## 1 Introduction  
Suturing is the traditional method used to seal biological tissues during surgical interventions. Although many medical glues [1] have been developed to replace suturing, conventional suturing is still dominantly adopted in preference to glues in medical centers due to ease of use and secure tissue sealing without mechanical delamination after surgery. Such suture threads, composed of biodegradable polymers (e.g., poly (lactic-co-glycolic acid)), may potentially result in acute/chronic inflammation by evoking continuous frictional stress on the tissues, and due to their mechanical stiffness and to acidic byproducts after degradation [2]. Furthermore, suturing suffers from the critical challenges associated with undesired issues such as the application of excessive pressure on the site of election, low biocompatibility, and difficult biodegradation time control [3, 4, 5]. Thus, to replace this conventional sealing method, a new type of adhesive material with thread formulation would be suitable.  
As an alternative to existing sutures, various studies have been focused on developing bioadhesives, with improved biocompatibility [6, 7], controllable degradation [8, 9], and drug encapsulation [10, 11]. However, these adhesives still have weak mechanical properties and low adhesiveness when compared to those of conventional suturing. To overcome these issues, many researchers have been interested in fabricating bio-adhesivefibers with strong mechanical strength [12]. For instance, one of the studies showed that a wet-spinning technique applied to alginate/chitosan fibers achieved the mechanical properties of a Young's modulus 200% higher than that of the natural materials [13]. Another study showed the formation of tough hydrogels, rehydrated from organogels, allowing further spinning to fibrous structures [14]. Furthermore, a mechanically robust hydrogel fiber fabricated by using a dynamic host-guest crosslinking that mimics the silk fibers of a spider web [15], and a viscoelastic hydrogel with a nanofiber structure similar to the sundew adhesive have been developed [16]. Nevertheless, these hydrogels cannot be adopted for versatile tissues with different physiological pHs, for instance, the strong acidity of the stomach and the mild basicity of the intestines.  
In this study, we report a pH-responsive adhesive hydrogel thread that mimics sundew adhesive mucus (Figure 1a). By simply mixing boronic acid-conjugated alginate (AlBA) and tamic acid (TA), which is an abundant polyphenol found in plants, viscoelastic gelation of the adhesive hydrogel thread occurs via reversible boronic acid-cis-diol complexation (Figure 1b, left) [17], further allowing facile spinning to fabricate hydrogel fibers (Figure 1b, right). After complete dehydration of the hydrogel fiber, the thread with strong mechanical strength is prepared for use in suturing like conventional threads. The thickness of the thread can be precisely controlled through a layer-by-layer stacking process of the hydrogel fibers. In particular, after suturing, the adhesive threads on the tissues can be swollen under exposure to a physiological buffer (pH 7.4), and their mechanical modulus is higher at acidic pH than that at neutral pH, owing to a pronounced precipitation tendency of the alginate as alginic acid in acidic conditions [18].  
Figure 1: (**a**) Schematic of a hydrogel formed from sundew and an AlBA/TA hydrogel inspired by it. It is composed of mono-/polysaccharides, which are the main components of the sticky hydrogel, and a crosslinking agent that helps the shape to be thread-like. (**b**) The chemical structure of the complexation between boronic acid and cis-diol in TA (**left**) and overall scheme for the manufacturing process of the AlBA/TA hydrogel threads (**right**).