*Review* **An Update on the Clinical Efficacy and Safety of Collagen Injectables for Aesthetic and Regenerative Medicine Applications**

**Luca Salvatore <sup>1</sup> , Maria Lucia Natali 1,2, Chiara Brunetti <sup>1</sup> , Alessandro Sannino <sup>2</sup> and Nunzia Gallo 1,2,\***


**Abstract:** Soft tissues diseases significantly affect patients quality of life and usually require targeted, costly and sometimes constant interventions. With the average lifetime increase, a proportional increase of age-related soft tissues diseases has been witnessed. Due to this, the last two decades have seen a tremendous demand for minimally invasive one-step resolutive procedures. Intensive scientific and industrial research has led to the recognition of injectable formulations as a new advantageous approach in the management of complex diseases that are challenging to treat with conventional strategies. Among them, collagen-based products are revealed to be one of the most promising among bioactive biomaterials-based formulations. Collagen is the most abundant structural protein of vertebrate connective tissues and, because of its structural and non-structural role, is one of the most widely used multifunctional biomaterials in the health-related sectors, including medical care and cosmetics. Indeed, collagen-based formulations are historically considered as the "gold standard" and from 1981 have been paving the way for the development of a new generation of fillers. A huge number of collagen-based injectable products have been approved worldwide for clinical use and have routinely been introduced in many clinical settings for both aesthetic and regenerative surgery. In this context, this review article aims to be an update on the clinical outcomes of approved collagen-based injectables for both aesthetic and regenerative medicine of the last 20 years with an in-depth focus on their safety and effectiveness for the treatment of diseases of the integumental, gastrointestinal, musculoskeletal, and urogenital apparatus.

**Keywords:** collagen; injectable collagen; medical devices

### **1. Introduction**

Soft tissues loss could be due to iatrogenic, traumatic, pathological, or physiological reasons. Aside from significantly affecting patients' quality of life, their surgical management requires targeted, costly and sometimes constant interventions. With the average life increase, a proportional increase of age-related soft tissues diseases has been witnessed. Due to this, recent decades have seen a tremendous demand for soft tissue reconstruction strategies and one step resolutive procedures. Intense scientific and industrial research has been conducted to develop innovative approaches or optimize current solutions. Among them, in the last two decades injectable formulations have attracted even more interest for both aesthetic and regenerative surgery for their versatility and multifunctionality (Figure 1). Indeed, injectable scaffolds could be used in large and irregularly shaped lesions for a huge variety of damaged tissues, as well as providing temporary pain relief and functional improvement with a single treatment. Thus, injectable formulations could reduce the number of surgical procedures, costs, times and accelerate healing rate and quality.

The popularity of minimally invasive techniques increased rapidly for several reasons. A principal factor is the acceptance of soft tissue fillers among patients that are not ready for permanent treatments [1]. In the case of patients not wishing to undergo surgery, an easier procedure would generally be more accepted. Moreover, compared to undergoing

**Citation:** Salvatore, L.; Natali, M.L.; Brunetti, C.; Sannino, A.; Gallo, N. An Update on the Clinical Efficacy and Safety of Collagen Injectables for Aesthetic and Regenerative Medicine Applications. *Polymers* **2023**, *15*, 1020. https://doi.org/10.3390/ polym15041020

Academic Editors: Jianxun Ding and Donatella Duraccio

Received: 19 December 2022 Revised: 19 January 2023 Accepted: 13 February 2023 Published: 17 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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more invasive surgery, fillers offer the patient less discomfort and a shorter recovery time, making them very practical in the resolution of minor-serious disease and allowing patients to return immediately to their daily routine [1,2]. Minimally invasive therapies would give a better quality of life also for that part of population that would otherwise not survive the trauma induced by conventional surgeries. Moreover, they could delay the execution of invasive surgical procedures for the implantation of permanent devices [3]. In the case of a staged surgical intervention, the use of injectable systems may avoid the need for multiple invasive operations, thus reducing the related morbidities and negative aesthetic effects associated with repeated procedures [4]. With regard to aesthetic treatments, minimally invasive therapies are preferred as they are less impacting and give a more natural look. Moreover, the lack of an external incision or an autologous tissue donor site is preferred because the absence of scarring is usually socially and psychologically more accepted.

**Figure 1.** The increasing research interest in injectable formulations and dermal filler. Articles indexed in Scopus (www.scopus.com) with the keywords 'injectable' and 'dermal filler' and published from 2000 to 2022 (last accessed on 27 May 2022).

From the surgeon's point of view, the advantages of minimally invasive procedures include principally the need for fewer resources (e.g., operating room, staff, equipment, and time). Being simpler, transcutaneous injections require less operating room staff and time. The pro-regenerative action of injectables would reduce operating room time also because they would be able to restore physiological conditions with a single injection. However, it should not be forgotten that simpler procedures are not less exhausting and do not require less experience. Like any surgical procedure, minimally invasive therapies require adequate knowledge in order to reach the best outcome and avoid unwanted adverse events.

Thus, not only clinicians' but especially patients' preference for fewer invasive and expensive procedures has undoubtedly promoted their use [4–10]. An injectable formulation for soft tissues reconstruction currently relies on two main approaches, involving autologous tissue displacement (e.g., lipofilling, platelet-rich plasma) or biomaterials-based filling [5]. Both approaches have some advantages and drawbacks. Autologous materials provide the most physiological solution (no adverse events or immune reactions) but suffer from donor site morbidity, volume resorption rate variability, and double surgery requirements. Moreover, their harvesting is a time-consuming procedure that requires double intervention. Alternatively, biomaterials offer an off-the-shelf solution with immediate results and should be distinguished as non-resorbable and resorbable, depending on their half-life. Non-resorbable solutions (e.g., silicone, poly(methyl methacrylate), polyvinylpyrrolidone, polyacrylamide), are permanent (last more than 2 years) but usually suffer from mild-severe adverse reactions (i.e., granuloma, implant encapsulation, persistent pain or rejection) that limit patient satisfaction and could require implant removal surgery [6–12]. Contrarily, resorbable formulations are usually based on natural biomaterials (i.e., collagen, hyaluronic acid, calcium hydroxyl apatite) and last 6–18 months [13–16]. Their durability depends on many factors such as the raw material type, product cross-linking degree, lost tissue

extension, disease site and etiology, and patient metabolism, age and co-morbidities. The most used resorbable dermal fillers are collagen or hyaluronic acid based.

Collagen is the most abundant structural protein of vertebrate connective tissues [17–25] and plays a crucial structural role for the maintenance of tissues' architecture, shape and mechanical properties [20]. Moreover, by mediating a fundamental inter- and intracellular signaling it dictates specialized regulatory functions, especially during development and repair processes [26–32]. Type I collagen is one of the most widely used biomaterials in the health-related sectors, including medical care and cosmetics [17–25,33]. Several collagen-based injectable products have been approved for clinical use and used in many clinical settings.

This review will specifically focus on the clinical efficacy of collagen-based injectables for both aesthetic and regenerative medicine from 2000 until today. In particular, collagen extraction sources for injectables development and relative applications are discussed. To the best of our knowledge, we collected and discussed all pertinent research reports, commercial products data, and clinical trials about approved collagen-based injectable formulations, in order to underline the advantages and disadvantages related to their use. Accordingly, the available clinical results of the last 20 years about some of the leading collagen-based approved products were gathered and discussed according to the body site and pathology. In particular, this review focused on collagen-based injectables currently used for the regeneration of the musculoskeletal, urogenital, gastrointestinal, and integumental systems as well as for non standard clinical applications by presenting exemplary attempts to improve tissues' regenerative performance. Finally, collagen-based products adverse events rate and their regulation are discussed.

### **2. Methodology**

A deep search was undertaken on studies about injectable collagen alone or in combination with other materials for cosmetic and medical applications. The electronic search engines used were PubMed (https://pubmed.ncbi.nlm.nih.gov, accessed on 4 January 2023), ScienceDirect (https://www.sciencedirect.com, accessed on 4 January 2023), Google Scholar (https://scholar.google.com, accessed on 4 January 2023) and U.S. National Library of Medicine (https://clinicaltrials.gov/, accessed on 22 December 2022). The keywords used were 'injectable' and 'collagen'. Several synonyms were searched for each component (i.e., injection, hydrolysate, gelatin, dermal filler, solution, colloid, infusion, hydrogel). The search included all studies related to injectable collagen-based formulations, including clinical trials, prospective case series, retrospective reviews, and case reports, independently from their level of evidence. A total of 125 studies were screened from 2000 to 2022 and reviewed.

### **3. Collagen as Biomaterial**

Collagen is the most abundant structural protein of vertebrate connective tissues, and accounts for about the 30% of the total body protein content [17–25]. The collagen family is a group of proteins that share a unique molecular fingerprint that is characterized by the presence of a right-handed triple-helical domain formed by three left-handed polyproline-II helices [26,34,35]. This superfamily accounts for 28 members, named from type I to XXVIII according to the discovery order [34,36]. Type I collagen was the first to be discovered and accounts for the 70% of the total collagen found in the human body [26]. This protein is a hetero trimer of about 400 kDa consisting of two identical α1 (≈139 kDa) chains and one α2 (≈129 kDa) chain of about 1000 amino acid residues [20,37]. Both chains are characterized by the repetition of the Glycine-X-Y triplet, where the X and Y positions are usually represented by proline and hydroxyproline, respectively [34,37]. Hydroxylation of proline residues is a typical modification of collagen and, because it accounts for about 11–14% of total residues, it is commonly used as a marker to detect and quantify collagen in tissues [35,38]. Another peculiarity of fibril-forming type I collagen molecules is their ability to spontaneously assemble to form fibrils in which molecules are quasi-hexagonally packed and super-twisted in a right-handed structure along the longitudinal axis of the fibril [39–41]. Thus, collagen molecules are aligned parallel to one another with a staggering of about 67 nm (D-banding) and can assemble into fibrils that can be greater than 500 µm in length and 500 nm in diameter [25,34,42,43]. Then, fibrils assemble in fibers whose 3D arrangement is tissue specific.

Type I collagen not only covers a crucial structural role in tissue architecture maintenance but is actively involved in several biological and pathological processes [44]. The involvement of collagen in numerous cellular processes prompted research towards the use of collagen as biomaterial for the development of simplified ECM-like structures [20,35]. To this, several companies isolate medical-grade type I collagen from several sources (Table 1) and manufacture collagen-based implantable devices that are currently used in many clinical settings. Besides its advantages in term of biocompatibility for its physiological structural and non-structural functions, the use of collagen as biomaterial offers several advantages including low immunogenicity, tunable properties, and biodegradability. The low evolutionary gap and the high conservation of type I collagen amino acid composition among vertebrates make that homology up to 95% [19,45–48].


**Table 1.** Most important world companies producing clinical grade collagen and related extraction sources.

The possibility to define specific scaffolds properties (i.e., by tuning protein concentration, solvent type and concentration, protein molecular weight, superficial morphology, 3D organization, and by pre- and post-production processing) offers a great opportunity

to modulate the structure-related biological activity of the scaffolds in order to optimize their capability to induce and sustain tissue regeneration [20,35,43,49–55]. Moreover, the use of collagen is advantageous for regenerative medicine and tissue engineering applications because, being recognized as a self-molecule, it is metabolized by the natural body enzymatic apparatus that gradually breaks down collagen molecules and substitutes it with newly synthetized one. Molecular pathways that mediate collagen degradation are several and are tissue and cell specific [39]. In general, the human body has several collagen degrading enzymes including the matrix metalloproteinases (in particular, matrix metalloproteinase 1), and cathepsins and neutrophil elastase that cleave collagen molecules which undergo a successive proteolytic process that depends on several factors (i.e., triple helix stability, protein amino acid sequence, crosslinking) [56–61]. Generally, collagen fragments resulting from the action of collagenases are further degraded by gelatinases and non-specific proteases. Thus, the presence of an accurate and complex degradation system for the endogenous collagen makes the exogenous collagen highly biodegradable and low immunogenic. Recently, the attention on collagen degradation pathways has grown for the even more evident collagen critical role in tissue homeostasis [39]. Evidence about collagen and its degradation products could also be helpful in promoting the restoration of tissue structure and function [62].

### **4. Historical Overview on Collagen-Based Injectable Formulations**

The history of biomaterials used as soft tissues filler dates from before the 19th century. The first injectable filler, which was autologous fat, was used in 1893 for forearm scar filling [63]. Since then, several materials have been used for the development of injectable formulations. Some of them were abandoned because of the development of mediumsevere adverse reactions (e.g., paraffin: embolization, granuloma formation, migration; silicone: granuloma formation; teflon: inflammatory reaction) [15,64]. Among them, autologous fat is still used as filler for its biocompatibility, availability and low cost. However, its long-term variable and unpredictable results limited its employment [10,15].

A strong turning point happened in 1981 with the development and Food and Drug Administration (FDA) approval of the first collagen filler, Zyderm® (Inamed Corporation, Santa Barbara, CA, USA). A new aesthetic procedure category of injectable treatments known as "fillers" was created. This paved the way for research into and development of biomaterials-based injectable formulations. However, the risk of immunogenic and hypersensitivity reactions soon decreased the popularity of animal-derived collagen fillers [64]. Moreover, the fear that the protein extracted from some animal tissues can be a vector for prion infections precluded their use. However, it should be taken into account that the first registered adverse events were not only related to material properties but also to the implantation methods. Proper patient selection and optimal methods of treatment delivery are crucial factors for therapeutic success and patient satisfaction [65]. Unfortunately, due to this, in the 1990s many collagen injection therapies failed because of the lack of data. Thereafter, surgeons were even more reluctant to perform collagen injections because they were commonly considered as ineffective therapies.

Thus, despite the growth of research interest in new fillers development, Zyderm® remained the only FDA approved injectable formulation until 2003 when the first hyaluronic acid based dermal filler, Restylane® (Galderma, Fort Worth, TX, USA, www.galderma.com, accessed on 14 February 2023), was approved. Since 2003, there has been an exponential increase in the number of FDA approved fillers. Indeed, both permanent (e.g., poly(methyl methacrylate), polyacrylamide, polyvinylpyrrolidone) and resorbable (e.g., collagen, hyaluronic acid, calcium hydroxyl apatite, poly(L-lactic acid) materials-based fillers were developed and clinically approved. Although synthetic compounds gained popularity as soft-tissue augmentation for their cost-effectiveness, mass production, limited immunogenicity and long-term effects, they also raised concerns over their long-term safety due to the growing data on long-term side effects or adverse events such as tissue necrosis, infection, granulomas, chronic inflammatory reaction [6–12].

In this context, resorbable fillers caught on even more for their relative safety in terms of local immunological reactions and ability to actively restore soft tissues volume. Indeed, the advantages offered by the use of minimally invasive therapies and the spread of the idea of regenerating damaged tissues pushed towards the development of temporary injectable hydrogels with specific properties. In particular, as argued by Cho et al., injectable bioactive formulations should: (i) be biocompatible without toxicity or immunogenic phenomenon after degradation; (ii) have mechanical properties compliant with the targeted tissue; (iii) be able to keep drugs and cells in the injected area; (iv) have adequate permeability, pore size and interconnectivity for mass transport and cell colonization; (v) be cost-effective; (vi) be easily handled; (vii) be biodegradable, allowing replacement by the newly formed functional tissue [66]. Indeed, an ideal injectable formulation should form a natural open pore 3D scaffold that should allow cell migration, and slowly break down stimulating growth factors and cytokines to promote neocollagenesis, elastic fiber production, neovascularization, and the wound healing response/repair [67]. Thus, ideal injectables should not only provide immediate and stable results, but also recreate natural-like extracellular matrix (ECM) for bio-dermal restoration and a long-lasting effect. However, one of the main disadvantages of resorbable filler is their short half-life. An inadequate reabsorption rate may not be sufficient to support the regenerative processes and therefore may lead to form loss. Thus, resorbable fillers-based approaches may require multiple applications to maintain their effect.

For this reason, in the last two decades, type I collagen-based products and derivates (i.e., hydrolysates, gelatin, peptides) came back into vogue because of the spreading idea of developing multifunctional fillers able to fill soft tissue defects and restore deficient tissue physiological functions [4,12,14,32,66,68,69]. The use of heterologous collagen as a medical product spread also as results of the development of both accurate extraction processes and effective sterilization procedures that improved their safety profile. Indeed, advances in purification processes allowed creation of collagen preparations with minimum immunogenicity and infection risks, with high purity levels [19,25]. Moreover, with the definition of adequate implantation protocols, collagen-based injectable therapies were re-evaluated as a minimally invasive and effective strategies for the treatment of different types of diseases. Therefore, on account of collagen's intrinsic structural and non-structural properties due to which it is historically considered as the "gold standard" material for the development of health-care related products, collagen-based injectable formulations have proved to be a promising strategy in many applicative areas. Despite the wellknown effectiveness of collagen in tissue regeneration, the recent discovery of new ECM homeostasis molecular mechanisms raised again the interest in the mechanism of action of collagen. Indeed, lately it has been discovered that type I collagen operates a traction on the type VI collagen fibrils, which forms a network of fibrils in the immediate vicinity of the cell membranes [70]. The mechanical stress that results on the cells stimulates the production of new ECM (mechano-transduction process).

### **5. Collagen-Based Injectable Formulations**

More than 60 kinds of collagen-based fillers are available on the market, according to the end-use and they have routinely been introduced in many clinical settings (Table 2). The most common collagen extraction sources for the manufacture of collagen based injectable formulations are bovine, swine, porcine, equine and human derived, whose advantages and disadvantages are described in depth elsewhere [19,20,25]. Bovine collagen is one of the most commonly used fillers for effectively reducing wrinkles and other facial imperfections. More famous branded bovine-based collagen fillers are Zyderm®, Zyplast®, Contigen® (Allergan Inc., Dublin, Ireland), Artefill® (Suneva Medical, San Diego, CA, USA), and Artecoll® (Canderm Pharma Inc., Saint-Laurent, QB, Canada). Others include CHondroGrid® (Bioteck Spa, Arcugnano, Italy), Integra Flowable Wound Matrix® (Integra LifeScience Corp., Princeton, NJ, USA), Resoplast® (Rofil Medical International, Breda, The Netherlands), Atelocell® (KOKEN Co., Ltd., Bunkyo-ku, Tokyo, Japan). However, bovine

collagen is known to be exposed to zoonosis (e.g., the foot and mouth disease and the group of the bovine spongiform encephalopathies, among which the most dangerous for humans is the transmissible spongiform encephalopathy) and to trigger allergies (about 2–4% of population) [71–73]. In addition to the strict regulation to which all implantable products are subjected, two consecutive negative patient skin tests at 6 and 2 weeks are required before use [73,74]. This sensitivity has been considered generally acceptable for implants for human use and actually bovine collagen is principally used for the treatment of the integumental [6,75–96] (NCT01060943) and musculoskeletal apparatus [97–112] and to a minor extent for the gastrointestinal [113–120], urinary [65,121–125] and cardiovascular [126–128] systems. Recently, bovine collagen in fibrillar form has been employed as an organ protection system during thermal ablation of hepatic malignancies [129].

Porcine collagen is the second most used. There are several products derived from porcine collagen, including GUNA® (GUNA, Milan, Italy) products, CartiRegen® (Joint Biomaterials S.r.l., Mestre, Italy), COLTRIX CartiRegen® and TendoRegen® (Ubiosis, Gyeonggido, Republic of Korea), CartiFill®, CartiZol®, RegenSeal® and TheraFill® (Sewon Cellontech Co., Ltd., Seoul, Republic of Korea), Dermicol-P35 (Evolence, Ortho Dermatologics, Skillman, NJ, USA), Fibroquel® (Aspid S.A. de C.V., Mexico City, Mexico), Fibrel® (Mentor Corporation, Santa Barbara, CA, USA), Permacol® (Tissue Science Labs., Aldershot, UK) and RPC Pure Collagen® (EternoGen LLC, Columbia, MO, USA). Among them, Dermicol-P35®, was withdrawn from the market in 2009. Compared to other animal derived collagens, porcine collagen-based injections are said to be rather painful and may cause allergic reactions [17]. While bovine collagen is used for many purposes, porcine collagen is almost exclusively used for the treatment of diseases belonging to the musculoskeletal apparatus [130–146] (NCT02539030, NCT02519881, NCT02539095), followed by the integumental apparatus [67,76,86,87,147–152] (NCT03844529, NCT00891774, NCT00929071) and gastrointestinal apparatus [116,117,153–159]. Only recently porcine collagen potential has been explored for the treatment of facial nerve palsy [160] and for the treatment of COVID-19 due hyperinflammation [161,162] (NCT04517162). However, despite their wide use and effectiveness, bovine and porcine collagens suffer from cultural or religious concerns (bovine collagen: Sikh, Buddhism; porcine collagen: Jewish, Islamic faiths), which restricted their applicative potential [18,19].

Equine collagen is the third most used collagen. It is free from the risks of triggering immune reaction and of zoonosis transmission, as reported elsewhere [19]. This kind of collagen is less used than bovine and porcine derived collagen for the manufacture of injectable formulations because of its naturally high hierarchical organization that made it more compliant for other applications (i.e., sponges, thin substrates). Thus, less injectable products from horse collagen are available but recently discovered advantages deriving from its use [19] are driving the development of new equine collagen-based products. Among them, Nithya®, Linerase® (Euroresearch, Milan, Italy), Salvecoll-E® (Nearmedic Italy S.r.l., Como, Italy), Biocollagen® and ActivaBone® (Bioteck Spa, Arcugnano, Italy) are commercially available and are mainly used for the treatment of diseases belonging to the integumental [163,164], urogenital [165] and gastrointestinal [160] apparatus. Its potential has also been recently explored for the treatment of periodontal tissues, with encouraging outcomes [166,167].

Human collagen fillers were developed in the early 2000s and are principally used for the integumental apparatus (e.g., facial soft tissues augmentation, wrinkles, scars, fat atrophy, diffuse depressions, paralyzed lips and tongues, nasolabial folds, and others) [6,168–172] and have been investigated for diseases of the gastrointestinal apparatus (e. g., vocal folds) [118,173–175]. In particular, there are three kinds of human collagen based injectables: autologous reconstituted collagen formulation (Autologen® and Dermologen®, Collagenesis, Inc., Beverly, MA, USA) [173], autologous collagen formulations from in vitro cultured cells (Isolagen therapy® from Fibrocell Science, Exton, Pennsylvania, USA,; Cosmoplast® and Cosmoderm® from Inamed Corporation, Santa Barbara, CA, USA)(NCT00655356)[6,169], and reconstituted collagen formulation from deceased humans

(Fascian® from Fascia Biosystem, Beverly Hills, CA, USA; Dermalogen® and Cymetra® from Life Cell Corp., Branchburg, NJ, USA) [6,118,168,170–173]. Autologous reconstituted collagen formulations are produced from collagen harvested from patients' skin small biopsy, harvested during an earlier procedure, and liquefied for future re-injection. Two square inches of donor material are required to formulate a 1-mL syringe of injectable material, which can be stored for 6 months [176]. This procedure was developed by Collagenesis Inc. (Beverly, MA, USA) and is commercially known as Autologen®. As previously noted, human collagen fillers can also be derived from in vitro cultured autologous cells. In particular, skin cells from behind the human ear could be harvested, cloned, and derived collagen could be then harvested, liquefied, and injected. This procedure was developed by Fibrocell Science Inc. (Exton, PA, USA) and is commercially known as azfibrocel-T (formerly Isolagen Therapy®). Being autologous collagen, these kinds of formulations are allergy free, making them an excellent alternative to animal-derived treatments. Apart from general mild disorders (bruising 5%, erythema 15%, hemorrhage 10%, with numbers comparable to placebo groups), this kind of human derived formulation does not trigger serious adverse events (NCT00655356). Human collagen fillers could also be prepared from deceased human donors, with the main advantages of extensive raw material availability and the reduced preparation time compared to both autologous reconstituted collagen formulation and autologous collagen formulations from patients' own in vitro cultured cells. Injectables from human donors (Dermalogen®) were firstly developed by Life Cell Corporation (Branchburg, NJ, USA). Because Dermalogen® originates from humans, also the deceased human-derived collagen-based injectables do not need an allergy test. Although humanderived collagens proved to be a good alternative, they have some disadvantages such as long preparation times, non-availability of sufficient donor tissue and high management costs (i.e., harvesting, donor tissue availability, isolation, manufacturing, need for highly specialized teams and instruments, refrigerated and limited storage, shipping) [7,176,177]. Moreover, while no efficacy differences emerged between the use of autologous collagen and animal-derived collagen, a 2–3 folds greater injection of cadaveric collagen is needed for similar augmentation results to those achieved with bovine collagen [176]. Thee mentioned drawbacks, together with the insubstantial difference in terms of efficacy compared to animal-derived collagen-based injectables, led to the progressive abandonment of human collagen for large scale applications and its exclusive use for patients with hypersensitivity to animal derived collagens.

In the last decade, new solutions were offered by recombinant collagens. Indeed, two injectable fillers, consisting of collagen, hyaluronic acid and carboxymethylcellulose, are now commercially available. In particular, Fillagen® (Monodermà, Milan, Italy), made with recombinant polypeptide of collagen α1-chain from silkworm [178], and Karisma® (Taumed, Rome, Italy), made with unspecified recombinant collagen were proposed. More recently a photocurable collagen-based regenerative dermal and soft tissues filler was developed by CollPlant Biotechnologies Ltd (Rehovot, Israel, www.collplant.com, accessed on 14 February 2023), comprising a recombinant type I collagen from tobacco plant (not currently commercially available).

