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The present protocol describes a non-emulsion-based method for the fabrication of chitosan-genipin microgels. The size of these microgels can be precisely controlled, and they can display pH-dependent swelling, degrade in vivo, and be loaded with therapeutic molecules that release over time in a sustained manner, making them highly relevant for tissue engineering applications.
Chitosan microgels are of significant interest in tissue engineering due to their wide range of applications, low cost, and immunogenicity. However, chitosan microgels are commonly fabricated using emulsion methods that require organic solvent rinses, which are toxic and harmful to the environment. The present protocol presents a rapid, non-cytotoxic, non-emulsion-based method for fabricating chitosan-genipin microgels without the need for organic solvent rinses. The microgels described herein can be fabricated with precise size control. They exhibit sustained release of biomolecules, making them highly relevant for tissue engineering, biomaterials, and regenerative medicine. Chitosan is crosslinked with genipin to form a hydrogel network, then passed through a syringe filter to produce the microgels. The microgels can be filtered to create a range of sizes, and they show pH-dependent swelling and degrade over time enzymatically. These microgels have been employed in a rat growth plate injury model and were demonstrated to promote increased cartilage tissue repair and to show complete degradation at 28 days in vivo. Due to their low cost, high convenience, and ease of fabrication with cytocompatible materials, these chitosan microgels present an exciting and unique technology in tissue engineering.
The growth plate, also known as the physis, is the cartilage structure located at the end of long bones that mediates growth in children. If the growth plate becomes injured, repair tissue known as a "bony bar" can form, which interrupts normal growth and can cause growth defects or angular deformities. Epidemiological data have shown that 15%-30% of all childhood skeletal injuries are related to the growth plate. Bony bar formation occurs in up to 30% of these injuries, making growth plate injuries and their associated treatment a significant clinical manifestation issue1,2,3,4. When bony bar formation occurs, the most common treatment avenue involves resectioning the bony bar and inserting an interpositional material, such as silicon or adipose tissue5. However, patients that undergo bony bar resection surgery often have a poor prognosis for full recovery, as there is currently no treatment that can fully repair an injured growth plate6,7,8. In light of these shortcomings, there is a critical need for effective strategies for treating growth plate injuries, both in preventing the formation of a bony bar and regenerating healthy physeal cartilage tissue.
Hydrogel microparticles, or microgels, have recently gained interest as injectable scaffolds that can provide sustained release of therapeutics9. Due to their high tunability and biocompatibility, microgels are also well-suited for bioactive factor or cell encapsulation. Microgels can be made of various materials, ranging from synthetic polymers, such as polyethylene glycol (PEG), to natural polymers like alginate or chitosan10,11,12. Chitosan has been shown to have several beneficial effects for tissue engineering, such as its ability to destabilize the outer membrane of gram-negative bacteria, thereby offering inherent antimicrobial activity13,14. Additionally, chitosan is cost-effective, cell-interactive, and easily modified using its amine-containing structure. Chitosan-based microgels promise a biomaterial strategy for drug delivery and material signaling that can promote tissue regeneration while preventing bacterial infection. However, chitosan microgels are often fabricated with a wide range of techniques that require special equipment, emulsion techniques, or cytotoxic solvent rinses. For example, some studies have fabricated chitosan microgels with emulsion-based methods, but these protocols require solvent rinses and cytotoxic crosslinkers, potentially negating their translation to clinical settings15,16. Other studies have used microfluidics or electrospray approaches to fabricate chitosan microgels, which require special equipment, preparation, and training17,18. Chitosan microgels are also commonly made with a dropwise process of crosslinker into chitosan solution; however, this method is highly dependent on solution viscosity, polymer concentration, and flow rate, making it difficult to control the size and dispersity of the microgels19,20. Conversely, the method for microgel fabrication described herein requires no specialist equipment or solvent rinses, making these microgels viable for fabrication in nearly any lab or setting. Therefore, these microgels represent highly relevant biomaterials for a quick, cost-effective, and easy-to-produce drug delivery vehicle for many applications.
By modulating a microgel's composition and material characteristics, researchers can gain precise control over the cellular microenvironment, thus directing cell behavior in a material-dependent manner. Microgels can be employed on their own or combined with bulk biomaterial systems to impart specific functionalities, such as the extended release of bioactive factors or precise special signaling for native or exogenous cells. Biomaterials and microgels have emerged as attractive treatment avenues for growth plate injuries. Significant effort has been dedicated to developing alginate and chitosan-based biomaterials to treat growth plate injuries21,22,23,24,25. Due to the dynamic temporal nature of growth plate ossification and bone elongation, the mechanism of bony bar formation is not fully understood. Therefore, several animal models have been developed to better elucidate the mechanisms of endochondral ossification and bony bar formation, such as in rats, rabbits, and sheep26,27,28. One such model is a rat growth plate injury model, which uses a drill-hole defect in the rat tibia to produce a bony bar in a predictable and reproducible manner and mimics human injuries across all three zones of the growth plate29,30. Several biomaterial-based strategies for treating growth plate injuries have been tested using this model. Additionally, two different methods for fabricating chitosan microgels have been developed, which can be used as an injectable biomaterial system that releases therapeutics in a sustained manner10,31. These microgels have been employed in a rat physeal injury model, and they showed improved cartilage regeneration31 when releasing SDF-1a and TGF-b3. The techniques provided in this protocol describe methods developed to fabricate these chitosan microgels, which can then be employed in a wide variety of tissue engineering applications. For example, recent studies have used thermo- or magento-responsive chitosan microgels for controlled oncological drug delivery applications32,33.
All animal procedures were approved by the University of Colorado Denver Institutional Animal Care and Use Committee. 6-week old male Sprague-Dawley rats were used for the present study. The rat growth plate injury model was created following a previously published report30.
1. Preparation of the chitosan polymer
2. Fabrication of chitosan hydrogel
3. Preparation of microgels for in vitro or in vivo applications
NOTE: For the present study, cartilage regeneration in growth plate injuries was studied in a rat model. For details, see reference31.
Successful fabrication of chitosan microgels relies on the crosslinking reaction between genipin and chitosan, specifically involving the amines on the chitosan polymer chains. In contrast to other microgel fabrication techniques, this method does not require emulsions or solvent rinses and can be quickly and easily conducted with inexpensive equipment. A hallmark indicator for successful microgel fabrication is the distinct color change from off-white to dark blue after the chitosan and genipin have been mixed. The cros...
Microgels have been widely researched in recent years due to their high level of applicability for various purposes, such as drug delivery or cell encapsulation9. The ease of manufacturing of micro-scale biomaterial constructs is of significant relevance in tissue engineering, as it allows researchers to develop hydrogel-based strategies at a specific size and time scale. However, most methods for fabricating chitosan microgels require expensive equipment and reagents, emulsions, or cytotoxic solv...
The authors have nothing to disclose.
Research reported in this publication was supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institute of Health under award numbers R03AR068087 and R21AR071585 and by the Boettcher Foundation (#11219) to MDK. CBE was supported by NIH/NCATS Colorado CTSA Grant Number TL1 TR001081.
Name | Company | Catalog Number | Comments |
Acetic acid | SigmaAldrich | AX0073 | |
BD Luer-Lock Syringe | Fisher Scientific | 14-823-16E | |
Büchner Funnel | Fisher Scientific | FB966F | 100 mm diameter |
Chitosan (low molecular weight) | SigmaAldrich | 448869 | 75-80% deacetylation |
Dialysis Membrane Tubing | Fisher Scientific | 08-670-5C | 3500 MWCO |
Ethanol | SigmaAldrich | 493538 | |
Genipin | SigmaAldrich | G4796 | |
Heracell 150i Incubator | ThermoFisher | 50116047 | |
Parafilm | Fisher Scientific | 13-374-12 | |
Recombinant human SDF-1a | Peprotech | 300-28A | |
Recombinant human TGF-b3 | Peprotech | 100-36E | |
Whatman Filter Paper Grade 540 | SigmaAldrich | Z241547 | 8 mm pore size |
Whatman Filter Paper Grade 541 | SigmaAldrich | WHA1541055 | 22 mm pore size |
Whatman Filter paper Grade 542 | SigmaAldrich | WHA1542185 | 2.7 mm pore size |
Wire Mesh Sieve | McMaster-Carr | 9317T86 | No. 100 Mesh |
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