**1. Introduction**

As we know, human skin is soft, self-healable and stretchable, and has the ability to sense subtle external changes. This amazing property has attracted tremendous interest in artificial skin, especially wearable electronics for healthcare, artificial intelligence and soft robotics [1–3]. These artificial skin-like devices can monitor environmental stimuli such as pressure, strain, temperature, and deformation by detecting electric signals like current and voltage, or measuring electrical properties including resistance, and capacitance. "Electronic skin" is usually considered as a stretchable sheet with area above 10 cm<sup>2</sup> integrating sensors to detect different external stimuli [4]. Usually, electronic skin is made of stretchable electrical conductors including carbon grease [5], graphene sheets [6], carbon nanotubes [7], liquid metals [8] and metal nanostructures [9,10], which transmits signals via electrons. Although these materials present high conductivity and excellent stretchability, which meet the necessary requirements of electronics skin, they fail to meet other additional requirements like biocompatibility and transparency. On the other side, human skin can report signals via ions, which provides a potential pathway to develop ionic conductors based on a sensory sheet called "ionic skin" [4]. Hydrogels are three-dimensional networks composed of high-molecular weight polymer, large amounts of water, and crosslinkers [11]. As the water in hydrogels can dissolve ions, hydrogels can be employed as ionic conductors [4,12], which may have potential applications in "ionic skin". In addition, hydrogels are highly stretchable and biocompatible [13]. Furthermore, high transparency of hydrogels allows these sensory sheets to report electrical signals without impeding optical signals [4]. They can behave as tough as elastomers due to recent developments [14–17], which can monitor large deformation, like finger bending.

Three-dimensional (3D) printing, also known as an additive manufacturing process, is an emerging technology [18]. Due to its rapid production with high shape fidelity, 3D printing technology has attracted tremendous attention since it was first proposed by Charles W. Hull in 1986 [19]. Recently, volumetric additive manufacturing has been developed and has received lots of attention due to its excellent performance to overcome limitations of low speed and geometric constraints [20–22]. However, the 3D printing technology has been recently introduced to fabricate hydrogels. Extrusion printing method is a modified fused deposition modeling method that extrudes continuous liquid inks to achieve layered structures. As the extrusion printing method has lots of advantages including simple fabrication procedure, large range of materials, good balance between printer's cost and printing quality, and high cell deposition in bioprinting, it has been considered as an excellent choice to print hydrogels [17,23].

Various polymers have relatively high viscosity to maintain their pattern in printing process, and have crosslinking abilities allowing for 3D structures maintenance after printing, like collagen [24], hyaluronic acid (HA) [25], chitosan [26] and alginate [18], have been employed in 3D printing technology to achieve 3D printed hydrogels. Usually, physical crosslinking can be induced by temperature change [15,24] and ionic crosslinking [11,27], whereas chemical crosslinking can be formed by polymerization [28]. Sodium alginate (SA), an anionic polymer isolated from brown algae, has the ability to crosslink assisting by divalent or trivalent ions [11]. Due to its high biocompatibility, hydrophilicity and biodegradability under normal physiological conditions, SA has received increasing attention as an instant gel for tissue engineering. Although there are lots of conventional methods to fabricate SA hydrogel constructs including the injection molding method and solution casting method, SA solution has a certain viscosity and its limited flowability leads to a poor dispersion in molds [11]. Compared to these conventional methods, 3D printing technology has one main advantage to fabricate customized constructs, which can fit various requirements of wearable sensors applied on di fferent body parts on di fferent humans [17]. In addition, 3D printing technology has the potential to fabricate hydrogels with hierarchically porous structures or gradient properties, which may improve sensitivity and sensing range of wearable sensors [29]. Although 3D printing technology has these advantages to fabricate SA hydrogel constructs, several challenges have not been well addressed, which limits its development. One of the common challenges of 3D printing hydrogels is to achieve printed constructs with high shape fidelity due to low viscosity leading to a collapse tendency of the printed constructs. Various methods have been proposed to increase the viscosity of SA solution, such as increasing the SA ink concentration or varying molecular weight [30], combining with other materials including nanocellulose [18] or gelatin [31], employing a supporting sacrificial polymer [32] and partially crosslinking alginate with calcium ions [33].

To improve the printing resolution and mechanical property, the hybrid agar/calcium alginate (CA)/polyacrylamide(PAAm) hydrogels combining brittle thermo-responsive agar and ionic-responsive alginate and soft polyacrylamide network is proposed in this manuscript. During printing, mixture of thermo-responsive agar and ionic-responsive alginate was extruded as a continuous stripe to enhance printing resolution. Meanwhile, due to the increasing viscosity, the mixture can maintain its shape during printing. After photopolymerization and solution soaking, the 3D printed tough hydrogels were achieved. With calcium crosslinking of alginate, tensile strength and fracture energy of 3D printed hydrogels are greatly improved. Furthermore, the water in 3D printed hydrogels dissolves

lots of calcium ions, which make them work as ionic conductors. A wearable soft resistive strain sensor was developed by a 3D printed hydrogel film. This resistive strain sensor exhibits quick and accurate detection of changes of finger bending, which demonstrates that the conductive, transparent, stretchable hydrogels can be used as wearable resistive strain sensor to monitor human motion.
