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Intangible assets, net (Tables) | 6 Months Ended
--- | ---
Jun. 30, 2022
Intangible assets, net |
Schedule of intangible assets, net | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | June 30, |  | December 31,
​ | ​ | 2022 | ​ | 2021
Software | ​ | $ | 29,345 | ​ | $ | 30,838
Land use rights* | ​ |  | 299,224 | ​ |  | 314,456
Patents | ​ |  | 4,479,000 | ​ |  | 4,707,000
Subtotal | ​ |  | 4,807,569 | ​ |  | 5,052,294
Less: Accumulated amortization | ​ |  | (4,615,716) | ​ |  | (4,846,323)
Intangible assets, net | ​ | $ | 191,853 | ​ | $ | 205,971
*There is no private ownership of land in China. Land is usually owned by the local government and the government grants land use rights for specified terms. The Company acquired land use rights from the local government in December 2002 for period of 50 years. As of June 30,2022 and December 31, 2021, land use rights with net book value of $187,514 and $200,204, respectively, were pledged as collateral for bank loans (Note 11).

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- Definition Tabular disclosure of goodwill and intangible assets, which may be broken down by segment or major class. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -URI https://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267 + Details
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