Share-Based Compensation - Stock option and restricted stock unit activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Amount, Outstanding	1,893,201
Amount, Granted	650,000
Amount, Outstanding	2,543,201	1,893,201
Weighted-average exercise price, Outstanding	$ 6.44
Weighted-average exercise price, Granted	2.00
Weighted-average exercise price, Outstanding	$ 5.30	$ 6.44
Biostage2013 Equity Incentive Plan [Member] \| Stock Options [Member]
Amount, Outstanding	1,599,720
Amount, Granted	301,674
Amount, Canceled	(398,212)
Amount, Outstanding	1,503,182	1,599,720
Amount, Options vested	895,850
Weighted-average exercise price, Outstanding	$ 6.33
Weighted-average exercise price, Granted	1.49
Weighted-average exercise price, Canceled	9.20
Weighted-average exercise price, Outstanding	4.63	$ 6.33
Weighted-average exercise price, Options vested	$ 6.21
Weighted-average contractual life, Outstanding	7 years 4 months 28 days	5 years 9 months 7 days
Weighted-average contractual life, Options Vested	6 years 11 months 19 days
Outstanding at the beginning (in dollars)	$ 0
Outstanding at the end (in dollars)	876	$ 0
Vested at the end (in dollars)	$ 376

The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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