Recent advances of collagen composite biomaterials for biomedical engineering: antibacterial functionalization and 3D-printed architecturalization

Collagen possesses high biocompatibility with all tissue and cell types in the body, enabling the creation of multifunctional composite materials for medical applications. In biomedical engineering, naturally-sourced collagen is often combined with diverse organic and inorganic bioactive components to eliminate defects and disorders in fields including orthopedics, dermatology, and more. At the same time, medical-related infection issues and the precise treatment needs of patients require collagen composite biomaterials to have antibacterial properties and customized structures. This paper reviews the antibacterial functionalization of collagen composite biomaterials in recent years, including the combination with inorganic or organic antibacterial agents, which is beneficial for preventing and controlling biological contamination in medical applications. Then, the existing problems and future development directions for the architecturalization of collagen composite materials with 3D printing were discussed, providing guidance for personalized customization of multifunctional materials to meet the specific needs of patients in the future.


Graphical Abstract 1 Introduction
Due to the increasing aging population, various degenerative diseases, and related complications, the demand for repair and alternative medical materials is steadily increasing.Collagen accounts for 35% of all proteins in the human body and has received special attention in the fields of repair and regenerative medical engineering.As the most ubiquitous protein in the body, collagen is biocompatible, biodegradable, weakly antigenic, and mass-extractable.Moreover, it also induces or regulates numerous cellular functions like differentiation, mobility, communication, and apoptosis.Based on the high biocompatibility between collagen and different tissues and cells, multifunctional collagenbased materials have been widely designed for medical applications [1][2][3].At the same time, collagen plays an important role in maintaining the structural integrity and normal physiological functions of the human body [4].It is closely related to many disease treatment [5], including skin repair, orthopedic disease, tumors, etc.The research and utilization of collagen will bring many breakthroughs and progress to the medical and health industry.However, when collagen is used alone in the biomedical field, there may be defects such as insufficient mechanical strength, rapid degradation, and lack of antibacterial properties [6].Therefore, it is a development trend to combine various natural or synthetic materials with collagen to obtain composite biomaterials with optimized performance.
Collagen composite materials are often used in medical fields such as tissue engineering scaffolds, bone repair, and wound repair dressings.There is a potential risk of bacterial infection in these application environments, and once an infection occurs, it will cause serious consequences.In addition, bacteria can accelerate the degradation of the collagen material and shorten its service life [7].Meanwhile, due to the abuse of antibiotics, some bacteria have developed drug resistance [8], and endowing materials with their own antibacterial activity can help address this challenge.To enhance the safety, efficacy, and durability of materials for applications in medical and biological fields, it is crucial that collagen composite materials possess antibacterial properties.At present, combining collagen with various organic and inorganic antibacterial active ingredients is a practical and feasible strategy [9].
In addition, there are many problems with the traditional manufacturing methods of collagen composite biomaterials, such as poor personalization ability, simple internal structure, and difficulty in integrating bioactive factors.Standardized structure cannot adapt well to the individual request of different patients, and simple structure is difficult to simulate and reproduce the complex gradient structure of natural tissues [10].3D printing technology can effectively solve these problems, providing a powerful mean for the preparation of personalized collagen composite scaffolds and having broad application prospects in the field of tissue engineering [11][12][13].By adjusting the composition of collagen-based bioink, and selecting different printing strategies, personalized scaffold materials with composite functions (such as cell adhesion, bone conduction, and vascularization) that simulate natural tissue structures can be obtained for repairing damage to different tissues and organs [14].
This review introduces the applications of collagen composite materials in the biomedical field.Especially, the latest development in the antibacterial functionalization of collagen composite materials is reviewed, which expands the application of various antibacterial active ingredients in collagen medical materials.Besides, the architecturalization of collagen composite materials with 3D printing is explored to create structurally adjustable tissue engineering scaffolds.

Collagen
Collagen (Col) is a protein family with a characteristic triple helix structure and is the main component of the extracellular matrix (ECM).The ECM structure and composition vary across different tissues and organs, as does the collagen type present.The collagen family contains 28 types, differing in amino acid sequence and degree of hydroxylation or glycosylation.All collagen types possess one or more triple-helix domains.Over 90% of collagen is type I, II, III, and IV.Col-IV is part of the basement membrane underlying epithelia and surrounding most organs [15].Col-III is found in the skin, uterus, gums, and blood vessels.Col-II is present in the cartilage, and Col-I is present in the bone matrix, dermis, dentin, and internal organs.Col-I exists in almost all human connective tissue types (Fig. 1).Collagen is the most abundant structural protein in the human body, capable of forming molecular scaffolds and providing mechanical strength to tissues and organs.At the same time, collagen can bind various cell surface receptors and regulate cellular activities [16].It plays a vital role in tissue regeneration by modulating the dynamic balance between synthesis and degradation, ultimately guiding the formation of tissues or organs with specific structures and functions.Additionally, collagen demonstrates excellent biocompatibility, biodegradability, absorbability, suitable porosity, and beneficial cell growth and metabolic interconnection structure [17,18].These qualities make collagen one of the most widely utilized biomaterials in biomedicine.Currently, it has been widely used in fields such as bone repair, wound dressings, and drug sustained-release carriers.

Collagen composite biomaterials
Due to the low mechanical properties and fast degradation rate of collagen, its potential as a standalone biomaterial is limited.Therefore, combining collagen with other organic and inorganic materials to multiply its characteristics and obtain composite materials suitable for specific applications is a current research hotspot (Table 1).Currently, collagen composite materials that simulate bone and cartilage tissues have gradually developed.Among them, hydroxyapatite (HAp) and calcium phosphate are effective bone inducers that are often added as active ingredients to collagen materials.Meanwhile, natural polysaccharides, such as chitosan, alginate, and hyaluronic acid (HA), can be added to give collagen more properties of promoting cell proliferation, increasing glycosaminoglycan levels in chondrocytes, and relieving joint swelling and pain.The addition of some synthetic polymers or natural polymer networks also can further enhance the mechanical properties of collagen composite materials and avoid collagen rapid degradation.Therefore, by combining these organic/inorganic active ingredients with collagen through physical or chemical means, the resulting composite material can better meet the needs of bone and cartilage tissue repair (Table 1).
Collagen has the advantages of good biocompatibility, hemostasis, and promoting healing, making it very suitable as a material for skin tissue repair.Numerous investigations have confirmed that organic compounds of plant and animal origin, as well as some inorganic compounds, are highly effective biologically active components of wound repair [19,20].Commonly used active ingredients, such as cellulose, chitosan, and sodium alginate (SA), possess desirable properties including biocompatibility, biodegradability, environmental friendliness, and cost-effectiveness.Meanwhile, they can effectively promote wound healing, reduce the risk of infection, and provide other benefits like inflammation reduction and angiogenesis stimulation [21].Effectively combining collagen with these materials and balancing their interaction relationships are beneficial for achieving synergistic effects and obtaining high-performance skin repair materials.The most recent developments are presented in Table 1.

Discussion and brief summary
In concluding research on collagen composite materials, the adding functional components can be mainly classified into the following categories: (1) bioactive molecules, including growth factors, cells, etc., mainly induce and regulate specific behaviors of cells; (2) antibacterial agents, including various organic and inorganic antibacterial materials, mainly endow materials with antibacterial functions; (3) mineralization inducers, such as HAp, calcium phosphate, and bioactive glass, primarily facilitate bone tissue regeneration by inducing biomineralization processes within the materials; (4) drug molecules that target specific diseases or injuries; (5) other performance-enhancing components, which mainly endow materials with functions such as mechanics, electronics, and optics.
These functional components can exert various biological, physicochemical, and other effects by binding to collagen.Simultaneously, they can achieve regulation of material properties and tissue repair processes, thereby greatly expanding the application range of collagen materials.At the same time, there are intricate interactions between functional components and collagen, and it is necessary to deeply study the acting mechanisms to comprehensively enhance the overall performance of the composite materials.These mechanisms mainly include the loading methods of functional components in the collagen network, the influence of collagen network structure on the release behavior of functional components, the biological effects of composite materials, and the balance relationship between the various properties of composite materials.A deeper understanding of these mechanisms will help in the precise design and optimization of functionalized collagen composites.

M/S3 Shark collagen + squid chitosan + brown seaweed fucoidan solution
Marine polymeric membrane imitating cartilage Membranes have demonstrated significant water and gas uptake, along with the ability to withstand mechanical loads similar to native tissues [23].

M/J5S3
Jellyfish collagen + shark collagen + squid chitosan + brown seaweed fucoidan Marine polymeric membrane imitating cartilage Membranes have demonstrated significant water and gas uptake, along with the ability to withstand mechanical loads similar to native tissues [24].

CoChHa1 and CoChHa2
Col-I + chitosan + hydroxyapatite Bone-mimetic scaffold CoChHa2 (Ha was introduced into the scaffold by the ex situ method) stimulated bone formation more efficiently than CoChHa1 (Ha was introduced in situ) [27].for bone tissue regeneration The patches supported mesenchymal stem cell adhesion, colonization, and early differentiation [29].

Coral collagen fibers + alginate
Treatment of intervertebral disc degeneration The mechanical behavior was found to reproduce the full stress-strain behavior of the human AF single lamella in several regions of the AF in vitro and in vivo [30].

CPC-P-15
Calcium phosphate cement + the collagen I mimetic P-15 Composite for pedicle screw augmentation in osteoporotic bone CPC-P-15 induced osteoblastic differentiation of human MES and improved the pullout resistance of pedicle screws in osteoporotic bone [31].

Spinal cord injury
The composite has a mechanical strength of 0.1281 MPa, similar to that of the native human spinal cord, and stimulates HAC differentiation into neuronal cells [33].

Bone reconstruction
The scaffold has good mechanical properties with a controlled degradation rate, as well as osteogenic induction in vitro and repaired of 5 mm cranial defects in vivo [34].

Wound dressing
Composition stimulated fibroblasts to produce VEGF-A and promoted the closure of splinted wounds in mice [37].

Bilayered antimicrobial collagen-based scaffold
Collagen/chitosan + collagen/glycosaminoglycan Treatment of diabetic ulcers Scaffold inhibited the growth of S. aureus, stimulated the proliferation of fibroblasts and endothelial cells, and accelerated the healing of ulcers [38].

Antibacterial functionalized collagen composite biomaterials
Through summarizing the latest developments in collagen composite biomaterials mentioned above, it is found that composite materials need to have good mechanical properties, biocompatibility, and a certain promoting effect on repair.Simultaneously, we realize that materials utilized in biomedical applications require greater consideration of bacterial infection issues.Nonetheless, most composite materials do not possess antibacterial properties currently.Therefore, in order to prepare collagen composite materials with antibacterial properties, combining Col with organic/inorganic antibacterial agents is a common strategy.

Combination with organic antimicrobials
Antibiotics are well-known drugs that play an important role in the treatment of bacterial infections.The most commonly used antibiotics in the biological field are ciprofloxacin, cefotriaxone, tetracycline hydrochloride, amoxicillin, and so on [44] Among them, antibiotics that combine with collagen mainly include aminoglycosides and tetracyclines [45].For example, mupirocin/ collagen sponges used for skin wound healing treatment showed significant antibacterial activity against methicillin-resistant S. aureus (MRSA) and B. subtilis [46].Collagen-based scaffolds containing doxycycline are beneficial for improving the gap closure rate of rat bone defects [47].Tripathi et al. [48] prepared collagen-based sponges loaded with calcium peroxide and ciprofloxacin.
The sponges accelerated wound closure with the help of antibacterial activity and hypoxia conditions.In addition, regulating collagen concentration was beneficial for better sustained release of the antibiotic cefazolin, thereby accelerating wound healing [49].Collagen has amino and carboxyl groups, which can combine with antibiotic molecules through chemical or physical interactions to form new antibacterial composite materials [50].By adjusting the concentration and crosslinking degree of collagen, controlled release of antibiotics can be achieved.However, the interplay between them could also potentially elicit some adverse effects [51,52], such as diminishing or inhibiting the intrinsic antibacterial activity of the antibiotics, altering collagen conformation to affect its physicochemical properties and biological functions, heightening cytotoxicity and immunogenicity, as well as facilitating the emergence and dissemination of drugresistant strains.
Other organic antibacterial materials from natural sources, such as chitosan and curcumin, have good biocompatibility, biodegradability, and antibacterial properties, and the composite with collagen has great potential in biomedical applications.Chitosan (Chi) can resist various microorganisms and has the ability to chelate metal ions.Multifunctional Chi/Col composite materials have been widely studied in fields such as wound dressings, drug delivery, and tissue engineering.For instance, chitosan/collagen sponge prepared by freeze-drying method had good antibacterial and hemostatic effects [53].Xiong et al. [54] fabricated a series of hydrogels with promising antioxidative and antibacterial traits based on recombinant human collagen type III and chitosan.Curcumin (Cur) is a natural polyphenol compound, also a hydrophobic drug, with antibacterial activity and wound healing ability.Cur/Col/cellulose nanocrystal sponge dressings provided complete wound closure on full-thickness burn wounds within 21 days, due to the long and sustained release of curcumin with antibacterial, antioxidant, and anti-inflammatory characteristics [55].Cur-loaded polycaprolactone/Col composite membranes are promising candidates for use as antibacterial dressings to enhance clinical wound management [56].In addition, some other herbal extracts with antibacterial and anti-inflammatory effects are also commonly used to prepare collagen composite materials, which is an ideal alternative strategy for antimicrobial biomaterials [57].Moreover, these antibacterial agents possess a narrow antimicrobial spectrum and lack long-lasting efficacy, which may fail to satisfy the demands of clinical applications [58].Therefore, it is necessary to comprehensively consider various issues, such as interactions between materials, antibacterial activity and durability, material properties and biosafety, in order to achieve optimal antibacterial effects and maximize clinical application potential.

Combination with inorganic antimicrobials
Antibiotic overuse has led many bacteria to develop antibiotic resistance, greatly reducing antibacterial efficacy.To find new antibacterial agents less prone to resistance and with excellent antibacterial effects as antibiotic alternatives, researchers have focused on inorganic antibacterial agents like metal or metal oxide nanoparticles.Metal/metal oxide nanoparticles can bind to bacterial cell walls through electrostatic interactions, hydrophobic forces, Van der Waals forces, etc [59][60][61][62].The presence of functional groups on the surface of collagen makes it possible to improve its properties by combining metal or metal oxide nanoparticles.These composites acquire new functional properties such as antibacterial activity and the ability to interact with cells and other biomolecules.Metal-containing nanocomposites are a class of hybrid materials combining the properties of biopolymer matrices with metal-containing nanoadditive properties.Typically, metal and metal oxide nanoparticles (NPs) are frequently used as additives [63].Incorporating metal or metal oxide nanoparticles into collagen can improve the mechanical strength, antibacterial activity, electrochemical performance, barrier qualities, or biological capabilities of materials.
Silver nanoparticles (Ag NPs) have attracted much attention due to their broad-spectrum antibacterial properties and effectiveness in fighting various infections and diseases [64].A great deal of research has shown that these nanoparticles can kill bacteria, but the antibacterial mechanism of Ag NPs is still unclear.Some researchers believe that when silver ions released from Ag NPs reach a certain concentration range, they will inhibit the growth of bacteria, destroy cell membranes, and prevent DNA replication and transcription.Composite gels consisting of collagen and silver help to eliminate infection of skin wounds and promote the survival and growth of skin-related cells (fibroblasts, endothelial cells, keratinocytes, etc.).Previous studies have introduced Col-Ag nanocomposites through surface modification for the clinical treatment of vascular regeneration [65](Fig.2).An effective coating for the treatment of purulent wounds was obtained on the basis of collagen and Ag NPs [66].Collagen nanofibers containing Ag NPs increased the rate of collagen deposition at the wound site [67], while collagen hydrogels containing Ag NPs reduced the expression of the proinflammatory cytokines IL-6 [68].In addition, a Col/ ORC/Ag composite was used for the healing of chronic wounds in patients with diabetes foot ulcers.The composite improved the wound microenvironment and prevented bacterial infection, thus accelerating wound healing [69].Ag NPs/polyvinyl acetate/Col nanofiber membranes had enhanced antibacterial activity against drug-resistant bacteria and could be used as wound dressings [70].
Gold nanoparticles (Au NPs) have many unique properties, like low toxicity, photothermal effect, large specific surface area, multifunctional surface modification, multivalent effect, etc. Due to the excellent photothermal effect of Au NPs, local heat is generated under laser irradiation, thus destroying the cell structure.In addition, the collagen/Au NPs composite showed good vascular regeneration, mechanical properties, and thermal stability [71].Collagen could be used as a reducing agent to reduce [AuCl 4 ] − in situ to obtain Au NPs and to obtain self-assembled collagen hybrid hydrogels with enhanced mechanical properties [72].
Fig. 2 Functionalized collagen-silver nanocomposites for vascular regeneration application [65], copyright 2021, with permission from Elsevier Besides Ag NPs and Au NPs, other metal nanoparticles have also been actively studied due to their antibacterial and anti-inflammatory properties.For example, Col-ZrO 2 hybrid scaffolds had biocompatibility, which could prolong drug release time, and provide long-term effective treatment [73].Col/ZnTiO 3 nanocomposites exhibited antibacterial activity, and their antibacterial mechanisms included physical damage, metal ions, and oxygen-free radical sterilization [74].Corn alcohol-soluble protein/PCL/collagen composite materials containing ZnO and aloe vera nanoparticles were fabricated.They exhibited good cell compatibility and considerable antibacterial activity, making them suitable for application as antibacterial dressings for skin wounds [75].Col/PCL/ZnO composite materials prepared by electrospinning could be used for skin wound regeneration [76].Moreover, copper has bactericidal effects on fungi, Gram-positive bacteria, and Gram-negative bacteria [77].Collagen scaffolds containing copper-coated bioactive glass (CuBG-Col) could induce bone growth and angiogenesis while preventing infection [78].The CuBG-Col scaffold exhibited good antibacterial activity by inhibiting the growth of S. aureus.At the same time, copper had a certain positive effect on stimulating the production of vascular endothelial growth factor by bone marrow stem cells [79].
HAp, collagen fibers, and some trace elements play important roles in bone formation [80][81][82].HAp [Ca 10 (PO 4 ) 6 (OH) 2 ] has a hexagonal crystal structure where the (PO 4 ) 3− group can bind to other foreign ions.Thus, many metal cations (Ag + , Na + , Mn 2+ , Mg 2+ , and Sr 2+ ) are used to replace calcium ions in the HAp lattice [83][84][85].For example, materials obtained by combining HAp modified with metals such as gold [86], silver [87], and iron [88] with collagen exhibit high antibacterial effects and promote bone tissue regeneration.HAp-metal/Col composite materials have been proven to have excellent biocompatibility and antibacterial properties and can be used as bone repair materials [89].HAp-Ag NPs/Col composite gel had better inhibitory effect on bacterial growth with the increase of silver ions released [90].Gold nanoparticles were deposited on the surface of HAp, and Au-HAp was produced quickly and easily via microwave procedures (Fig. 3).Using doxorubicin as a model drug, Au-HAp/Col nanocomposites were investigated for drug loading and release efficiency [91].Furthermore, research has been conducted to fix Col onto Au NPs via in situ chemical reduction and then combine it with HAp to form composite materials for bone tissue regeneration [92].Ti-HAp/ Col composite material had high antibacterial activity against S. aureus and E. coli [93].The good biocompatibility between antibacterial ZnSi/HAp/Col scaffold and bone marrow stromal cells contributed to bone tissue regeneration [94].Graphene oxide (GO) is a potential antibacterial material with good biocompatibility and mechanical properties.It can achieve antibacterial effects by physical damage, generated ROS, or local heat under different stimuli.GO nanoparticles were used as antibacterial additives to enhance the marine collagen membrane and overcome some of its limitations [95].Collagen sponge combined with GO and Ag NPs can be used for skin wound healing to prevent external infections after implantation [96].
Owing to the presence of hydroxyproline in its molecular sequence, collagen can act as a reducing agent and stabilizer for the synthesis of inorganic metal nanoparticles [97].Inorganic antibacterial agents typically have long-lasting and broad-spectrum antibacterial activity and can be combined with collagen under mild conditions.In the initial stage, composite materials should release sufficient concentrations of antibacterial active ingredients to kill bacteria while maintaining sustained release to prevent subsequent infections [98].Controlled Fig. 3 A Rapid microwave-assisted synthesis of Au-HAp nanoparticles; B Schematic representation of collagen coating and electrostatic interaction with Au-HAp nanostructure.Reprinted from ref [91], copyright 2018, with permission from Elsevier release of antibacterial agents can be attained by modulating the interactions between inorganic particles and collagen, the dispersion state of inorganic particles within the matrix, the collagen degradation rate, and other factors [99].Meanwhile, the introduction of an appropriate amount of inorganic phase can improve the mechanical strength and toughness of collagen materials, as well as endow them with other functions such as promoting mineralization and repair.However, some inorganic antibacterial agents like Ag NPs pose a certain toxic risk to human cells at high doses.Introducing some precious metal ions, including silver, gold, etc., will increase the manufacturing cost of composite materials.Therefore, it is necessary to consider many complex factors such as the release kinetics of antibacterial agents and biocompatibility, and optimize the composition, structure, and preparation process of the materials on this basis to obtain ideal antibacterial collagen composite materials.

Combination with both inorganic and organic antimicrobials
The organic/inorganic composite antibacterial system is also a common antibacterial functionalization strategy that can maximize the synergistic advantages to achieve efficient, broad-spectrum, long-lasting, and low toxicity antibacterial functions.The research on metal/Col/ Chi composite materials has always been of great concern [71,100].For example, studies have shown that Ag NPs/Col/Chi composite scaffolds were beneficial for promoting wound healing and inhibiting scar formation [101] (Fig. 4).The Col/Chi composite sponge containing Ag NPs and silymarin nanoparticles significantly improved the wound healing rate [102].Moreover, Chi/ Col composite biomaterials can be used to improve the biocompatibility of metal alloy implants.Implants used for bone tissue engineering typically contain metallic elements such as Mg, Zn, Cu, Ti, Nb, Zr, and Co [103][104][105].
Copper-doped phosphate glass/Chi/Col composites were Fig. 4 Graphic illustration of Ag NPs/collagen/chitosan hybrid scaffolds implanted into the defects of male SD rats and their possible mechanisms to accelerate cutaneous wound healing [101].Copyright 2017, The Author(s), under the terms of the Creative Commons CC-BY license utilized to create implants with bone-like characteristics, which could increase bone conductivity by accelerating healing and fighting infections [106].Similarly, Curloaded nano graphene oxide-reinforced sponge dressings with superior anti-inflammatory and antibacterial features accelerated the wound closure in vivo [107].
Combining organic and inorganic antibacterial agents with collagen to form a multi-component synergistic antibacterial system can leverage various advantages.Hybrid usage can expand the antibacterial spectrum and form a broad-spectrum and efficient antibacterial system.Different types of antibacterial agents have different antibacterial mechanisms, and hybrid usage can have a synergistic effect [108].Concurrently, the fabrication process of composite systems will become more intricate, the interactions between components will be more arduous to equilibrate, the release kinetics of antibacterial agents will exhibit more complexity, and the degradation byproducts along with potential toxicity risks will pose a greater challenge for evaluation.

Discussion and brief summary
In summary, the main way to obtain antibacterial functionalized collagen composite materials is to composite collagen with organic or inorganic antibacterial agents.Different types of antibacterial agents have different advantages and limitations.Organic antibacterial agents have good biocompatibility, but they can easily lead to drug resistance issues or insufficient antibacterial activity.Inorganic antibacterial agents have excellent antibacterial performance, but there are potential toxicity issues.Organic and inorganic multiple antibacterial agents can synergistically enhance efficacy, but the process is more complex.Therefore, it is very important to balance the advantages and disadvantages of antibacterial agents, as well as the interaction behavior between the structure and properties of antibacterial agents and collagen.
In recent years, with the rapid development of nanotechnology and intelligent controlled release technology, research on the antibacterial functionalization of composite materials has gradually shifted towards functionalization based on nanotechnology, synergistic antimicrobial therapy, and intelligently responsive antibacterial functionalization [109].Nanomaterials are loaded onto collagen through physical or chemical interactions to achieve different sterilization mechanisms [110], such as physical structure sterilization, ion sterilization, photothermal sterilization, and photodynamic sterilization.Alternatively, the antibacterial agent can be prepared to nanoscale size, which is more conducive to penetration into cells and improves sterilization efficiency [111].Synergistic therapy includes the combination of multiple antibacterial agents and the combination of antibacterial agents and enzyme inhibitors to achieve synergistic effects [112].The methods of intelligent response control for releasing antibacterial agents include microbial-triggered controlled release, pH/temperature/ force stimulation response release, and biodegradable response-controlled release [113,114].These emerging technologies and innovative strategies provide more guidance for antibacterial functionalized collagen composite materials and will also give birth to more innovative antibacterial functionalized technologies applied in the field of biomedical materials in the future.

3D-printed architecturalization of collagen composite biomaterials
Due to illness, age, limb defects, and other reasons, patients need to receive specially customized medical devices, and traditional manufactured materials cannot meet the needs of patients well.In the context of big data, information, and intelligence, personalized and precise biomedical materials have gradually become a trend.3D printing is an additive manufacturing technology that can directly print 3D structures layer by layer based on computer data to produce any shape.At present, it is widely used in various fields of medicine.Collagen is an important component of the ECM in animal tissues.Because of its excellent biocompatibility, it has been used as various bioinks for tissue regeneration.Based on different damage repair purposes, structured collagen composite scaffolds can be obtained for tissue regeneration and soft and hard tissue damage repair.

Bio-inks 4.1.1 Collagen-only ink
The concentration of collagen in the ink severely affects the success of printing [115].Collagen hydrogels with low concentrations exhibit weak mechanical strength and thermal stability.And the low viscosity of collagen makes printing difficult.To construct scaffolds through 3D printing, the collagen gels or solution inks must exhibit high concentration, adequate viscosity, shear-thinning properties, and so on [116,117].The lower concentration limits printing to 1-2 mm high flat forms, while high-density single-component collagen bioink (> 20 mg/ mL) enables precise 3D structures [118].Nocera et al. [115] isolated printable collagen from bovine Achilles tendon.Rheological measurements of collagen dispersions exhibited a viscosity of 35.62 ± 1.42 Pa s at a shear rate of 10 s −1 and shear thinning behavior.Maxson et al. [119] evaluated the recellularization potential of an aortic heart valve scaffold printed with highly concentrated Col-I hydrogel (Lifeink ® 200) and MSCs.Also, except for increasing the collagen concentration, it is possible to increase the storage modulus by increasing the NaCl concentration and temperature in the solution.

Collagen hybrid ink
At present, there are several limitations to scaffolds using collagen-only ink, such as insufficient mechanical strength, limited biological activity, and a rapid biodegradation rate.Collagen typically needs to be combined with other materials to impart better printing performance, mechanical properties, and biological functionalities to the ink, thereby meeting the requirements for tissue engineering applications.The composite, which combines the organic component collagen with the inorganic component calcium phosphate, has been extensively studied to promote cell adhesion and bone regeneration.
For instance, Aydogdu et al. [120] used a PCL/β-TCP/ collagen polymer solution to 3D-print porous composite scaffolds for hard tissue engineering.Inzana et al. [121] dissolved collagen in a calcium phosphate solution and obtained cytocompatibility-enhanced calcium phosphate scaffold materials by low-temperature 3D printing.Cellloaded collagen scaffolds crosslinked with tannic acid solution, α-TCP/collagen scaffolds immersed in cellloaded collagen solution, and cell-printed α-TCP/colscaffolds had high cytocompatibility [122] (Fig. 5).Furthermore, a bioink consisting of cell-laden PLGA porous microspheres with agarose-collagen composite hydrogel (AC hydrogel) was developed.Collagen fibril formation in the AC hydrogel improved cell affinity and cell spreading compared to pure agarose hydrogel [123].
Alginate, PLGA microspheres, etc. are also frequently blended with collagen as bioinks for 3D bioprinting scaffolds.Alginate enables rapid gelation when exposed to Ca 2+ or other divalent cations, facilitating cell encapsulation and interlayer adhesion during layer-by-layer printing [12].Yang et al. [124] utilized Col-I/SA/chondrocytes as bioinks to construct in vitro 3D-printed cartilage tissues.The Col-I/SA composite markedly enhanced cell adhesion, accelerated cell proliferation, and boosted the expression of cartilage-specific genes.Functionalized Fig. 5 Fabrication schematics of cell-loaded scaffolds.A a cell-laden collagen scaffold; B a cell-laden α-TCP/collagen scaffold loaded using a dipping method; C a 3D cell-laden α-TCP/collagen scaffold loaded using cell printing.Reprinted from ref [122], Copyright 2017, The Author(s), under the terms of the Creative Commons CC-BY license alginate-collagen hybrid networks also demonstrate synergistic bioink effects.They could be used to make 3D-printed scaffolds with superior mechanical properties and cytocompatibility [125].Additionally, anti-VEGF bevacizumab was embedded in PLGA particles and then mixed with collagen as a hybrid ink.The ink was successfully printed in shape-stable meniscus-, nose-, and auricle-like structures [126].
In summary, materials combined with collagen can be roughly divided into several categories: 1 Rheological additives, which form a physical cross-linking network with collagen, can improve ink rheological properties, printing performance, mechanical properties, etc.; 2 Mechanical additives, which enhance the cross-linking between collagen or between collagen and other components, can strengthen the mechanical strength of scaffolds and slow down the degradation rate; 3 Bioactive ingredients, which are loaded into the collagen network through physical or chemical reactions, endow the scaffold with other biological activities.These materials have diversity in mechanical properties, biological activity, and degradability, providing a foundation for personalized design.

Printing strategies 4.2.1 Direct ink writing (DIW)
Direct ink writing (DIW) is a more refined and specialized extrusion molding process that can achieve higher printing resolution and is widely used in biomedical and tissue engineering fields [127].Though collagen solution's low viscosity challenges, adjusting ink viscosity, pH, and temperature enable DIW of collagen [115].A simple method utilizing three-dimensional mapping of high-viscosity, high-density collagen dispersions is developed.The swollen state of collagen fibers at pH 4 leads to uniform extrusion into homogeneous strands, ultimately constructing 3D scaffolds [128].
Further, collagen can be crosslinked through various methods to form fibers with excellent strength and stability.Glutaraldehyde, a synthetic crosslinking agent, has been widely utilized for crosslinking collagen-based constructs [129].EDC, a covalent, zero-length crosslinker, has not been reported to cause cytotoxic reactions.Nayak et al. [130] used collagen gels with suitable viscoelasticity to produce tissue-engineered scaffolds by DIW (Fig. 6A).They also examined the effects of EDC on the mechanical and biological properties of 3D-printed scaffold structures.Furthermore, UV irradiation can provide relatively low levels of collagen crosslinking without losing biological function.Davidenko et al. [131] employed short-wavelength (254 nm) UV irradiation to crosslink collagen-based scaffolds.However, higher-intensity UV irradiation has also been shown to induce conformational changes in the collagen triple helix structure, potentially causing denaturation.
Collagen remains liquid at room temperature.The low viscosity of collagen solutions makes direct 3D printing onto dry substrates challenging.To enable reproducible 3D collagen scaffolds supporting cell migration and infiltration, a researcher developed a cryogenic directplotting system [116].The resulting 3D collagen scaffolds permitted complete migration and favorable differentiation of co-cultured keratinocyte and fibroblast cells.Alternatively, the low-temperature gelation characteristics of agar could be leveraged for bioprinting on frozen platforms [123](Fig.6B).Printable bioinks based on collagen and chitosan exhibited good shear-thinning behavior.Their gelling temperatures were 7-10 °C, rendering them suitable for low-temperature 3D printing [132].
Support baths/sacrificial baths are commonly used to provide support and protection for structures during the DIW printing process [133].The support bath is enhanced with 10 mM HEPES to maintain a pH of 7.4 and neutralize the acetic acid, ensuring the crosslinking of collagen into a gel after extrusion.In order to further crosslink the collagen and melt the support bath, scaffolds are kept at 37 °C for at least one hour after printing [134].High-resolution printing of 3D collagen organ scaffolds is possible by using microgel bath containing trisodium citrate.3D collagen organ structures such as hands, ears, and hearts are successfully constructed with high shape fidelity [135].By layer-by-layer extrusion and a thermo-reversible gelatin support solution, the freeform reversible embedding of suspended hydrogels (FRESH) technology enables 3D printing of soft tissue structures [136,137].Sousa et al. achieved a stabilized microporous collagen scaffold by combining FRESH and freeze-fillcast methods [136].The collagen/alginate nerve-guiding catheter with the FRESH method promoted Schwann cell proliferation and secretion of neurotrophic protein [138].

Inkjet printing
Inkjet printing is a non-contact, precision 3D printing technique that employs tiny nozzles to precisely deposit biological inks in a layered manner, enabling the construction of intricate three-dimensional structures.Researchers have dissolved collagen in an adhesive solution to manufacture collagen-calcium phosphate composite materials and then achieved reliable thermal inkjet printing of the collagen solution by reducing viscosity and surface tension through physiological heat treatment and Tween 80, respectively [121].A method of precise and automatic deposition of collagen using highthroughput inkjet printing has been proposed to control cell adhesion and proliferation.This method demonstrates a combination of off-the-shelf inkjet printing and biomaterials and has the potential to be adapted to tissue engineering applications [139].Park et al. [140] fabricated a cancer microtissue array in a multi-well format by continuous deposition of collagen-suspended Hela cells on a fibroblast-layered nanofibrous membrane via inkjet printing (Fig. 6C).Additionally, the use of aerosol jet printing mitigates the limitations of using soluble collagen inks for extrusion-based 3D printing, particularly if microfibrillar collagen suspensions are used as an ink source [141].

Laser-assisted bioprinting (LAB)
Laser-assisted bioprinting (LAB) is a high-precision light-cured 3D printing technology that focuses a laser beam on the surface of a photosensitive material, selectively solidifies the material along the scanning path, and cooperates with a mobile platform to achieve layerby-layer molding [142].Researchers have used collagen/ nano HAp/cell composite ink and LAB to in-situ print a bone substitute for in vivo bone regeneration applications [143].Layered 3D bioprinted tissues mimicking the structure of native corneal tissue were successfully fabricated with recombinant human laminin/human sourced collagen I/human stem cells bioinks using LAB [144].

Digital light processing (DLP)
Digital light processing (DLP) is an efficient and precise photocuring 3D printing process with broad application Fig. 6 A 3D collagen scaffolds were obtained by the extrusion of 5% w/v collagen colloidal gel with the DIW printer [130].Copyright 2023, The Author(s), under the terms of the Creative Commons CC-BY license.B A cryogenic platform aided in the 3D bioprinting of scaffolds [123].Copyright 2016, The Author(s), under the terms of the Creative Commons CC-BY license.C Schematic illustration of cancer microtissue array with inkjet printing [140].Copyright 2024, The Author(s), under the terms of the Creative Commons CC-BY license.D Comparison between DLP and extension bioprinting with CMA bioink [146].Copyright 2023, with permission from Elsevier prospects in biomedical fields [145].Methacrylated collagen (CMA) has been developed as a photocrosslinkable bioink with good biological activity, but its low printability and biocompatibility limit its application in tissue engineering.Shi et al. [146] optimized the synthesis procedure of CMA, thereby enhancing its printability in extrusion bioprinting and improving its formability in DLP (Fig. 6D).A UV-curable bioink composed of CMA and a small amount of procyanidins is used in commercial DLP printers.This collagen-based ink can effectively print structures at micrometer resolution, and the fidelity of 3D structures can reach over 90%.The low concentration of CMA endows the bioink with good fluidity and excellent biocompatibility, while procyanidins increase the crosslinking density of the system to obtain better mechanical properties [147].
DIW technology is low-cost, widely available, and capable of printing viscous bioinks with very high cell density (printable viscosity range of 30 − 6 × 10 7 mPa•s, cell density of 10 5 -10 9 cells/mL), while allowing for the printing of relatively large-scale structures in a short period of time [148,149].Inkjet printing can print in the form of droplets at high shear rates, and bioinks have relatively low viscosity (< 10 mPa•s) and low cell density (< 10 6 cells/ mL) [118,150].LAB and DLP can print bioinks with high cell density (> 10 7 cells/mL) and relatively high viscosity (up to 300 mPa•s) [151].In terms of printing speed and resolution, DIW is low-speed and medium resolution, inkjet printing is high-speed and high resolution, and LAB and DLP are medium-speed and high resolution [148].Therefore, before selecting a suitable printing strategy, the following factors need to be comprehensively considered: (1) the properties of collagen bioink; (2) the geometric complexity and physicochemical properties of the bracket; (3) cell planting needs; (4) production efficiency and economic costs.Meanwhile, the correct printing strategy requires a systematic consideration of scaffold application requirements and process feasibility to achieve customized 3D printing manufacturing.

Cartilage/bone repair
Reconstruction of bone defects remains a clinical challenge.3D bioprinting is a manufacturing technique for treating bone defects through tissue engineering.Collagen is currently the most popular cellular scaffold for tissue engineering [152,153].Collagen-based scaffolds were fabricated with the aid of gelatin support baths.HAp and bone marrow mesenchymal stem cells were added to mimic the composition of bone, which provided a new avenue for 3D bioprinting bone tissue engineering [154].A multifunctional 3D-printed scaffold made of PLA, collagen, minocycline, and nanohydroxyapatite had both antibacterial and osteogenic properties [155].3D-printed crocin-loaded Chi/Col/HAp scaffolds were engineered for bone regeneration.Among all artificial bone repair materials, implantable scaffolds made from a mineralized collagen had the strongest bionic properties [156].Mineralized collagen/polylactic acid-glycolic acid copolymer/ Mg scaffolds had excellent biocompatibility, anti-inflammatory, osteogenic, and angiogenic activities.They could promote the healing and regeneration of bone tissues for the treatment of bone defects [157].
Meniscal tear is one of the most common knee injuries.Based on meniscal biochemical components, including Col-I, Col-II, and chondroitin sulfate, bioink with shear-thinning property is developed.After 3D printing, the shape of the meniscal fabricated has high fidelity and maintains dimensional stability for up to 4 weeks.This 3D-printed anisotropic meniscus has the potential to be a tool for the treatment of meniscal injuries, since it mimics the biochemical makeup of the natural human meniscus [158].Printing dECM inks at the appropriate pH promoted preferential alignment of collagen fibers.Related research demonstrated the potential for 3D printing of meniscus tissues with ECM inks containing high concentration collagen [159].Gradual wear and tear of articular cartilage leads to progressive tissue damage.Cartilage has a low regenerative capacity, making it more difficult to heal when injured.Collagen/proanthocyanidin/oxidized hyaluronic acid composite scaffolds for articular cartilage repair had high biocompatibility and excellent mechanical properties [160].
The above research indicates that bone/cartilage repair has certain requirements for material mechanics and osteogenic activity, usually requiring the combination of collagen and other materials to endow more physicochemical properties.For bone tissue repair, inorganic phases such as HAp and tricalcium phosphate (TCP), have excellent cell mineralization and bone conductivity and are often used as one of the collagen composite ink formulas.Other natural or synthetic polymers such as gelatin, alginate, and polylactic acid, which are often used as one of the formulas, can regulate rheological properties of collagen ink and improve mechanical properties of collagen scaffolds.In addition, growth factors and osteogenic-related cells can endow biological activity and are also one of the commonly used formulas for collagen composite inks.The commonly used printing strategies of collagen composite scaffolds for bone tissue repair are DIW and DLP.The current main challenge is to enhance the mechanical properties and bone conduction capacity of the scaffolds [161].One of the current research hotspots is the design of multi-level and multiscale structures of collagen composite scaffolds to simulate the hierarchical structure of natural bone tissue [162].The bioink formula and printing strategy of collagen composite inks for cartilage tissue repair are similar to those for bone tissue, but inorganic phases are generally not added.The main challenge is to enhance the mechanical properties of collagen scaffold materials and construct a realistic simulation of the biological microenvironment and gradient structure of cartilage [163].

Spinal cord injury (SCI) and traumatic brain injury (TBI)
Spinal cord injury (SCI) is a serious disabling disease that can lead to motor, sensory, and functional impairments.Col/Chi scaffolds fabricated with 3D printing technology showed remarkable therapeutic effects in a rat model of complete SCI, providing a promising and innovative treatment [164].To axially guide axonal growth, a 3D Col/silk fibroin scaffold was designed which mimicking the corticospinal tract.The scaffold improved axonal regeneration, promoted orderly connections within the neural network, and provided a novel approach for tissue repair after SCI [165].Moreover, the Col/Chi scaffold was integrated with brain-derived neurotrophic factor by low-temperature 3D printing technology.This novel artificial controlled-release system could prolong the release of BDNF for the treatment of SCI [166].
3D-printed tissue engineering material is considered promising for repairing traumatic brain injury (TBI).A 3D scaffold prepared with Col and heparan sulfate had good physical properties, a suitable degradation rate, and satisfactory cytocompatibility.Also, the scaffold had excellent performance in both structural repair and functional improvement and might provide a new strategy for TBI repair [167].Liu et al. fabricated exosome/collagen/ heparan sulfate scaffolds by 3D printing for neurological recovery in rats after TBI [168].Exosome/collagen/chitosan bioscaffolds were obtained using 3D printing technology for neural network remodeling after TBI [169].
For neural tissue repair, natural polymers like heparin, silk fibroin, and chitosan can regulate the rheological properties of collagen inks and improve the mechanical properties of collagen scaffolds, so they are commonly used as one of the formulas.In addition, growth factors, neurotrophic factors, and cells endow nerve cell growth induction ability, which is also one of the commonly used formulas for collagen composite inks.The commonly used printing strategies of collagen composite scaffolds for neural tissue repair are DIW and inkjet printing.The current main challenge is to build a biomimetic neural environment with collagen scaffolds and ensure that the printing process does not cause damage or inactivation to neurons and growth factors [170].One of the current research hotspots is the strategy of combining electrospinning with 3D printing to construct a highly biologically simulated collagen scaffold system [171].

Other applications
Recent advances in biomaterials and 3D printing/culture methods have enabled various tissue-engineered tumor models.Collagen is the main ECM protein in solid tumors, but the low viscosity of collagen solutions makes 3D bioprinting of cancer models challenging.The use of silk fibrin hydrogel as a support bath combined with hyaluronic acid enables bioprinting breast cancer cells and tumor-like organ models with low concentrations of Col-I bioinks [172].Cheng et al. [173] utilized a bicellular bioink of collagen-poly(N-isopropylacrylamide-co-methylmethacrylate) mixtures to inkjet print the morphogenesis of a tumor-stroma interface (Fig. 7).The resulting structures formed a mechanically active tumor-stroma interface with extremely high cell density due to cell-derived compaction.For the application of collagen scaffolds in tumors, bioinks often introduce synthetic polymers to improve the mechanical strength of the scaffolds.Adding tumor cell lines to collagen inks to construct tumor models or using drug molecules for drug screening research is also a universal ink formula.The most commonly used printing strategies of collagen composite scaffolds for tumors are DIW and inkjet printing.The main challenge is to precisely control the spatial distribution of tumor cells in collagen scaffolds as well as to construct heterogeneous, gradient, and complex microenvironments that simulate actual tumors [174].The research hotspot is to develop new types of collagen bioinks that integrate biological and diagnostic functions [175].
3D bioprinting of collagen holds promise for reconstructing components of the human heart [176].Lee et al. offered a method for 3D bioprinting collagen utilizing FRESH, which allows the design of human heart components at various scales [137].pH modification was utilized to control collagen gelation, and the printing resolution reached 10 μm.Early research in bioprinting vascular tissue has also utilized collagen.Smith et al. demonstrated early that five layers of arteries could be constructed from bovine aortic endothelial cells suspended in Col-I [177].Norbornene-functionalized collagen (Col-Nor) had better solubility and rheological properties and exhibited better shape fidelity in 3D printing.Photo-crosslinked Col-Nor hydrogels provided structural support and promoted adhesion, proliferation, and differentiation of various cells, resulting in a centimeter-scale liver tissue model.This highly generalizable methodology expands printable, versatile, and tunable hydrogels developed from the natural ECM, allowing the biofabrication of 3D liver tissue models with branched vascular networks [178].According to the specific application needs of biomimetic organs, natural polymers, synthetic polymers, cells, or growth factors can be added to collagen bioinks.The most commonly used printing strategies of collagen scaffolds in this application field are DIW and inkjet printing.The main challenge and research hotspot is how to use collagen scaffolds to simulate the complex physiological functions and dynamic environment at the organ level [179].
HA and Col are the most abundant proteins in the skin's ECM and are known to support a variety of cellular behaviors.Tyramine-conjugated HA and Col (HA-Tyr/ Col-Tyr) hydrogel bioinks could be photo-crosslinked in the presence of riboflavin and ammonium peroxydisulfate.They were developed to fabricate dermal-mimetic constructs [180].Lee et al. used collagen to represent the dermal matrix of the skin.They obtained skin tissue through 3D bioprinting, which was similar in morphology and biology to human skin tissue in vivo [181].In addition, the Col/SA hydrogels exhibited suitable tunability and properties for usage as a bioink for bioprinting skin, aiming at finding applications as 3D models for wound healing research [182].This formula of collagen bioink applied in skin tissue is similar to other applications, but the difference is that the added cells are targeted at repairing skin tissue, and conductive polymers can also be introduced to provide electrical stimulation function.The commonly used printing strategies of collagen scaffolds for skin repair are DIW and LAB.The main challenge is to simulate the natural skin hierarchy with multi-layer collagen scaffolds and achieve large-scale skin replacement [183].At the same time, one of the research hotspots is to endow the collagen scaffolds with skin intelligent detection capabilities [184].

Personalized customization strategy with 3D printing
The combination of medical imaging, computer modeling, and 3D printing technology to create customized stents that meet the personalized needs of patients is very valuable.The main process is as follows (Fig. 8) [185]: Firstly, medical imaging devices such as computed tomography (CT) scans or magnetic resonance imaging (MRI) are used to obtain three-dimensional image data of the patient's lesion site, which is stored in DICOM format.Then, image analysis software such as computeraided design (CAD) is used to segment the DICOM data, and an accurate 3D digital model is reconstructed, which is then output as a 3D printable STL or AMF format file.Next, appropriate biomaterials and 3D printing strategies are selected to print out customized scaffolds.Finally, post-treatments of the scaffolds (cleaning, sterilization, packaging, etc.) are performed.Throughout the entire process, the printing ink and printing strategy need to be determined based on actual needs.
At present, relevant reports have proven successful implementation of treatment cases targeting patients.On April 14, 2022, Dongguan Xiegang Hospital successfully applied 3D printing technology to perform skull repair surgery on a patient with a skull defect.An artificial skull was customized using polyether ether Fig. 7 The hypothesis of inkjet-printed morphogenesis of tumoroids with a bicellular bioink of collagen-poly(N-isopropylacrylam ide-co-methylmethacrylate) mixtures [173], copyright 2023, The Author(s), with permission from Elsevier ketone (PEEK) material and 3D printing [186].On July 23, 2022, Xiangya Hospital of Central South University used 3D printing technology to repair multiple fractures in the patient's skull and maxillofacial region and printed a 3D titanium mesh model that was completely symmetrical with the patient's left skull using the mirror principle [187].However, the 3D printing materials currently utilized in clinical practice primarily consist of inorganic synthetic materials such as titanium alloy, PEEK, PLA, PCL, and ceramics.The clinical application of 3D-printed collagen scaffolds is still blank, possibly due to poor mechanical performance and fast degradation rate that do not match the clinical application needs.There are still significant challenges in perfectly integrating the actual pathological data of patients with advanced 3D printing manufacturing technology and collagen biomaterials to achieve truly personalized precision medicine.

Discussion and brief summary
Collagen, as the main natural biomaterial, has broad application prospects in the field of 3D bioprinting.Researchers have developed various bioink formulations and 3D printing strategies for different tissue repair needs.In terms of bioink formulation, collagen is often combined with other natural polymers including chitosan and alginate to optimize rheological properties and endow specific biological activities.At the same time, functional components such as inorganic phases, cells, and biomolecules are also introduced to construct composite systems with specific biological functions.Different printing strategies, including DIW, inkjet printing, LAB, and DLP, have different requirements for bioink, printing efficiency, printing accuracy, cost, etc., and need to be selected according to actual needs.At present, 3D-printed collagen scaffolds are expected to play an important role in the repair and regenerative medicine of multiple tissues and organs, such as bones, cartilage, nerves, Fig. 8 Workflow to produce 3D-printed customized scaffolds for tissue regeneration cardiovascular, liver, and skin.But it also faces some challenges, for instance, improving mechanical properties, finely controlling pores and gradient structures, and achieving personalized manufacturing.Its future development trends include developing new printable biomaterials, integrating multifunctional and biological signals, 4D intelligent manufacturing of biomimetic brackets, and implementing highly personalized design based on artificial intelligence algorithms and image data.The emerging trend and innovative strategy of 3D printing aim to develop a new generation of intelligent, biomimetic, and functional 3D printing tissue scaffolds for promoting the development of regenerative medicine towards precision and personalized medicine.

Conclusions and prospects
Collagen is a promising material for addressing the challenges in regenerative medicine.Its ability to interact with a wide range of components enables the development of new functional materials.Collagen composite materials have many excellent properties as medical biomaterials, such as good biocompatibility, biodegradability, and stable mechanical properties.In order to obtain antibacterial composite materials, different kinds of antibacterial substances could be added, including organic and inorganic antibacterial agents.Collagen molecular chains can stabilize antibacterial agents, slow their release rate, and overcome the biocompatibility issues of inorganic antibacterial agents.In addition, the introduction of antibacterial agents not only endows materials with antibacterial properties, but also improves the problems of poor mechanical properties and rapid degradation of collagen.Therefore, there is a synergistic effect between collagen and antibacterial agents, and the resulting composite materials have significant prospects in combating bacterial infections.
Meanwhile, the properties of the materials should vary depending on the disease and individual patient needs.In order to better meet the precise treatment needs of patients, 3D printing technology can be used to obtain collagen composite materials with customized structures, which is a topic of research with enormous potential in tissue engineering.Collagen and its complexes have been used as various bioinks for tissue regeneration, and different printing strategies have different requirements for bio-inks.Adjusting the spatial distribution of bioink based on different printing strategies can help obtain the unique structure of organs or tissues that patients need.The printed composite materials have also been applied in areas such as cartilage/bone repair, SCI, and TBI.It is hoped that the advancement of this field will result in the production of therapeutically accessible mimetic materials in the near future.

Fig. 1
Fig. 1 Schematic diagram of the distribution of various collagen types in human tissues and organs

Table
The latest developments in the field of collagen composite biomaterials