## 1 Introduction  
Caries (tooth decay or dental cavities) is a major global healthcare issue, and according to the 2019 Global Burden of Disease Study, it is estimated that 2.03 billion people have caries in their permanent teeth and that 520 million children have caries in their primary teeth, making it the most prevalent non-communicable disease in the world [1, 2]. Caries is a result of plaque formation on the tooth surface, where bacteria excrete acids while metabolizing fermentable carbohydrates. This causes demineralization of the enamel and dentin and, if allowed to progress, will cause the formation of cavities. In order to prevent caries, it is important to reduce the daily intake of dietary sugars and to exercise regular dental hygiene care, such as tooth brushing and flossing. Tooth brushing should be performed with a fluoridated toothpaste, which can prevent and arrest the progression of caries by forming fluorapatite, i.e., inhibiting demineralization [3, 4]. Demineralization of dentin and enamel may be countered by remineralization, in which calcium and phosphate ions present in the saliva can deposit to form new mineral. This requires a local supersaturation of ions to form hydroxyapatite. If fluoride is present, the newly formed mineral may be fluorapatite rather than hydroxyapatite, making it less soluble and more resistant to acid erosion [5, 6]. Repeated acid attacks or poor salivary flow may, however, tip the scale towards continued demineralization, leading to a net mineral loss. This has prompted the need for additional calcium phosphate sources and the development of specialized oral care products [7]. Different calcium phosphate technologies and particles have thus been introduced in the past 20 years to promote the remineralization of enamel as well as the occlusion of dentin tubules for the reduction of dentin hypersensitivity [8; 9]. Dentin hypersensitivity is generally triggered by thermal, mechanical, or evaporative stimuli of the tooth and is highly prevalent among adults, reported at 34% in a recent study [10]. It is a clinical condition that can cause significant oral discomfort and pain, and the underlying cause is that the dentin tubules have become exposed due to gingival recession or loss of enamel, which is in turn caused by, e.g., abrasion or acid erosion, excessive tooth brushing or flossing, pocket reduction surgery, or as a secondary reaction to periodontal disease [10; 11]. According to the generally accepted hydrodynamic theory, occluding the dentin tubules will hinder fluid movement within the tubules, which otherwise excites nerve endings in the pulp, causing the sensation of pain [12]. For a long-lasting and minimally invasive treatment of dentin hypersensitivity, the tubules should therefore ideally be deeply occluded with an acid-resistant mineralization layer resistant to both abrasion and erosion, and it should preferably be easy to apply at home or as a topical desensitizer product at the dental office. Successful treatment and prevention of both caries and dentin hypersensitivity thus rely on effective remineralization, requiring adequate access to calcium, phosphate, and fluoride ions. There are a number of products and technologies on the market today that deliver various forms of calcium phosphate to the tooth surface, the most prevalent being hydroxyapatite (as nano- or microparticles, available in many kinds of toothpaste), different bioglases (e.g., NovaMin(r)), and casein phosphopeptide-amorphous calcium phosphate (CPP-ACP, marketed as RECALDENT(tm)). All of these technologies have successful clinical track records and have been demonstrated to be effective as remineralization agents in multiple studies [9; 13; 14]. They are regularly combined with fluoride in oral care products for daily caries protection. Each of these technologies has drawbacks or limitations, however, ranging from poor solubility (hydroxapatite) to coarse particles (bioglass) [15] and milk protein allergy (CPP-ACP).