	September 30, 2020
	Estimated Useful Lives	Gross Carrying Value		Accumulated Amortization			Net Book Value
Internally developed patents	6-10 years	$	845,216	$	(473,314	)	$	371,902
Acquired licenses	16-20 years		285,000		(281,063	)		3,937
Total intangible assets subject to amortization			1,130,216		(754,377	)		375,839
IPR&D assets	Indefinite		12,418,967		—			12,418,967
Total		$	13,549,183	$	(754,377	)	$	12,794,806

	December 31, 2019
	Estimated Useful Lives	Gross Carrying Value		Accumulated Amortization			Net Book Value
Internally developed patents	6-10 years	$	746,323	$	(448,874	)	$	297,449
Acquired licenses	16-20 years		285,000		(269,221	)		15,779
Total intangible assets subject to amortization			1,031,323		(718,095	)		313,228
IPR&D assets	Indefinite		12,418,967		—			12,418,967
Total		$	13,450,290	$	(718,095	)	$	12,732,195

Amortization expense of intangible assets was $10,406 and $28,084 for the three months ended September 30, 2020 and 2019, and $36,282 and $135,664 for the nine months ended September 30, 2020 and 2019, respectively. Amortization expense was classified as research and development expenses in the accompanying unaudited consolidated statements of operations and comprehensive loss.

As of September 30, 2020, future estimated amortization expense was as follows:

Years ending December 31,
The remainder of 2020	$	14,539
2021		26,576
2022		26,576
2023		26,576
2024		22,669
2025 and thereafter		258,903
Total	$	375,839