微泡

微泡（英语：Micro bubbles）又称微气泡，是气泡直径小于1毫米（mm），但大于一微米（μm）的水中气泡。当把水与空气交互冲击，水里会增加大量空气泡，在一定条件下，会产生直径小于1毫米（mm），但大于一微米（μm）的气泡，数量够多时甚至会令蒸馏水的颜色化成乳白色。其特性广泛应用使用在工业、生命科学和医学中。气泡壳和其填充物质的组成决定了微气泡的特征，例如：浮力、抗压强度、热导率和声学特性。
医学中的微气泡.
微气泡在医学诊断中用作超声波成像的对比剂[1]。充满空气或是碳氟化合物的微泡，在应用于声能场时振荡和振动，并可能反射超声波。这将微泡与周围组织区分开来。实际上，由于液体中的气泡缺乏稳定性，因此很快就会溶解，而微气泡必须用固体外壳包裹起来。该壳由脂质或蛋白质制成，例如由血清白蛋白壳所包裹的八氟丙烷气体组成的微气泡。具有与血液互相影响的亲水性外层和容纳气体分子的疏水性内层在热力学上是最稳定的。空气、六氟化硫和碳氟化合物气体都可以作为微气泡内部的成分。为了新增血液中的稳定性和持久性，高分子量气体和在血液中的低溶解度气体是微气泡气核最有吸引力的候选气体[2]。
微气泡可用于药物输送[3]、生物薄膜去除[4]、膜清洗[5][6]、生物薄膜控制和水及废水处理等目的[7]，也由船体在水中的运动产生，形成气泡层；这可能会干扰声纳的使用，因为声纳层需要吸收或反射声波[8]。
超声波反映.
超声波成像中的对比度仰赖于包含超声波速度和组织密度函数[9]的声阻抗及组织或感兴趣区域之间[2]的差异。由于超声波所引起的声波与组织介面相互影响，一些声波被反射回换能器。差值越大，反射的波越多，讯号杂讯比就越高。因此，数量级比周围组织和血液低且更容易压缩周围组织和血液的微气泡核心，在成像中提供了高对比[2]。
治疗应用.
2.1物理反应
当暴露于超声波时，微气泡以两种方式之一响应入射的压力波而振荡。压力越低、频率越高、微气泡的直径越大，微气泡振荡或空化就会越稳定[2]。这会导致周围血管系统和组织附近产生音波流，产生剪应力，从而在内皮层上产生孔隙[10]。这种孔隙形成会增强内吞作用和渗透性[10]。频率越低，压力越高，微气泡直径越小，微气泡振荡就越惯性；它们剧烈膨胀和收缩，最终导致微泡塌陷[11]。这种现象会在血管壁上产生机械应力和微射流，这已被证明会破坏紧密的细胞连接并诱导细胞渗透性[10]。极高的压力会导致小血管破坏，但是压力可以调节到只在体内产生短暂的孔[2][11]。微气泡破坏对药物传输载体是一个理想方法。破坏产生的力可以使微泡上的治疗有效载荷移位，同时使周围细胞对药物的吸收敏感。
2.2药品传输
微气泡可以用多种方法作为药物传输载体。其中最显著的包括：（1）将亲脂性药物并入脂质单层，（2）将奈米颗粒和脂质体附著到微泡表面，（3）将微泡包裹在较大的脂质体内，以及（4）将核酸静电键合到微气泡表面[2][12][13][14]。
2.2.1亲脂性药物
微气泡可通过将这些药物并入微气泡脂质壳来促进疏水性药物的局部标靶[15][16][17][18][19][20][21][22]。这种封装技术降低了全身性毒性，新增了药物定位，并提高了疏水性药物的溶解性[16]。对于新增的定位，标靶配体可以附加到微气泡的外部[17][18][20][21][22]。这提高了治疗效果[18]。脂质包裹的微气泡作为药品传输载体的一个缺点是其有效载荷低。为了解决这个问题，可以在脂质单层的内部加入一个油壳来提高有效载荷的效率
2.2.2奈米微粒和脂质体附著
为了增加微气泡的有效载荷，人们开始探索脂质体[24][25][26][27]或奈米微粒 [10][28][29][30][31]附著在脂质微气泡的外部。超音波破坏微气泡后，这些小颗粒可以渗出到肿瘤组织中。此外，通过将这些颗粒附著在微气泡上而不是联合注射，药物被限制在血流中，而不是聚集在健康组织。这种治疗被降级到超音波治疗的位置[26]。这种微气泡的改版对一种脂质体剂已经在临床上使用的阿霉素特别有吸引力[26]。对微气泡破坏引起奈米微粒浸润的分析表明，更高的压力对于血管渗透性是必要的，并且可能通过促进局部液体运动和增强内吞作用来改善治疗[10]。
2.2.3脂质体内负载微气泡
另一种新的声学反应灵敏的微气泡系统是直接将微气泡封装在脂质体中。这些系统在体内的回圈时间比单用微气泡要长，因为这种包装方法可以防止微气泡溶解在血液中[32]。亲水性药物存留在脂质体内部的水介质中，而疏水性药物聚集在脂质双层中[32][33]。体外实验显示巨噬细胞不会吞噬这些颗粒[33]。
2.2.4透过静电相互作用进行基因传递
微气泡也透过带正电荷的微气泡外壳和带负电荷的核酸之间的静电键作为基因转染的非病毒载体。微气泡崩塌形成的短暂孔隙允许遗传物质以一种比当前治疗方法更安全和更具体的管道进入靶细胞[34]。微气泡已被用于传递小分子核糖核酸[35][36]，质体[37]，小分子干扰核糖核酸[38]，和信使核糖核酸[39][40]。
微气泡对药品传输的缺点.
(1)由于微气泡体积大，不易外渗，因此其作用仅限于血管系统。奈米液滴和全氟碳化合物液滴周围的脂质壳因为超音波脉冲而蒸发，提供了一个小直径来促进外渗，也提供微气泡一个的替代方案。
(2)微气泡的半衰期很短，约为循环的几分钟，因此限制了治疗时间。
(3)微气泡由肝脏和脾脏过滤，如果微气泡还没有释放出它们的货物，任何药物复合体也可能对这些器官构成毒性威胁。
(4)药物复合体对微气泡来说很难解释，且这些制剂很难大量制造以广泛使用。
(5)当微泡被用来破坏脑血管障壁时，脑组织会有少量出血，尽管这被认为是可逆的。
2.3微气泡对治疗的特殊应用.
用于药品传输的微气泡不仅可以作为药物载体，还可以作为一种渗透其他不可穿透的屏障（特别是血脑）及改变肿瘤微环境的方式。
2.3.1血脑屏障破坏
大脑由毛细管内的内皮细胞壁紧密连接而被保护，此称为血脑屏障（BBB）[41]。血脑屏障严格调节血液进入大脑的物质，虽然这功能对健康的人来说是非常理想的，但对治疗人员进入癌症患者的大脑构成了障碍。20世纪中期，超音波被显示能破坏血脑屏障[42]，而在2000年代初期，微气泡被证明有助于暂时的渗透[43]。此后，超音波和微气泡治疗被用于向大脑输送的治疗药物。由于血脑屏障破裂的超音波及微气泡治疗已证明是一种安全且有前景的临床治疗，两个临床试验正在检测阿霉素[44]和卡铂[45]与微气泡的传输，以提高局部药物浓度。
2.3.2免疫治疗
除了渗透血脑屏障外，超音波和微气泡治疗还可以改变肿瘤环境，并作为免疫治疗手段[46]。高强度聚焦超声（HIFU）单独触发免疫反应，推测是通过促进肿瘤抗原的释放，以获得免疫细胞识别，活化抗原呈现细胞，促进其浸润，抑制肿瘤免疫抑制，促进Th1辅助细胞反应[47][48]。通常，高强度聚焦超声用于肿瘤的热消融。低强度聚焦超声（LIFU）结合微气泡也显示出可刺激免疫刺激作用，抑制肿瘤生长，新增内生性白血球浸润[47][49]。此外，降低高强度聚焦超声所需的声学功率，可为患者提供更安全的治疗并缩短治疗时间[50]。尽管此治疗有潜力，但据推测，一个完整的治疗需要组合治疗。超音波和微气泡治疗不需要额外药物，阻碍了小肿瘤的生长，但需要联合药物治疗来影响中等肿瘤的生长[51]。由于其免疫刺激机制，超音波和微气泡提供了一种独特的能力，可以促进或加强免疫治疗，以获得更有效的癌症治疗。
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