Science Focus (issue 25)

13 如果將數滴液態金屬鎵(gallium)加入一個鋁罐會發 生甚麼事呢?高中化學課告訴我們甚麼都不會發生,但在 片刻之後,你會驚訝地發現只需輕輕一碰,鋁罐就會化為 碎片。 難道我們的化學老師弄錯了嗎?非也。鋁罐接觸鎵後 會碎裂確實不是由化學反應引起,而是由一種名為液態金 屬脆化(liquid metal embrittlement / LME)的物理現 象導致。 雖然金屬物體(例如鋁罐)看起來是一個整體,但實際 上是由許多叫晶粒(grains)的細小晶體組成。如圖一所 示,當鋁罐與例如鎵等特定的液態金屬接觸時,後者可以 穿透晶粒之間的空隙,亦即是晶粒邊界 [1],晶體之間的內 聚力因而會被顯著削弱,導致鋁罐的強度下降,變得容易 碎裂。 雖然 LME 一直是航空、航天和建築等業界中導致金屬 結構失效的常見原因,但麻省理工學院的研究人員最近卻 「以具建設性的方式利用了這種失效機制 [2, 3]」。 Figure 2 Smearing EGaIn paint onto a staple to remove the device. 1 Editor’s note: In addition to the camera to look inside the body, various tools can be attached to the tip of the endoscope, such as grasping forceps (for retrieving foreign objects), and biopsy forceps (for performing biopsies). It may take some time before these dissolvable metal devices are ready for clinical use, but the genuine creativity demonstrated in this study is immediately apparent. While most people perceive LME as a failure mechanism, the researchers thought out of the box to turn such a mechanism into a productive one. At times, good research does not require highly sophisticated methods; a touch of creativity can make all the difference. biomedical devices: They are strong, durable, and have excellent electrical and thermal conductivity. However, a major problem when using metal devices is the way to remove them when they are not required anymore. This can possibly be done by surgery or endoscopy (footnote 1), yet these invasive procedures may cause additional tissue damage. Therefore, the researchers started exploring devices that can disintegrate inside the patient’s body after use. Drawing inspiration from LME, the research team experimented on the use of a gallium alloy called eutectic gallium-indium (EGaIn) for the dissolution of different aluminum devices. Gallium stands out from other LME-inducing liquid metals for two reasons. First, it can prevent the formation of a surface oxide layer on the aluminum device upon application. This allows aluminum to react with water and enhances its degradation via dissolution. More importantly, gallium is biocompatible – acute toxicity studies showed EGaIn is non-toxic to rodents even at high doses. The next step is to deliver gallium-indium to aluminum devices, either directly or indirectly. The former involves smearing EGaIn paint onto devices such as staples used to hold the skin together (Figure 2). This may appear trivial, but it is not an easy task. Like water, EGaIn has high surface tension that hinders its ability to attach to and spread over metal surfaces. Knowing that gallium oxide has a much lower surface tension, the researchers applied a simple trick – physically stirring EGaIn beforehand – to increase the alloy’s exposure to air and hence the ratio of gallium oxide to EGaIn in the paint. Alternatively, nano- and microparticles of EGaIn were produced for delivery into patients’ bodies to trigger the dissolution remotely. The team treated different biomedical devices made of aluminum, such as staples on skin and stents implanted in the esophagus, with EGaIn suspensions and found that these metal structures were broken down shortly afterward. Although gallium-induced embrittlement works well for aluminum devices, what about devices made of other metals? For instance, esophageal stents are often made of metals such as nitinol, a nickel-titanium alloy, instead of aluminum. To widen the applicability of LME in the removal of biomedical devices, the researchers have also been exploring the possibility of creating dissolvable devices made of nitinol and other metals commonly used in medical settings.