3.3.3　Preparation methods for biomedical polymers

3.3.3　Preparation methods for biomedical polymers

The construction process of biomedical polymer materials can be divided into two stages：the first stage is the synthesis of polymer compounds，and the second stage is the molding of polymer materials.This textbook only focuses on the first stage of the construction of biomedical polymer materials.For the second stage of polymer material molding and processing knowledge，please refer to extended reading 4 & 5.

In the synthesis stage of polymer compounds，the raw materials for polymerization need to be prepared and refined first.Commonly-used raw materials include monomers，initiators，chain transfer agents，catalysts，and other components required for polymerization，such as solvents，dispersant，and emulsifier.The refining process refers to the purification steps of monomers，initiators，solvents，etc.The purpose is to remove polymerization inhibitors and other impurities，so as to avoid unforeseen side reactions and affect the smooth progress of the polymerization reaction.Generally speaking，liquid raw materials can take advantage of their relatively low boiling point and can be purified by atmospheric distillation or vacuum distillation，and can also be separated and purified by alkaline alumina column by using the polar difference of the components.For solid raw materials，the difference in solubility of components in different solvents can be used to purify by recrystallization.

The polymerization reaction is the process of repeatedly linking small molecules of monomeric compounds through covalent bonds to form polymers，and the final product is the polymer.It is well known that the structure of polymer materials determines the properties，which ultimately determine its usage and effects.Therefore，the synthesis of polymers is the basis and key link in the construction of polymer materials.According to the different principles of polymerization，it can be divided into two types：stepwise polymerization and chain polymerization，among which stepwise polymerization usually refers to bifunctional or multifunctional monomers，using condensation polymerization（e.g.，polycondensation）or addition polymerization.Chain polymerization implies that monomers containing carboncarbon double bonds and other unsaturated bonds generate polymer chains through chain polymerization.During the polymerization process，there are active centers（e.g.，radicals or ions），mainly including three elementary reactions of chain initiation，chain growth and chain termination.According to different active centers，chain polymerization can be subdivided into ionic polymerization（e.g.，anionic polymerization，cationic polymerization），coordination（directional）catalytic polymerization，and free radical polymerization.It should be pointed out that the initiation chain transfer terminator method，the nitroxide-mediated living free-radical polymerizations（NMP），atom transfer radical polymerization（ATRP），and reversible addition fragmentation chain transfer polymerization（RAFT），belong to living/controllable free radical polymerization.It is a research hotspot in the field of polymer synthesis and has been widely used in the preparation of biomedical polymer materials.Figure 3-4 shows the classification diagram of polymerization reactions.

Step polymerization is the formation of oligomers（e.g.，dimers and trimers）through condensation or addition reactions between monomer functional groups，and then the oligomers are gradually transformed into high polymers.Therefore，it usually takes a long reaction time to reach high molecular weight.In addition，most stepwise polymerization reactions are reversible，meaning the polymerization and depolymerization occurs at the same time，while the conversion rate and molecular weight increase proportionally with time.

Chain polymerization takes free radicals，anions，or cations as the active center for chain growth.Since the active center is extremely active，the chain polymerization can reach high molecular weight instantly，and the extension of the time only increases the conversion rate of the monomer.The characteristics and comparison of common chain polymerization are shown in Table 3-5.

Free radical polymerization reaction is a kind of chain polymerization reaction，which is the most common in the field of polymer synthesis，and the most convenient experimental operation and the highest degree of industrialization.However，common free radical polymerization has some defects or deficiencies.For example，the molecular weight distribution is relatively wide，and the polymer dispersibility index（PDI）is often higher than 2.0.The polymer molecular weight and molecular structure are difficult to control，and branching and cross-linking often occur.In order to overcome the above problems，active controllable free radical polymerization can effectively inhibit double radical termination and chain transfer reactions by reducing the concentration and reactivity of free radicals（e.g.，active center），making it achieves negligible purpose，thus，the chain growth reaction is absolutely dominant.Living controllable free radical polymerization，such as NMP，ATRP，RAFT，has been widely studied，which has become one of the important directions of academic significance and application value in the field of polymer science.Compared with common free radical polymerization，the most significant advantage of active controllable free radical polymerization is that the molecular weight of the polymer is controllable and the molecular weight distribution is very narrow（1.0＜PDI＜1.5）.At the same time，according to the specific monomer feeding order，it can design and synthesize block copolymers with specified segment structure and topological structures（e.g.，star，comb，cyclic，graft polymer）.

Because of the different polymerization mechanisms，the polymerization process is implemented in different ways（Figure 3-5）.The polymerization methods of step polymerization include melt condensation polymerization，interfacial condensation polymerization，solution condensation polymerization and solid-phase condensation polymerization.The polymerization methods of chain polymerization include bulk polymerization，suspension polymerization，solution polymerization and emulsion polymerization.Melt condensation polymerization is a polycondensation reaction that is carried out above the melting point of monomer and polymer（generally 10-25℃ higher than the melting point of the resulting polymer）.The reaction temperature is much higher than chain polymerization，generally above 200℃.Interfacial condensation polymerization refers to the polycondensation reaction of monomers and catalysts in different phases and the monomers at the phase interface of the multiphase system.Solution condensation polymerization is a polycondensation reaction in which monomers and catalysts are completely dissolved in the solvent.Solid-phase condensation polymerization refers to a polycondensation reaction in which raw materials（e.g.，monomers and catalysts）are below their melting point or softening point.The polycondensation reaction is characterized by the reaction between different functional groups to form the target polymer，and the formation of low molecular by-products（e.g.，water and ammonia）.

The crude product system synthesized by the polymerization reaction is generally a multi-component mixed system，including the target polymer，the remaining monomers that have not participated in the reaction，residual initiators，chain transfer agents，catalysts，reaction solvents，etc.In order to obtain a pure target polymer，the crude product system needs to be purified，the polymer must be separated from other impurities，and the solvent must be removed.Common laboratory separation is mainly divided into two steps.Firstly，the target polymer is separated from the liquid medium，and then the residual volatile components（e.g.，solvent）are removed.The commonly-used methods of separating target polymer from liquid medium include chemical destruction coagulation separation，centrifugal separation，dialysis separation，etc.The principle of chemical destruction coagulation separation is that the target polymer and the remaining impurity components interact with the precipitation solvent，acid，alkali，and salt to break the original mixed state，utilizing the significant difference in solubility between the target polymer and the impurities in the precipitation solvent.The solid polymer is precipitated in the form of precipitation or crystallization，thereby achieving the goal of separating the polymer.The principle of the centrifugal separation is to use the effect of centrifugal force and gravity，use the sedimentation of polymer and other solid particles in the liquid medium，or use the different specific gravity of each component of the heterogeneous system to separate the solid and liquid phases.Dialysis separation utilizes that monomers，catalysts，initiators and other small molecules in solution can pass through the dialysis membrane，while polymers and other large molecules cannot pass through the dialysis membrane to achieve separation and purification.The method to remove volatile components such as solvents is to change the volatile components from liquid to gas，and then remove the gaseous components through volatilization.The separation efficiency is determined by the concentration difference of the gas-liquid interface and the diffusion coefficient.The achieved concentration is determined by the vapor-liquid equilibrium.(https://www.daowen.com)

When biomedical materials are applied to the human body，human tissue cells or blood first come into contact with the surface of the material and produce a certain response.Unfortunately，most biomedical materials cannot adapt and accommodate each other with cells or blood.In other words，they do not have good histocompatibility and blood compatibility，which will eventually cause serious rejection of the body and lead to the loss of biomedical value of the materials.Therefore，in order to enable biological materials to stimulate appropriate response capabilities and exhibit good biocompatibility，the surface of the materials needs to be decorated and modified accordingly.Optimizing and improving biocompatibility has become a core issue in broadening the application of biomedical polymer materials in the biomedical field.Therefore，the surface modification of biomedical polymer materials has important research significance and practical value.Commonly-used modification methods include physical modification，chemical modification，and biomimetic modification，as shown in Table 3-6.

Physical modification usually refers to wrapping a layer of biocompatible film or polymer through layer-by-layer self-assembly technology or physical adsorption.For example，on the surface of biomedical polymer material，the anticoagulant coating can passivate the surface of the polymer material so that the blood will not contact directly with the surface of the polymer material，which is an effective method to improve the anticoagulant blood on the surface of biomedical polymer material.A variety of polymer materials processing methods are also effective means to prepare biomedical polymer materials with different morphological structures，physicochemical properties and surface interface properties，among which physical blending is an important method to construct functional biomedical polymer materials.For example，poly（lactic acid）is a biomedical polymer material with good biocompatibility，biodegradability and machinability，but its application scope in biomedical field is severely limited due to its slow degradation rate in vivo，high hydrophobicity and low impact strength.In order to solve the above problems，scientists have combined poly（lactic acid）with other kinds of biomedical polymers（e.g.，chitosan，hyaluronic acid，poly（glycolic acid），poly（ethylene glycol），and polycaprolactone）to construct the composite biomedical polymer materials with excellent performance，which have been successfully applied to drug carriers，shape memory materials and tissue engineering scaffolds.In addition，in order to improve the blood compatibility of polysulfone，scientists have synthesized the amphiphilic anticoagulant additive copolymer（2-methacryloyloxyethyl phosphorylcholine-n-dodecyl methacrylate ester and 2-methacryloyloxyethyl phosphorylcholine-n-butyl methacrylate copolymer）physically blended with polysulfone.The blended amphiphilic copolymer anticoagulant additive can enrich and wrap on the polysulfone surface to reduce the free energy of the interface，which can improve the blood compatibility of the polysulfone dialysis membrane，and construct an anticoagulant material with excellent performance.

Coating the surface of biomedical polymer materials（e.g.，surface coating technology）is also one of the main methods for surface modification of biomedical polymer materials.By adding an anticoagulant coating on the surface of biomedical polymer materials，the surface of sensitive biomaterials is passivated.Thus，blood cannot directly contact with it，thereby effectively improving the biocompatibility of the material.For example，by applying anticoagulant substances，fibrin is produced on the outer surface of the biomedical polymer，which reduces the coagulation of platelets，so that the medical polymer material has anticoagulant properties on the surface.Generally speaking，commonly-used anticoagulant substances mainly include styrene and its conversion products，epoxy resin，acrylate conversion products，etc.These substances have strong viscosity and can be used on the surface of medical devices to avoid falling off.Another method is to mix the anticoagulant substance with the basic material to make it have anticoagulant properties.Generally，the base material can aggregate on the surface and has a certain degree of anticoagulation to mix the copolymer，which can effectively play a role in medical treatment.

The physical modification has the advantage of simple and convenient operation，but it only acts on the surface of the biomedical polymer material through physical adsorption.The weak interaction force results in poor stability，and it may fall off the surface of the substrate.Surface grafting modification by chemical methods is an important way for biomedical polymer modification.The surface layer constructed by the improved method is covalently bonded to the substrate through a covalent bond，and will not easily fall off.Commonly-used modification methods include introduction of functional groups through efficient click chemistry and other reactions，bonding polymer chains，coupling biologically active macromolecules，etc.In addition，it is well known that many biomedical polymer materials have good biocompatibility（e.g.，common polysiloxane，polytetrafluoroethylene，poly（lactic acid），polycaprolactone）.Due to the long-chain structure of polymers，they are easily fused with blood.Utilizing the relatively stable performance of polyoxyethylene in hemolysis，the biomedical polymer and polyoxyethylene are fused together to form a watersoluble long-chain structure，so as to minimize the impact of the biomedical polymer on blood proteins and prevent the adsorption of blood.In addition，most biomedical polymers themselves cannot achieve antibacterial，self-repair，drug sustained release and other functions.At the same time，these polymers themselves do not have reactive sites or reactive chemical groups，which often makes it difficult to achieve the desired effect of using functional materials.Therefore，in order to construct biomedical polymer materials with multiple responsiveness and functionality，it is particularly important to modify the surface of the above-mentioned polymers.At present，commonly-used surface modification methods include chemical solution treatment（e.g.，oxidation，sulfonation，immersion，and hydrolysis），lowtemperature plasma treatment，ultraviolet/ray radiation，atomic layer deposition，electrochemistry，atomic force microsurgery and other technologies and strategies.

Plasma is the fourth state of matter after solid，liquid，and gas.When the applied voltage reaches the breakdown voltage，gas molecules are ionized to produce a mixture of a large number of active particles，including electrons，ions，atoms，radicals，and free radicals，which can provide conditions for plasma technology to modify the surface of materials through chemical reactions.The use of low temperature plasma to modify biomedical polymer materials means treating the surface of polymer materials with plasma.Typically，place them in methane，nitrogen，oxygen or rare gases，and generate plasma through the principle of high-voltage discharge.The energic particles and the active substance react with the surface of the polymer material，to improve the surface structure and to achieve the purpose of surface modification.In addition，polymerizable gases（e.g.，methane）can form a polymer film on the surface of biomedical polymer materials and improve the adhesion of the materials.The ultraviolet/ray radiation method uses ultraviolet or ray to radiate biomedical polymer materials，so that the surface is heat and active groups and active substances can be attached to the surface.The advantages of this method are simple operation and low cost，and its anticoagulation effect is good.

Surface biomimetic modification is one of the ideal methods to improve the blood compatibility of biomedical polymer materials.Biomimetic modification prevents polymer compounds from being treated as foreign bodies by the blood，thus they will not be cleared and metabolized in the body.There are three common biomimetic approaches：surface heparinization，surface phospholipidization，and surface endothelialization.

Heparin was firstly discovered from the liver and got its name.It is a natural anticoagulant substance in animals and one of the first natural anticoagulant drugs to be recognized and studied.Fixing heparin molecules on the surface of biomedical polymer materials is an important way to improve the anticoagulant properties of the materials.The methods used include physical adsorption and chemical bonding.Although the physical adsorption method is not very strong，it has the advantage of keeping the conformation of heparin unchanged.The structure of the chemical bonding method is stable，but it is not easy to maintain the original conformation of heparin，which reduces the anticoagulant performance.Phospholipids are the main components that make up the outer surface of biological membranes.The amphiphilic phosphorylcholine（PC）group in lecithin has strong anticoagulant activity.An effective way to improve the blood compatibility of biomedical polymer materials is to introduce phosphorylcholine groups into medical materials，which can make the surface of biomedical polymer materials phospholipidization.

Surface endothelialization is also called endothelial cell fixation.As we all know，because of the complex biological system components（e.g.，body fluid，enzymes，cells，free radicals）that biomedical polymer materials come into contact with，and the extremely complex biological environment，it is difficult to achieve great blood compatibility only by surface modification.Researchers have found that an important way to improve blood compatibility of materials is to cultivate human endothelial cells in situ on the surface of the material through the application of tissue engineering technology.The blood can flow normally without clotting through the physical barrier function of vascular endothelial cells and the regulation and maintenance of the dynamic balance between coagulation factors and anticoagulation factors.At present，the ideal method to improve the blood compatibility of materials is to plant and culture vascular endothelial cells on the surface of biomedical polymer materials.However，directly planting endothelial cells on the surface of the matrix materials not only proliferates slowly，but also easily falls off and separates.Therefore，endothelial cells can be fixed on the surface of the materials by covalent bonding，and then endothelial cells can be planted and cultured on it.