摘要
近年来,太赫兹(Terahertz,THz)技术在生物医学领域的应用研究发展迅速。科技工作者利用THz技术在生物分子、细胞、组织及生物个体等层面取得了一系列的重要研究成果,为生物医学研究和诊疗提供了新视角和手段。作者从THz光谱检测、成像及生物效应三个方面总结了THz技术在生物医学领域的研究现状,分析了THz技术在生物医学应用中所面临的挑战,并对未来的发展方向进行了展望。
先进光学技术不仅为获取生物体系的物化信息以及探索光与生物体系的相互作用机制提供了新的方法,而且也为生物医学诊断和治疗带来了新的思路和手段。作为21世纪一个重要的新兴前沿方向,太赫兹(Terahertz,THz)生物医学技术正在越来越多地受到大家的关注和重视。
THz波是介于微波和红外之间的电磁波,其频率范围通常是指0.1~10 THz,对应波长范围为3-0.03 mm
自上世纪90年代以来,得益于THz辐射源和探测器性能的不断提
近年来,研究者们在THz生物医学领域开展了积极探索性研究,在诸多方面取得了显著的成绩。THz生物医学研究范围不仅包括生物分子、细胞、组织、个体等生物体系的光谱和成像检测,还涉及到THz波辐射的生物效应研究(

图1 THz生物医学应用概览图。THz生物医学应用主要包括三方面:THz光谱、THz成像和THz波生物效应。
Fig. 1 Schematic overview of THz biomedical detection applications. THz biomedical applications mainly focuses on three aspects: THz spectroscopy, THz imaging, and THz biological effects.
THz光谱技术是一种重要的无标记生物样品波谱检测手段。目前应用最广泛的THz光谱技术是THz时域光谱技术(Terahertz time-domain spectroscopy,THz-TDS)。它使用飞秒脉冲激光作为光源,通过分束器将激光分成两束,其中一束作为泵浦光,经过聚焦后照射在光电导天线/非线性晶体上,通过光电导/光整流效应诱发产生THz脉冲辐射;另一束作为探测光,作用于光电导天线/非线性晶体,基于THz波产生原理的逆过程实现THz波探测。在探测过程中,通过调整光路延迟线改变泵浦光和探测光之间的相位关系,从而基于等效采样技术实现对样品作用前/后THz波时域电场信号的采集。将时域信号进行快速傅里叶变换,可获取THz波的频域信息(不同频率下的振幅和相位信息)。在此基础上,根据相关算法,这些信息可以进一步被转换为表征样品性质的介电常数、折射率和吸收系数等光学参数。在THz光谱检测中,常用的探测模式有透射、反射和衰减全反射(Attenuated total reflection,ATR)模式(

图2 THz光谱检测技术。(a)透射模式示意图;(b)固态 L-Tyr、D-Tyr 和 DL-Tyr 的THz吸收光谱。为了便于比较,将L-Tyr和D-Tyr的THz吸收光谱沿垂直轴进行了偏移;(c)反射模式示意图;(d)大鼠脑组织样品中肿瘤区域和正常区域的THz吸收光谱;(e)THz-ATR模式示意图;(f)经不同浓度人参皂苷Rg3处理后,U87细胞的THz吸收光谱。图(b)、(d)和(f)经授权转载自参考文献[
Fig. 2 THz spectroscopy detection techniques. (a) Schematic of the transmission mode. (b) THz absorption spectra of L-Tyr, D-Tyr and DL-Tyr samples in solid state. The spectra are offset along the vertical axis for the purpose of easier comparison. (c) Schematic of the reflection mode. (d) THz absorption spectra of tumor and normal regions of a rat brain tissue sample. (e) Schematic of the ATR mode. (f) THz absorption spectra of U87 cells treated with different concentrations of ginsenoside Rg3. Images reproduced with permission from Ref. [
在透射模式下(
透射模式可用于检测固态生物分子(氨基酸、核酸、蛋白质和碳水化合物等)的特征吸收峰,实现对样品成分和结构的分析鉴定。例如,Zhang团队检测了20种基本氨基酸,发现这些氨基酸在THz频率范围内具有不同的特征峰,可以利用THz光谱特征对它们进行分
除了生物分子,还可以利用透射模式THz光谱技术分析复杂度较高的生物样品(如细胞层和组织切片)的结构特性和功能信息
反射模式(
THz-ATR是一种特殊的反射模式(
特别需要说明的是,在THz-ATR技术中,传统方法是将溶液样品直接滴加在ATR棱镜上或位于其上的开放式液体池中。然而这种方法对样品的需求量较大,而且实验结果容易受到溶剂挥发的影响。为克服上述问题,Wang团队设计了一种聚二甲基硅氧烷微流道芯片,将该芯片与THz-ATR结合,可以实现对微量溶液样品的高灵敏检测。他们还基于衰减全反射理论,系统地分析了单界面模型和双界面模型对实验结果的影响,确定了理论模型正确应用的条
THz生物成像技术是一种无标记、非电离的可视化表征技术,是THz生物医学应用的重要研究方向之一。在该技术中,通过分析生物样品在THz波段的介电特征,可以揭示样品的结构和成分信息。为深入理解生物医学问题,需要从生物分子、细胞、组织、器官和生命个体不同层次开展THz成像研究,这就对THz成像技术的空间分辨率提出了要求。传统的远场THz成像技术具有毫米或亚毫米级的分辨率,主要应用于生物组织、器官及生命个体的成像检测;新兴的近场THz成像技术能够突破光学衍射极限,实现微米甚至纳米级的分辨率,因而在细胞、亚细胞结构以及生物大分子成像检测方面具有良好的应用前景。
基于THz-TDS系统的传统THz成像技术在生物医学检测中有着较为广泛应用,不仅可以用于离体生物组织或器官成像研究,还可用于在体成像探测。该成像技术需要将THz-TDS光谱系统与能荷载样品的精密位移平台相结合,通过位移平台的运动,逐点获取样品不同位置的THz信号,进而基于THz信号形成样品的THz图像。
在离体成像研究方面,研究人员采用不同THz成像模式对多种离体生物组织样品进行了检测(

图3 传统THz生物成像。(a)离体成像检测示意图;(b)荷有黑色素瘤小鼠皮肤组织切片的光学图像及对应的不同频率下的透射模式THz图像;(c)脑胶质瘤模型小鼠脑组织的光学图像(i,虚线区域为肿瘤)和对应的HE染色图像(ii)及THz-ATR图像(iii);(d)在体成像检测示意图;(e)人体皮肤疤痕不同恢复时长的反射模式THz图像;(f)正常人足底和糖尿病足患者足底的反射模式THz图像。图(b)、(c)、(e)和(f)经授权转载自文献[
Fig. 3 Conventional THz biomedical imaging techniques. (a) Schematic of ex vivo imaging detection. (b) Optical visual image and corresponding transmission-mode THz images of a mouse skin tissue sample containing melanoma. (c) Optical visual image (i, the tumor region is marked by the dotted line) and corresponding HE stained image (ii) and THz-ATR image of a mouse brain glioma tissue (iii). (d) Schematic of in vivo imaging detection. (e) Reflection-mode THz images of a human skin scar measured at different times during the recovery period. (f) Reflection-mode THz images of the plantar region of a normal human being (left), and a typical diabetic foot patient (right). Images reproduced with permission from Ref. [
在活体生物成像方面,研究人员已经利用传统反射式THz成像技术开展了一系列工作(
由于受限于光学衍射极限,传统THz成像的空间分辨率仅能达到亚毫米量级,无法满足对生物组织、细胞和生物大分子等样本的微细结构进行显微检测的实际需求。近年来,研究者们积极开发出各种方法提高THz成像的空间分辨率。其中,近场THz成像是主要的超光学衍射极限THz光谱成像方
根据近场探测方式的不同,目前用于生物样品检测的微米级近场THz成像技术主要可分为两种:第一种为基于光电导天线微型探针(Photoconductive antenna microprobe,PCAM)与THz-TDS结合的近场技术 (PCAM-THz-TDS);第二种是基于将样品贴附在光学晶体表面的近场技术。PCAM-THz-TDS近场探测方法是目前使用较为广泛的近场THz成像技术,它是通过利用探针在距离样品表面很近的位置(通常为微米量级,小于THz波长)探测透过样品的THz信号(

图4 THz近场生物成像。(a)微米级空间分辨PCAM-THz-TDS 近场系统原理图;(b)单个细胞的光学图像(i)及在脱水过程中不同时刻(ii-iv)的PCAM-THz-TDS图像;(c)纳米级空间分辨THz s-SNOM系统原理图;(d)免疫球蛋白(IgG)和铁蛋白(Ferritin)THz s-SNOM近场图像,左侧为AFM形貌图,右侧为对应的THz图像。图(b)和(d)经授权转载自文献[
Fig. 4 THz near-field biomedical imaging. (a) Schematic of the micron-resolution THz-PCAM system. (b) The optical visual image of a single cell (i), and the THz-PCAM images of the cell during the dehydration process (ii-iv). (c) Schematic of the nano-resolution THz s-SNOM system. (d) THz near-field images of IgG and ferritin measured by the s-SNOM, which include both atomic force microscopy (AFM) images (left) and THz images (right). Images reproduced with permission from Ref. [
将THz技术与原子力显微镜技术(Atomic force microscopy,AFM)结合形成的THz 散射式扫描近场光学显微镜(THz s-SNOM,
未来,通过对技术的持续优化和升级,上述微米级和纳米级分辨THz近场技术将有望在精准生物医学检测研究领域发挥更为重要的作用。
THz波作为一种电磁辐射,它的生物效应受到越来越多的关注。尽管THz波不会像X射线那样引起电离损伤,但它可能会对生物体系产生非电离的生物效应,如:改变生物大分子构象或功能、影响细胞成分及活性、引起组织的炎症反应或抑制肿瘤生长等。为了评估生物体在THz波辐照下的安全性,探索THz波辐照治疗疾病的可能性,研究人员从生物分子、细胞及生物组织等多个层面对THz波生物效应进行了深入研究。
在生物分子层面,THz波辐照能够引起生物分子的构象或功能发生变化(

图5 THz波辐照对生物分子产生的效应。(a)THz波辐照生物分子示意图;(b)BSA蛋白溶液在THz波辐照前后的圆二色谱图;(c)在 THz波辐照后,dsDNA长度对链交换实验荧光恢复的影响,*p < 0.05。图(b)和(c)经授权转载自文献[
Fig. 5 Effects of THz radiation on biomolecules. (a) Schematic of THz radiation on biomolecules. (b) Circular dichroism spectra of BSA solution before and after THz radiation. (c) After the exposure to THz radiation, the effect of dsDNA length on the fluorescence recovery in the strand exchange experiment, *p < 0.05. Images reproduced with permission from Ref. [
在细胞层面,THz波辐照可能会引起细胞膜通透

图6 THz波辐照对细胞产生的生物效应。(a)THz波辐照细胞示意图;(b)THz波辐照对精子活力影响的统计图,实验组(E)接受了THz辐照,而对照组(C)未接受。测试结果以中位数和平均值的标准差表示,“N” 表示正常人的精子样本,“Mi” 表示轻度弱精症患者的精子样本,“Mo ”代表中度弱精症患者的精子样本,*p < 0.05;(c)利用流式细胞术分析太赫兹波辐照对大鼠原发性海马神经元凋亡的影响,左侧为代表性图像,右侧为统计分析结果图,**p<0.01。图(b)和(c)经授权转载自参考文献[
Fig.6 Effects of THz radiation on cells. (a) Schematic of THz radiation on cells. (b) Statistical results for the effects of THz radiation on sperm motility. The experimental groups (E) received THz irradiation, while the control groups (C) did not. The results are presented as median with the standard error of the mean, “N” stands for results from normal patients, “Mi” stands for results from mild asthenospermia patients, and “Mo” stands for results from moderate asthenospermia patients, *p < 0.05. (c) Effects of THz radiation on the apoptosis of primary hippocampal neurons of rats analyzed by flow cytometry. The representative images are presented on the left while the statistical analysisis shown on the right, **p < 0.01. Images reproduced with permission from Ref.
在生物组织及个体层次,已有研究发现THz波可能会引起的变化主要包括局部升温、炎症反应以及肿瘤减小

图7 THz波辐照对活体组织产生的效应。(a)THz波辐照小鼠示意图;(b)THz波辐照前后活鼠耳皮肤中性粒细胞分布的荧光图像,红色表示中性粒细胞,绿色表示血管,品红色表示毛囊;(c)THz波辐照后小鼠皮肤的升温曲线(左)及组织切片HE染色图(右)。图(b)和(c)经授权转载自参考文献[
Fig.7 Effects of THz radiation on tumors in living organisms. (a) Schematic of THz radiation on a mouse. (b) Fluorescence images of neutrophil distribution in the ear skin of a live mouse before and after THz radiation. Red, green, and magenta colors indicate neutrophils, blood vessels, and hair follicles, respectively. (c) Temperature rise curves of the skin (left) and a HE stained image of a skin tissue slice (right) after THz radiation. Images reproduced with permission from Ref. [
由上述内容可以看出,THz波辐照对生物体系产生的影响与THz源类型、频率范围、辐照功率、辐照时间以及生物体系本身性质等因素密切相关,既可以产生不良影响,也可以产生有利的作用。
THz生物医学是一个新兴且快速发展的多种科学技术高度交叉融合的研究领域,通过研究THz波与生物体系的作用机制并建立精准的THz探测和分析方法,结合生物学和医学知识,揭示生命活动规律和发展疾病诊疗技术,为生物医学研究和诊疗提供了新视角和新手段。然而在THz生物医学的快速发展过程中,仍面临着一些挑战和问题亟待解决:
(1)水对THz波的强吸收影响。由于水分子对THz波有很强的吸收,生物样品中的水分会对THz探测产生影响。因此需要在实验设计和样品处理中采取措施,以减小水对THz信号的干扰。将性能良好的高功率THz源和高灵敏THz探测器结合使用可以提高对样品探测的信噪比和灵敏度(如采用气体激光器THz源和GeGa探测器),有望在一定程度上缓解水对THz波强吸收的影响。虽然目前THz TD-ATR技术可以用来检测生物溶液样品,但是其灵敏度及分辨率有限,难以对痕量样品进行检测,也无法对单生物分子或单细胞样品检测,因而需要进一步发展高灵敏和高分辨的液相THz检测技术。
(2)频谱分辨率不足。目前常用于生物检测的THz光谱和光谱成像系统的频谱分辨率通常在GHz量级,难以满足有效区分生物样品可能具有的特征峰的要求,可以考虑把具有高频谱分辨的THz光梳技术应用到THz生物医学检测领域
(3)数据分析及解读方法欠缺。虽然目前已经将小波分析、主成分分析、支持向量机、神经网络等方法应用到了THz数据分析,但是还比较初步,仍然需要发展高效和准确的数据分析方法,系统性地加强对THz数据的分析能力。与此同时,对于THz信号所反映生物学信息的深层次理解的研究还比较匮乏。已有的初步研究表明,将分子动力学模拟引入到THz生物检测研究,将有助于从探测到的THz信号中解读生物学信息。
(4)三维成像需求。现有的THz生物成像通常是利用透射模式获得样品的整体图像信息,或者是通过反射/散射模式获取样品的表面信息,关于生物样品的三维成像技术的研究还比较匮乏。虽然目前已经出现了THz计算机辅助和光学相干层析等三维成像技术,但是由于生物样品的复杂性,相关技术还难以用于生物样品的三维成像,需要进一步在成像速度、分辨率、图像重构算法等方面进行技术突
(5)THz波生物效应研究不充分。目前THz波生物效应的研究尚不充分,还需要进一步深入研究。在将来的研究中,需要特别注意以下几方面:首先,目前大多数实验给出的辐照功率密度是从THz源出射的功率密度,不是真正到达样品的功率密度,为了能够准确地评估辐射效应,应该给出作用在生物样品的辐照功率密度;其次,THz辐照的热效应和非热效应都有可能对生物体系产生影响,在排除热效应影响时,需要仔细地考虑THz辐照对样品局部的影响,而不是仅通过监测样品的宏观温度变化就做出结论;最后,现有辐照实验所采用的THz源、辐照参数、辐照方式、生物样品多种多样,不同研究团队的实验很难在不同团队之间重复开展,不利于系统化的THz波生物效应研究,因而急需建立标准的THz波生物效应研究规范。
综上所述,THz技术在生物医学领域展现出巨大的应用前景。尽管当前THz生物医学发展仍然很不充分,面临着多方面的挑战。但是随着现代科技的发展,优质的THz源和探测器不断地被开发出来,先进探测方法和计算光学技术也在蓬勃发展。 因此我们有理由相信,上述THz技术挑战将会逐渐得到解决,THz生物医学研究将进一步发展,并在国民经济发展和人民健康水平提升中发挥重要作用。
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