• Bark medicinals primarily consist of tree bark and root bark, but can also include fruit peels, seed coats, and the outer layers of fungal medicinal materials.
• The primary functions of bark medicinals include heat-clearing, qi-regulating, warming the interior, wind-dispelling and dampness-eliminating, diuretic and edema-reducing, and astringent effects. Modern research on plant barks focuses on identifying active ingredients and has found that they mostly have antibacterial, anti-inflammatory, antiallergic, and immune-regulating properties.
Bark medicinals refer to the parts of plants external to the cambium layer, including the periderm, cortex, primary phloem, and secondary phloem. In layman's terms, these are tree barks (including trunk and branch barks) and root barks. Additionally, fruit peels, seed coats, and the outer layers of fungal medicinals are sometimes classified as bark medicinals based on practical applications. These medicinals have broad clinical applications and distinct scopes and patterns of use. This article will review and analyze relevant literature.
Classification by Function
The Shennong Bencao Jing provides a detailed understanding of bark medicinals, classifying some like Wujiapi, Diguipi, Rougui, Duzhong, and Huangbai as superior, Baixianpi, Hehuapi, Mudanpi, Houpu, Qinpi as middling, and Kuanliapi as inferior.
Based on historical records and clinical practice, the functions of bark medicinals can be grouped into six main categories:
Heat-clearing: Mudanpi, Diguipi, Baixianpi, Kuanliapi, Huangbai, Xiaobopai, Qinpi, Sankexin, Gualluopi, etc.
Qi-regulating: Houpu, Zhike, Xiangyuanpi, Qingpi, Chenpi, Dafupi, Ningmengpi, Hongmuxiang, Zhaoshuipi, etc.
Warming the interior: Rougui, Yinxiangpi, Guipi, Gansui, etc.
Wind-dispelling and dampness-eliminating: Wujiapi, Haitongpi, Difengpi, Chuanjinpi, Zushima, Chashuipi, etc.
Diuretic and edema-reducing: Dongguapi, Sangbaipi, Yubaipi, FuLingpi, Xigua Pi, Jiangpi, Hulupi, Wujiumugenpi, etc.
Astringent: Liuzhipi, Zongzhupi, Chungenbaipi, Zao shu pi, Suanzaogenpi, etc.
Like other medicinal categories, many bark medicinals exhibit multiple functions. For example, both Northern and Southern Wujiapi can dispel wind-dampness and strengthen the sinews and bones. Southern Wujiapi also tonifies the liver and kidneys and reduces edema. Duzhong can tonify the liver and kidneys and strengthen the sinews and bones, and is also used for stabilizing pregnancy. Rougui can replenish primordial yang, warm the spleen and stomach, eliminate cold accumulation, and promote blood circulation. Qinpi can clear heat and dry dampness, act as an astringent to stop dysentery, and also stop leukorrhea and improve vision. Other examples include Dafupi, which can regulate qi and reduce distention, promote urination and reduce edema; Baixianpi, which can clear heat and dry dampness, dispel wind and detoxify; Yinxiangpi, which can warm the middle and stop pain, dispel wind and cold, detoxify and reduce swelling, and stop bleeding; Haitongpi, which can dispel wind and dampness and relax tendons and meridians; Huamupi, which can clear heat and drain dampness, expel phlegm and stop coughing, and detoxify and reduce swelling; Shazhaoshupi, which can clear heat and stop coughing, drain dampness and stop pain, detoxify, and stop bleeding; Chu shubai pi, which can promote urination and stop bleeding; Luohansonggenpi, which can invigorate blood and remove blood stasis, dispel wind and dampness, and kill parasites and stop itching; Mumianpi, which can clear heat and detoxify, disperse blood stasis and stop bleeding; Wujiumugenpi, which can promote urination, eliminate accumulation, kill parasites, and detoxify; Jiubiying, which can clear heat and detoxify, reduce swelling and stop pain; Baomazi pi, which can clear the lungs, expel phlegm, stop coughing, and promote urination; Shanhuangpi, which can dispel wind and cold, regulate qi and stop pain, and eliminate dampness and reduce swelling; Sankexin, which can clear heat, drain dampness, and disperse blood stasis; Yajiaomu pi, which can clear heat and expel exterior pathogens, dispel wind and dampness, relax tendons and promote circulation; Hehuapi, which can resolve depression, harmonize blood, calm the mind, and eliminate carbuncles and swellings; Zapishu pi, which can clear heat and drain dampness, and detoxify and reduce swelling; Yanfumugenpi, which can clear heat and drain dampness, detoxify and disperse blood stasis; Muhudie shupi, which can clear heat and drain dampness to resolve jaundice, and relieve sore throat and reduce swelling; Kushipi, which can clear heat and drain dampness and kill parasites; Yubaipi, which can promote urination, relieve stranguria, reduce swelling, and kill parasites and stop itching; Mumijinpi, which can clear heat and drain dampness, detoxify, and stop itching; Longyan shu pi, which can kill parasites, eliminate accumulation, detoxify, and consolidate sores, among others.
Mechanism of Action
In biology, the epidermis is an important component of plant nutritive organs. Tree bark and stem bark provide significant protection, while root bark absorbs and transports nutrients needed for growth.
For a plant, the bark acts like an outer coat or armor, serving a protective function. Bark functions similarly to a heat sink, controlling temperature fluctuations within the trunk and effectively defending against extreme cold, heat, and humidity, making some of them wind-dispelling and dampness-eliminating drugs. Bark has a wrapping and restraining function, keeping the plant erect and less prone to breaking, making certain plant barks useful for bone and joint diseases. Bark has a certain degree of impermeability and water resistance, making some of them diuretic and edema-reducing drugs. Most barks taste bitter or have some toxicity, and bitterness clears heat, making them either heat-clearing or insecticidal drugs. Bark or fruit peels have numerous pores, allowing air permeability, making some of them qi-regulating drugs. Bark has strong self-repair capabilities, capable of self-repair after damage, making some of them astringent drugs for stopping bleeding and diarrhea.
Based on the “correlative thinking” approach, traditional Chinese medicine considers bark medicinals to correspond to the lungs, as the lungs govern skin and hair. They belong to the metal element in the five elements theory, implying restraint and constriction. Therefore, most barks can regulate water pathways or dispel dampness or promote urination, regulating excess subcutaneous fluid and directing it downward. According to the principle of analogy, medicinals like Dafupi, Chenpi, and Chen hulu piao, which are large and round, can treat large abdominal edema. Their spaciousness also allows them to regulate qi and reduce distension. Furthermore, their constraining nature seems to enable them to counteract their own toxicity, such as apricot bark decoction relieving apricot kernel poisoning, white fruit shell decoction relieving white fruit poisoning, and Sangbaipi decoction relieving dizziness and facial heat from overeating mulberry fruits. Badaoren has a strong purgative effect, but badao shell can stop diarrhea. Citrus and pomelo can produce phlegm, but their peels can transform phlegm, and so on. Moreover, an interesting phenomenon was observed regarding ginger: “retaining the ginger peel makes it cool, removing the ginger peel makes it hot.” The Compendium of Materia Medica describes the ginger peel as “spicy, cool, non-toxic,” able to “reduce edema, abdominal distension, fullness, and regulate the spleen and stomach, and eliminate eye cloudiness.” The Yilin Zuan Yao further explains: “The ginger peel is spicy and cold. Most peels are contrary to the nature of their base material, hence it is cold. It reaches the skin through the skin, spiciness promotes circulation, thus treating edema and eliminating wind and heat from the skin. Ginger induces sweating, while ginger peel stops sweating due to its slight coldness.” This reflects the complexity of the