Dynamic covalent polymer networks
(DCPN) have historically attracted
attention for their unique roles in chemical recycling and self-healing,
which are both relevant for sustainable societal development. Efforts
in these directions have intensified in the past decade with notable
progress in newly discovered dynamic covalent chemistry, fundamental
material concepts, and extension toward emerging applications including
energy and electronic devices. Beyond that, the values of DCPN in
discovering/designing functional properties not offered by classical
thermoplastic and thermoset polymers have recently gained traction.
In particular, the dynamic bond exchangeability of DCPN has shown
unparalleled design versatility in various areas including shape-shifting
materials/devices, artificial muscles, and microfabrication. Going
beyond this basic bond exchangeability, various molecular mechanisms
to manipulate network topologies (topological transformation) have
led to opportunities to program polymers, with notable concepts such
as living networks and topological isomerization. In this review,
we provide an overview of the above progress with particular focuses
on molecular design strategies for the exploitation of functional
material properties. Based on this, we point out the remaining issues
and offer perspectives on how this class of materials can shape the
future in ways that are complementary with classical thermoplastic
and thermoset polymers.
Thermoset polymers are known for their superior thermomechanical properties, but the chemical crosslinking typically leads to intractability. This is reflected in the great differences between thermoset and thermoplastic shape-memory polymers; the former exhibit a robust shape memory but are not capable of redefining the permanent shape. Contrary to current knowledge, we reveal here that a classical thermoset shape-memory polyurethane is readily capable of permanent reshaping (plasticity) after a topological network rearrangement that is induced by transcarbamoylation. By employing the Jianzhi technique (also known as kirigami), unexpected shape-shifting versatility was observed for this otherwise classical material. As the essential carbamate moiety in polyurethanes is one of the most common polymer building units, we anticipate that our finding will have significant benefits beyond shape shifting.
Peripheral neuromodulation has been widely used throughout clinical practices and basic neuroscience research. However, the mechanical and geometrical mismatches at current electrode-nerve interfaces and complicated surgical implantation often induce irreversible neural damage, such as axonal degradation. Here, compatible with traditional 2D planar processing, we propose a 3D twining electrode by integrating stretchable mesh serpentine wires onto a flexible shape memory substrate, which has permanent shape reconfigurability (from 2D to 3D), distinct elastic modulus controllability (from ~100 MPa to ~300 kPa), and shape memory recoverability at body temperature. Similar to the climbing process of twining plants, the temporarily flattened 2D stiff twining electrode can naturally self-climb onto nerves driven by 37°C normal saline and form 3D flexible neural interfaces with minimal constraint on the deforming nerves. In vivo animal experiments, including right vagus nerve stimulation for reducing the heart rate and action potential recording of the sciatic nerve, demonstrate the potential clinical utility.
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