材料导论 (56)

In the previous lectures, we have already know that degradation is a vital properites of biomaterials. Biodegradable polymers normally possess labile functional groups accessible to biological or non-biological mechanisms of degradation. Biodegradable polymeric biomaterials will gradually degrade into water soluble oligomers and monomers that eliminate from a human body after fulfilling their intended use. The use of biodegradable synthetic polymers as biomaterials is also due to that their mechanical and physical properties can readily be adjusted by varying preparation techniques and molecular structure. Therefore, biodegradable polymeric biomaterials are intensively studied and used in biomedical and tissue engineering applications.

在之前的课程中，我们已经知道降解是生物材料的一个重要特性。生物可降解聚合物通常具有生物可降解或非生物可降解机制的不稳定官能团。生物降解聚合物生物材料将逐渐降解为水溶性低聚物和单体，在完成预期的用途后可以从人体中去除。生物可降解合成聚合物作为生物材料的使用，还因为它们的机械和物理性能可以通过不同的制备技术和分子结构很容易地进行调整。因此，生物降解聚合物生物材料在生物医学和组织工程应用中得到了广泛的研究和应用。

合成可降解材料在人体中完成使命以后呢，可以被降解成为水溶性的低聚物或者单体然后被人体排出，而且我们可以通过分子设计，可以实现合成可降解材料的不同力学和物理性质，以满足应用需要，因此此类生物材料非常的重要。

According to the mechanisms of erosion, degradable polymers can

be classified into surface eroding and bulk eroding ones, The term “degradation” specifically refers to bond cleavage, whereas “erosion”refers to depletion of material. Biodegradation is the destruction and assimilation of organic polymers by living organisms and their enzymes, particularly fungi and bacteria. Degradation is a chemical process, whereas erosion is a physical phenomenon depending on dissolution and diffusion processes. Surface and bulk erosion are two different forms of erosion that describe how a degrading polymer erodes. In surface erosion, the polymer degrades from the exterior surface. The inside of the material does not degrade until all the surrounding material around it has been degraded. In bulk erosion, degradation occurs throughout the whole material equally. Both the surface and the inside of the material degrade.

根据侵蚀（溶蚀）的不同机制,可降解聚合物可分为表面侵蚀和本体侵蚀,术语“降解”主要指化学键的断裂,而“侵蚀”是指材料的消耗。生物降解是生物体及其酶(特别是真菌和细菌)对有机聚合物的破坏和同化。降解是一个化学过程，而侵蚀是一个物理现象，取决于溶解和扩散过程。表面侵蚀和本体侵蚀是描述降解聚合物侵蚀的两种不同形式。在表面侵蚀中，聚合物会从表面降解。材料的内部不会降解，直到周围所有的材料都被降解。在本体侵蚀的过程中，整个材料均发生降解。材料的表面和内部都降解了。有缘学习更多驾卫星ｙｇｄ３０７６那么degradation降解和erosion 侵蚀这两个概念还是有一定的不同，降解指的是化学键的断裂，是一个化学过程；而侵蚀指的是有溶解和扩散产生的损耗，是一个物理过程。侵蚀又分为表面侵蚀和本体侵蚀。

In surface erosion, the erosion rate is directly proportional to the surface area of the material. 也就是侵蚀的速率和材料的表面积相关。For very thin materials, the surface area remains relatively constant when the material degrades, which allows surface erosion to be characterized as zero order release since the rate of degradation is constant. 当材料的表面积相对恒定时，随着表面侵蚀，可以得到接近零阶释放的效果。In bulk erosion, the erosion rate depends on the volume of the material. Due to degradation, the volume of the material decreases during bulk erosion causing the erosion rate to decrease over time. 本体侵蚀和材料的体积相关，当材料的体积减少的时候，侵蚀的速率也会相应降低。Therefore, bulk erosion rates are difficult to control since it is not zero order. T o determine whether a polymer will undergo surface or bulk erosion, the degradation rate of the polymer in water and the rate of diffusion of water penetrating through the material must be considered. 那么降解速率的衡量就需要同时考量材料在水中的降解速度以及材料在水中的渗透速率If the degradation process is faster than the diffusion process, surface erosion will occur since the material’s surface will quickly degrade before water has time to diffuse and penetrate through the material. If the diffusion process is faster than the degradation process，bulk erosion will occur because water penetrates through the material before significant erosion occurs on the surface. 如果降解速率大于渗透速率将会以表面侵蚀为主，相反的则以本体侵蚀为主。A kinetics of the erosion of a polymer can be modified by

changing the diffusion process or the degradation process. For example, blending a polymer with another polymer that is very reactive to water will speed up the degradation process and cause surface erosion. On the other hand, decreasing the dimensions of a material will allow water to travel to the center of the material more quickly, which speeds up the diffusion process and causes bulk erosion.

在表面侵蚀中，侵蚀速率与材料的表面积成正比。对于非常薄的材料，当材料退化时，表面积保持相对恒定，这使得表面侵蚀被描述为零级释放，因为降解速率是恒定的。在体积侵蚀中，侵蚀速率取决于材料的体积。由于降解，材料的体积在本体侵蚀过程中减少，导致侵蚀速率随时间的减少。因此，由于不是零阶速率，因此很难控制体积侵蚀率。为了确定聚合物是否会发生表面或本体侵蚀，聚合物在水中的降解速率和穿透材料的水扩散速率必须被考虑。如果降解过程比扩散过程快，那么表面的侵蚀将会发生，因为在水有时间扩散并穿透物质之前，材料表面会迅速降解。如果扩散过程快于降解过程中,那么本体侵蚀将会发生，因为水渗透通过材料表面严重侵蚀发生之前。通过改变扩散过程或降解过程，可以对聚合物的侵蚀动力学进行修正。例如，将一种聚合物与另一种对水非常敏感的聚合物混合将加速降解过程并导致表面侵蚀。另一方面，减小材料的尺寸将使水更快地流到材料的中心，从而加速扩散过程并导致大量的侵蚀。

In practical terms, both mechanisms will coexist in the erosion of most biodegradable polymers, but the relative extent of surface or bulk erosion varies radically with the chemical structure of the polymer backbone.

在实际情况中，大多数生物可降解聚合物的腐蚀过程中两种机制都会共存，但是表面或本体腐蚀的相对程度会随着聚合物骨架的化学结构发生根本性的变化。

The widely used synthetic biodegradable polymers are aliphatic polyesters 脂肪族聚酯, poly(ortho esters) 聚原酸酯, poly(anhydrides)聚酸酐, and poly(amides)聚酰胺. Aliphatic polyesters have good mechanical properties, hydrolyzability, and biocompatibility. Depending on their structure, these polymers are widely used in medical and pharmaceutical applications as artificial skin, dental implants, vascular grafts, bone screws, and plates for temporary internal fracture fixation, biodegradable bone repair materials, tissue engineering scaffolds, and as a carrier material for controlled release. The forms of such controlled release devices are also versatile, ranging from nanoparticles, lamellar particles, microparticles to matrix tablets and implants.

广泛使用的合成生物可降解聚合物包括脂肪族聚酯,聚原酸酯、聚酸酐,和聚酰胺。脂肪族聚酯具有良好的力学性能、水解性和生物相容性。根据其结构，这些聚合物被广泛应用于医疗和制药应用领域，如人造皮肤、植牙、血管移植、骨螺钉和钢板，用于临时内骨折固定、生物降解骨修复材料、组织工程支架，以及作为控制释放的载体材料。这种控制释放装置的形式也多种多样，从纳米颗粒、片状颗粒、微颗粒到基质片和植入物。

Aliphatic polyesters are generally prepared by ring opening polymerization (ROP) of lactones and lactides. 通过内酯和交酯的开环聚合

得到。The mechanical properties, hydrophilicity and degradability of aliphatic polyesters can be further improved by synthesizing polyesters with more complex architectures such as stars, brushes, cyclics, and cross-linked materials. 一些复杂聚合物的结构也会影响材料的降解性The degradation rate is influenced by the degree of crystallinity, molecular weight, copolymer ratios, stress factors, and the site of implantation . For example, poly (lactic acid) (PLA) has a much slower degradation rate than poly(glycolic acid) (PGA) since PLA is much more hydrophobic than the highly crystalline PGA. 聚乳酸由于疏水性和较高的结晶度，和聚乙醇酸相比就具有较慢的降解速率。The hydrophobicity of PLA has been improved by copolymerization of PLA and polyethylene glycol (PEG). The cell interactions have been modified by inclusion of collagen, chitin or its derivatives into the polymer PLA to form blends.

脂肪族聚酯通常是通过内酯和交酯的开环聚合(ROP)来制备的。脂肪族聚酯的力学性能、亲水性和可降解性可以通过合成具有更复杂结构的聚酯如星形、刷形、环型和交联材料来进一步提高。降解速率受结晶度、分子量、共聚物比、应力的因素和植入位置的影响。例如，聚乳酸(PLA)比聚乙醇酸(PGA)降解速度慢得多，因为聚乳酸比高结晶的PGA更加疏水。聚乳酸的疏水性可以通过与聚乙二醇的共聚得到改善。通过将胶原蛋白、几丁质或其衍生物加入聚乳酸中形成共混物，也可以改变细胞间的相互作用。

Polyorthoesters or polyanhydrides contain hydrophobic repeating units with water-labile linkages. These polymers possess controllable

degradation, and degrade through the hydrolysis of the surface, as opposed to the bulk degradation profile seen with PLGA. Polyorthoesters can be used in orthopaedic applications and drug delivery systems. 聚原酸酯和聚酸酐是通过表面侵蚀降解的。

聚丙烯酸酯或聚酸酐的疏水重复单位由可水解的键相连接。这些聚合物表现出可控降解的特性。与PLGA本体侵蚀不同，这些聚合物通过表面侵蚀降解。聚醚酯可用于骨科应用和药物输送系统。

Polyanhydrides have hydrophobic backbone with hydrolytically labile anhydride linkages such that hydrolytic degradation can be controlled by manipulation of the polymer composition. The attractiveness of polyanhydrides lies in that they show no evidence of inflammability and have excellent controlled release characteristics. Other advantages of polyanhydrides are well-defined structure, and controllable degradability, processibility, sterilization endurance. Polyanhydrides have been used as carriers in short-term controlled delivery of bioactive agents due to their rapid degradation and limited mechanical properties.快速降解的特性These polymers are now being used to treat brain cancers.

聚酸酐所具有的疏水性骨架由可水解的酸酐连接，并可以通过控制聚合物的组成来控制水解速度。聚酐的优势在于它们不会引起炎症，并且具有良好的控制释放特性。聚酸酐的其他优点是结构良好，可降解性、可加工性、杀菌耐久性。聚酸酐由于其快速降解和力学性能，已被作为短期药物控制释放的载体。这些聚合物快速降解的特性目前已经被用于治疗脑癌。

Poly (amides) are composed of amino acids joined together through peptide linkage, which can be broken down enzymatically. Poly (amino acids) are the most interesting class of poly (amides). They are highly insoluble, non-processible, and antigenic when the polymers contain three or more amino acids. 聚氨基酸不溶解，不易加工同时还具有抗原性。To circumvent these problems, “pseudo”-poly (amino acids) has been synthesized from tyrosine dipeptide.

聚酰胺是氨基酸通过肽键连接在一起组成，它们可以被酶分解。聚氨基酸是最有趣的聚酰胺类聚合物。当聚合物含有三个或更多的氨基酸基团时，它们的溶解性和加工性很差并具有抗原性的。目前以酪氨酸构建的模仿的聚氨基酸结构来解决这些问题。

To summerized this topic, we have known that 1. Biodegradable polymers normally possess labile functional groups include ester, amide, and ether functional groups which accessible to biological or non-biological mechanisms of degradation. 可降解聚合物都包裹脂键，酰胺，醚键等可以被降解的化学键2. Degradation is a chemical process, whereas erosion is a physical phenomenon depending on dissolution and diffusion processes. Surface and bulk erosion are two different forms of erosion that describe how a degrading polymer erodes. 降解是一个化学过程，侵蚀是一个物理过程，通过溶解和扩散的速率不同，侵蚀分为表面侵蚀和本体侵蚀 3. The widely used synthetic biodegradable polymers are aliphatic polyesters, poly(ortho esters), poly(anhydrides), poly(amides),

and they all have their own corresponding applications. 在最后我们介绍了一些常见的可降解聚合物材料，包括脂肪族聚酯，聚酸酐等等。

总结来说，1.生物可降解聚合物通常具有不稳定的官能团，包括酯、酰胺、醚官能团，可用于生物或非生物降解机制。2. 降解是一个化学过程，而侵蚀是一个物理现象，取决于溶解和扩散过程。表面侵蚀和本体侵蚀是描述降解聚合物侵蚀的两种不同形式。3。广泛使用的合成生物可降解聚合物是脂族聚酯、聚原酸酯、聚酸酐、聚酰胺，它们作为生物材料都有广泛的应用。