Surface modification technology
Chemical modification refers to the enhancement of biomedical materials by changing the chemical composition of their surfaces. Common chemical modification methods include surface grafting, surface biochemistry, layer-by-layer assembly and coating techniques.
Among them, surface grafting is the most commonly used method, usually by grafting different polymers or bioactive molecules on the surface of a material to give it specific functions and properties.
Future research will focus on the development of surface-initiated graft polymerization techniques that can be carried out at ambient or low temperatures, in a variety of media and in oxygen environments.
1) Surface grafting
Surface grafting is a technique of graft polymerization by creating active graft sites on the surface of a material.
Surface grafting methods can be classified into photochemical grafting, plasma polymerization grafting, radiation grafting, ozonation grafting, and reactive polymerization grafting according to the different ways of generating reactive grafting sites.
Photoinitiated grafting initiates the polymerization of monomers by adsorbing a photoinitiator on the surface of the material and using its absorption of light energy to generate active centers such as free radicals, cations or anions.
This method has the advantages of mild reaction conditions, simple process, convenient operation and high reaction efficiency, so it is widely used in the surface modification of polymer materials.
Photoinitiated grafting is particularly suitable for low temperatures and can initiate the polymerization of monomers with poor thermal stability.
Common photoinitiators include anthraquinones, benzophenones, and azides, which impart excellent physicochemical properties such as hydrophilicity, biocompatibility, antimicrobial, and anticoagulant properties to material surfaces.
Plasma grafting is done by treating the surface of the polymer material with an inert gas, such as helium or argon, to generate reactive free radicals in situ, and then polymerizing and grafting vinyl monomers containing specific functions onto the surface of the material.
This method improves the hydrophilicity, adhesion, biocompatibility, breathability and anticoagulant properties of the material surface.
Plasma grafting is easy to operate, does not pollute the environment, and does not damage the structure and properties of the substrate material, so it has received more and more attention.
However, it still faces certain challenges in industrial applications due to its need to be carried out in a vacuum environment, the complexity of the equipment and the difficulty of realizing large-scale continuous production.
Radiation grafting, on the other hand, utilizes high-energy radiation, such as X-rays, gamma rays, or heavy ions, to excite the polymer chain to produce ions or free radicals, triggering a grafting reaction.
Radiation grafting has the advantages of wide applicability, fast reaction speed and high reproducibility, so it has been widely used in the surface modification and functionalization of materials.
Ozonation grafting involves the introduction of peroxyl radicals on the surface of biomedical materials to generate peroxides on the surface of the material through ozonation, which generates free radicals and triggers the graft polymerization of monomers.
Unlike other methods, ozone is able to penetrate into the interior of the material, which in turn affects its degradation rate and biological properties.
2) Biologization of material surfaces
Surface bioconversion of biomaterials is achieved by immobilizing biologically active macromolecules (e.g., gelatin, enzymes, growth factors, proteins, etc.) on the surface of the material in order to improve its biocompatibility and promote cell attachment or functional expression, while reducing adverse reactions such as allergies, inflammation, and blood clots.
Currently, common surface biochemical methods include physical adsorption and chemical bonding.
Physical adsorption is simple and easy to perform, usually by contacting the material with biologically active molecules so that a physical adsorption reaction occurs on the surface.
The most common means is impregnation, i.e., the material is immersed in a chemical reagent containing biologically active molecules, which induces a chemical reaction or physical change on the surface to achieve the modification effect.
In order to improve the hydrophobicity and biological properties of polymeric materials, they are often impregnated with acidic or alkaline solutions.
Although degradable polyesters are widely used in biomedical materials, they often lack bioactive sites, limiting their application in tissue engineering.
By impregnation, free amino groups or other functional groups can be introduced to enhance the density of active sites on the surface, improve hydrophilicity, and alleviate the acidification that occurs when polyester is hydrolyzed.
The chemical bonding method, on the other hand, firmly immobilizes functional groups or biomolecules on the surface of the material through covalent bonds so that they remain biologically active for a long period of time.
For example, permanent anticoagulant materials can be prepared by combining heparin with polymeric materials through chemical covalent bonding.
Although the chemical bonding method is more complicated to operate, the immobilized bioactive molecules are more stable and less likely to detach, overcoming the problem of molecules being easily detached in the physical adsorption method.
3) Layer by layer assembly
Layer-by-layer assembly technology mainly relies on electrostatic, hydrogen bonding or covalent bonding interactions, and is categorized into single-layer assembly and multilayer assembly.
The method does not require high material morphology and can be modified by adjusting the thickness of the film layer as needed.
Since many bioactive macromolecules or drugs are inherently charged, layer-by-layer assembly techniques are widely used in the field of drug delivery and delivery of bioactive molecules.
By constructing multilayered films containing drugs and bioactive molecules, the surface of the material can be effectively functionalized to change its surface properties and enhance the slow-release properties of the material.
4) Solid Coating
Surface firm coating modifications include plasma spraying, electrochemical coating, ultrasonic spraying, plasma deposition and biomimetic mineralization.
Plasma spraying utilizes the high temperature, high speed and large temperature gradient characteristics of plasma to heat the material to a molten or semi-molten state, and then sprays it onto the surface of the treated workpiece to form a solid coating layer.
This method enables the material to have excellent properties such as high temperature resistance, oxidation resistance, corrosion resistance and abrasion resistance, which can significantly extend the service life of biomedical materials and is especially suitable for the surface modification of alloy materials.
The electrochemical coating technology utilizes the migration of ions in the electrolyte solution under the action of an external electric field to form a coating by a redox reaction on the electrode surface.
This process is usually carried out at room temperature or slightly higher, and the mild conditions help to maintain the activity of the biomolecules, making it suitable for use with structurally complex materials.
Plasma deposition is a plasma treatment process in which ionized precursor fragments accumulate on the surface of a biomedical material to form a thin film.
The film is capable of imparting antimicrobial and anticoagulant properties to the material, improving its biological performance.
Biomimetic mineralization promotes the formation of biominerals by mimicking the biological environment through strict control of physical and chemical conditions, and is often applied to bone tissue engineering.
Through biomimetic mineralization, composite materials with unique structure and excellent biological properties can be prepared to enhance the bioactivity of the materials and improve the interaction between the materials and the biological tissues, which is especially suitable for the field of bone tissue repair and regeneration and has a broad application prospect.
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