Another device, called Scentee, plugs into a smartphone.

plug单独使用是个及物动词。但plug into是个短语动词，这时的plug可以是及物动词也可以是不及物动词，into是个介词。因此，可以用于A plugs into B这样的用法，表示电器或设备连接在一起。如朗文词典的解释和例句所示。

《朗文词典》plug into: phrasal verb

plug (something) into something： to connect one piece of electrical equipment to another, or to be connected 

Your phone can be plugged into the cigarette lighter socket in your car. 

Games consoles plug into the back of the TV. 游戏操纵板连接到电视机的背后。

类似的用法如下：

The material washes easily. 这种布料很容易洗。

有的语法书将动词的这种用法称为主动形式表示被动意义。其实也可以不这样理解，直接将动词按不及物理解。

Cheok said different tastes are triggered by molecules that cause chemical ionization on the surface of the tongue, sending the brain electrical signals of a specific amplitude and frequency that are possible to reproduce with two electrodes.

sending the brain electrical signals...为现在分词短语作补充性状语，对定语从句内容作补充说明。sending的逻辑主语不是molecules，而是that (malecules) cause chemical ionization on the surface of the tongue整个分句内容。这个分词短语可以改写为非限制性定语从句：which sends the brain...，which的先行词为定语从句整句内容。

译文：切奥克说，触发不同味觉的原因是：这些分子引起舌头表面发生化学电离变化，向大脑发出特定振幅和频率的电信号，这些信号可以通过两个电极复制。

这个句子中的that are possible to reproduce...是electrical signals的定语从句。something is possible to do这种用法十分罕见，有人认为possible不能这么用。但我在COCA语料库找到两个类似的例句：

I think there can be synergies that are possible to create to essentially advance these causes. 

We make calculations of prudence based on the goals that are possible to achieve in the situation.