## 3 Stimuli-Responsive Mechanisms: Avenues for 4D (Bio-)printing
### Electrically and Mechanoelectrically Responsive Materials  
Most smart electrically responsive materials usually depend on their electrically conductive characteristics. Upon an electric field, a hydrogel will swell or contract depending on its charge. Electric field responsiveness is, in fact, a combination of several electrical interactions occurring in hydrogels, including Coulombic, electrophoretic, electroosmotic, and piezoelectric phenomena. Electrical properties of these types of hydrogels can be regulated by the intensity or direction of the externally applied electric field. This feature provides new applications for biomimetic systems, since materials can change their size and/or shape in response to electrical stimulation [16]. Electrical and electrochemical stimuli have been widely used in research and applications due to their advantages of precise control through the magnitude of the current, the duration of an electric pulse or the pulse interval [65]. As an external stimulus, an electric field presents some advantages such as: (i) the availability of equipment that allows a precise control considering the magnitude of the current; (ii) the duration of the electric pulses, and (iii) the intervals during pulses [41]. In Table 3, the reported trials using electricity as a predefined stimulus are collected. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material(s)** & **Mechanism(s)** & **Application(s)** & **Ref.** \\ \hline Poly(ether-amide) grafting with & & & \\ PNIPAAm and PEG & Temperature & Controlled drug release & [79] \\ \hline Poly(acrylic acid)–PNIPAAm & & & \\ and ceramic powder (Al\({}_{2}\)O\({}_{3}\)) & Temperature & & \\ \hline Poly(t-lactic acid)-poly(ethylene & & & \\ glycol) Poly(t-lactic acid) & Temperature & & \\ (PLLA-PEG-PLLA) & & & \\ \hline Poly(ethylene oxide)–poly & & & \\ (propylene oxide)–poly & & & \\ (ethylene oxide) & & & \\ (PEO-PPO-PEO) & & & \\ \hline Epoxy based UV curable shape & & & \\ memory polymers (SMP) & Temperature & & \\ \hline Poly-(D,L-lactide-co-glycolide)- & & & \\ block-poly(ethylene glycol)- & & & \\ block-poly(D,L-lactide-co-glycolide) & & & \\ (PLGAA-b-PEG-b-PLGA) triblock & & & \\ copolymer thermogels & & & \\ \hline Methacrylate based copolymer & Temperature & Biomedical devices & [92] \\ \hline \hline \end{tabular}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Mechanical** \\ formulations, skeletal \\ muscle-like linear & & & \\ actuation & & \\ \hline Methyl cellulose hydrogel & Temperature & Cell sheet engineering & [93] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._Electrically responsive hydrogels are prepared from polyelectrolytes (i.e., polymers that contain high concentrations of ionizable groups along the chain and, therefore, are sensitive to electrical stimuli) and electroresponsive polymers can be used to prepare materials that swell, contract or bend in response to an electric field. The polyelectrolyte hydrogels are electroresponsive polymers and can deform under an electric field, due to swelling or contract because the charged ions are directed to the anode or cathode of the gel [41]. It should be considered that also electrically unresponsive hydrogels may show an electric responsiveness in presence of dielectric liquids or in combination with electrically responsive nanoparticles [43; 103]. Electrically responsive polymers are also called conducting polymers, such as polythiophene (PT) [94; 95] and sulfonated-polystyrene (PSS) [96], which may undergo swelling, shrinkage, or flexion in response to an electrical stimulus. The most employed natural polymers used to prepare electroresponsive materials are chitosan, chondro sulfate, hyaluronic acid, and alginate [14; 104]. Major synthetic polymers have been based on vinyl alcohol, allylamine, acrylonitrile, 2-acrylamido-2-methylpropane sulfonic acid (PAMPS), aniline, poly(2-hydroxyethyl methacrylate) (PHEMA), methacrylate acid, acrylic acid, and vinyl sulfonic acids [41].