6. INTANGIBLE ASSETS

Intangible assets consisted of the following:

For the nine months ended September 30, 2021 and 2020, total amortization expense was $1,304 and $562, respectively. For the three months ended September 30, 2021 and 2020, total amortization expense was $410 and $212, respectively.

The following table summarizes estimated future amortization expense of intangible assets placed in service at September 30, 2021 for the years ending: