## 1 Introduction  
Skin aging is a gradual process influenced by prolonged exposure to sunlight, pollution, smoking, stress, and other environmental factors, all of which accelerate the aging process [1]. As skin ages, it generally becomes less elastic and more fragile and experiences persistent barrier dysfunction, resulting in wrinkles, fine lines, and hyperpigmentation or age spots [2]. Wrinkle formation is closely linked to increased production of matrix metalloproteinases (MMPs), particularly MMP-1, which play a role in breaking down the extracellular matrix (ECM) through collagen degradation [3]. This breakdown of collagen leads to a loss of skin elasticity and firmness, contributing to the development of fine lines and wrinkles [4, 5]. Environmental factors, especially UV light, further upregulate MMP-1 expression, promoting processes like photoaging [6]. Additionally, oxidative stress from sources such as UV exposure, pollution, and inflammation increases free radical production, which can degrade ECM components like collagen and elastin and stimulate MMP-1 expression, accelerating the skin aging process [6]. The most common approach to counteracting skin aging is to simultaneously inhibit MMP-1 expression and increase collagen production in skin fibroblasts by reducing oxidative stress using antioxidants. Many natural resources with high antioxidant efficacy have been considered for this purpose because their use in cosmetics is considered safe and more eco-friendly compared with that of synthetic chemicals [7]. _Spirulina (Arthospira species)_ is a prime candidate for use in antiaging approaches, considering its long history of use in treating various diseases and its reputation as a \"superfood,\" suitable even for consumption in outer space [8]. It is rich in proteins and contains many biologically active compounds, such as c-phycocyanin (cPC), beta-carotene, and chlorophylls [9]. cPC, a two-subunit complex comprising a blue chromophore and apoprotein, is the most abundant bioactive component in spirulina with versatile cosmeceutical activities, such as reducing oxidative stress, photoproduction, anti-skin wrinkling, anti-inflammation, and skin hydration effects [10; 11]. However, the potential skin health benefits of cPC have not been realized because of its light and temperature sensitivity [12]. Owing to this critical limitation, cPC is only used in value-added products distributed under low-temperature conditions, such as ice cream and chocolate colorants, and it is rarely used in cosmetics as an active ingredient [12]. To overcome this limitation, cPC can be further broken down into its constituent chromophore and protein subunits [12; 13]. This chromophore is known as phycocyanobilin (PCB), which is a royal blue tetrapyrrole pigment mainly responsible for the health benefits of cPC, such as effective reduction of oxidative stress, anti-inflammatory, immunomodulatory, anticancer, and anti-diabetic nephropathy effects, and protection against neural tissue degeneration [14; 15; 16]. PCB is more stable than cPC as it can simultaneously attach to any protein (for example, bovine and human serum albumin) with free SH groups, such as those provided by cysteine and ethanethiol, without loss of its biological activities [16; 17]. Furthermore, whey proteins and polysaccharides such as carrageenan, with high antioxidant effects, have been used to improve the stability of PCB from spirulina [18; 19]. Although PCB, derived from spirulina, shows excellent blue color and exerts many beneficial effects on human skin, its cosmetic applications have not been extensively investigated [20; 21]. In recent times, trends in cosmeceutical development have been shifting toward vegan cosmetics, driven by the increasing number of consumers seeking plant-based ingredients. For promoting the use of PCB in vegan cosmetics, the development of plant-derived resources that stabilize this pigment is essential.