Diabetes is a severe disease impacting nearly 10% of the whole population, which is still increasing in the predictable future due to the trend of population aging globally [1]. In the context of COVID-19 pandemic, it is imperative to pay more attention to diabetic patient management [2]. Hyperglycemia is a prominent trait of diabetes ascribed to delayed or insufficient insulin secretion. Metformin and many other drugs are usually prescribed to control the high blood glucose, which inevitably causes some side effects [3].

Insulin is still the most effective drug to control blood glucose levels, especially for severe diabetic management. Compared with injection, oral insulin administration is much preferred [4]. However, the gastrointestinal (GI) tract presents a hostile environment for oral insulin administration, accompanied with a strongly acidic environment (pH 1.2–2.0) and digestive enzymes, which significantly denature and degrade insulin and reduce its bioactivity [5]. The mucus lines on the surface of the GI tract and introduces a barrier against protein drugs like insulin. Effective adhesion and quick penetration of intestinal mucus are important for drugs to avoid elimination by the continuously secreted mucus [6].

To address these challenges, various types of carriers were applied to protect insulin against the harsh environment in the GI tract and improve its oral bio-efficacy. Liposome is a typical drug carrier that reduces systemic toxicity and improves tolerable doses [7]. Since the 1970s, liposomes with different properties have been developed as carriers for oral insulin administration [8]. However, conventional liposomes are destabilized in an acidic environment, leading to an early release of payloads and low bio-efficacy. To improve the release pattern and bio-efficacy, complicated modifications of liposomes are required which indeed restrict their practical applications for oral insulin administraiton.

Hydrogel is another extensively investigated carrier for insulin [9]. Hydrogel is a water-swollen 3D polymer network that contains a high amount of water but remains the structure owing to physical or chemical crosslinks, which resembles the extracellular matrix. Alginate (Alg) hydrogel is commonly considered as a biocompatible platform for oral insulin administration that is stable in an acidic environment [10]. A carrier that precisely coordinates the release kinetics of insulin in specific locations is critical for improving the bio-efficacy of insulin [11]. However, the porous structure of Alg hydrogel leads to early burst release of payloads and poor intestinal absorption [12]. Besides, simple Alg hydrogel provides only weak adhesion to intestinal mucosa, offers a short residence time on the surface of the small intestine, and leads to a low absorption rate [13]. Hence, modified Alg hydrogel with stronger adhesion ability on the intestinal mucosa is promising for oral insulin administration.

A single type of carrier, such as liposome and hydrogel, usually focuses on specific problems of oral insulin administration, such as burst release in stomach, weak mucosal retention on the intestinal mucosa and low intestinal absorption [[14], [15], [16], [17], [18]]. In recent years, complex delivery platforms combining two or more carriers have proved more efficient as delivery tools. In our previous study, Alg microbeads were applied to load insulin-encapsulated chitosan nanoparticles for oral insulin administration [19]. Meanwhile, chitosan-based hydrogel was applied to incorporate chlorhexidine-loaded liposomes, which significantly improved the biofilm eradication ability [20]. Another study integrated different modular liposomes into the hydrogel to form a liposome-in-hydrogel complex delivery system as an injectable protein drug carrier [11]. Liposome-in-hydrogel combines the strength of both liposome and hydrogel as drug carriers and provides an effective solution for oral insulin delivery. However, to the best of our knowledge, there is still no report on using the liposome-in-hydrogel complex for oral insulin delivery.

Based on previous research and our understanding, we hypothesized that the liposome-in-alginate hydrogel complex is an effective delivery tool to encapsulate, protect, and promote the insulin absorption for oral administration.

In this study, we aim to develop a multi-functional oral delivery system for improving the bio-efficacy of insulin. As a proof of concept, the liposome-in-alginate hydrogel complex is designed as the delivery system. To promote intestinal permeation and absorption efficiency, instead of pristine insulin, an arginine-insulin complex was prepared by electrostatic attraction [21,22]. Liposome (Lip) was applied to encapsulate the arginine-insulin complex (AINS) to protect insulin and further improve intestinal permeation and absorption. Furthermore, instead of Alg, cysteine-modified alginate (Cys-Alg) was used to form hydrogel to improve the adhesive ability of the intestinal mucosa [13], which is beneficial to the absorption of Lip by the epithelial cells. Cys-Alg hydrogel avoids burst release of insulin from the Lip in an acidic environment. In the intestine, Cys-Alg hydrogel would swell and release the negatively charged Lip, which would be absorbed by intestinal cells. The oral bio-efficacy of insulin could be significantly improved.