## 1 Introduction  
Recurrent aphthous stomatitis (RAS) is the most common ulcerative disease in the oral mucosa in humans that often results from trauma, hormonal changes, physical or psychological stress, chemical irritation, genetic factors, and allergies [1]. To date, effective RAS therapy has not been found, and its treatment is aimed at treating symptoms with a focus on suppressing local immune responses, reducing pain, discomfort, and preventing secondary infections and recurrences. If the ulcer is left unchecked, it will cause secondary infection because the oral cavity can be the ideal place for bacterial growth [2].  
The use of drugs in RAS aims to accelerate the regeneration of tissue cells. The drugs used in RAS therapy can be modern drugs or natural medicines derived from plants and spices [3]. Topical applications of antiseptics (chlorhexidine, triclosan, benzylamine hydrochloride, amlexanox), local anesthetics (lidocaine, dicholine hydrochloride), or corticosteroids (triamcinolone, betamethasone) have been shown to have positive effects for treatment but have harmful side effects when they are used too often and over a long period [4].  
Mangosteen rind (_Garcinia mangostana_ L.), an Indonesian native plant, is used as traditional medicine, such as abdominal pain, chronic ulcers, skin infections, wounds healing, analgesic and anti-inflammatory properties [5]. The substances contained therein are phenol derivatives namely xanthones or xanthen-9H-ones [6]. Mangosteen rind (_Garcinia mangostana_ L.) contains various of xanthones, including \(\alpha\)-, \(\beta\)-, \(\gamma\)-magnostin which have pharmacological activities, such as antioxidant, antitumor, antibacterial, antiviral, antifungal, antialergic, and anti-inflammatory [7].  
Recently, \(\alpha\)-mangostin (\(\alpha\)-M) has been known as an agent for oral cavity therapy. However, \(\alpha\)-M is difficult to be delivered directly to the site because it is insoluble in water, therefore, a carrier for delivering \(\alpha\)-M is strongly needed. Hydrogel film (HF) can be used as a patch designed to separate mucosal lesions from the oral environment, and thus reduce the risk of getting the drug carried by saliva so that it can increase the time of contact between active substances and lesions, improve the effectiveness of therapy, and reduce patient discomfort [8; 9].  
In previous studies, HF based chitosan (Ch) derived from chitin has high resistance, biocompatibility, low toxicity, high fluids absorption, and has an antibacterial activity that will accelerate healing. However, to the superior nature of the Ch above, chitosan turns out to have a deficiency that is sensitive to acid solutions. Therefore, it is important to increase its chemical resistance, as well as long-term biological hydrophilicity by a combination of other polysaccharides [10]. Alginate (Alg), a natural polysaccharide extracted from brown seaweed, has been commonly used in the food industry as an additive or thickener [11]. Alg and Ch dissolve together in the right conditions and interact with each other through carbonyl groups of Alg and protonated amino groups of Ch. It was reported that the formation of hydrogels using Alg and Ch can be achieved by adjusting the pH of the mixing solution without adding calcium cations [12; 13]. [PERSON] et al. proved that hydrogel based on Ch-Alg formulations had a higher mucoadhesive strength compared to hydrogel based on Alg [14]. In addition, Ch molecular weight in Ch-Alg polyelectrolyte complex film containing clindamycin phosphate affected to adhesiveness of complex films [15]. Mucoadhesive ability of Ch closely related to the structure of Ch, which can interact with mucin predominantly by electrostatic binding [16].  
In this study, we prepared and characterized the \(\alpha\)-M HF based on Ch and Alg (ChAlg/\(\alpha\)-M HF) as a new RAS therapy.