIPAM) or poly(acrylic acid) (PAA) and sodium
alginate (SA) and are reinforced by a high content of cellulose nanocrystals
(CNC) (14 wt%) and nanofibers (CNF) (1 wt%). These composites exhibit a
simultaneously tunable response to external stimuli, such as temperature, pH,
and ion concentration, enabling precise control over their swelling and
shrinking behavior, shape, and mechanical properties over time. Bilayer
hydrogel actuators are designed to display bidirectional bending in response to
various stimuli scenarios. Finally, to leverage the multi-responsiveness and
programmability of this new composite, Boolean algebra concepts are used to
design and execute NOT, YES, OR, and AND logic gates, paving the way for
self-actuating materials with embodied logic.
