The Company leases various office and research and development facilities under operating lease agreements that expire at various dates through October 2050. Under the terms of the agreements, the Company is responsible for certain insurance, property taxes and maintenance expenses. The Company recognizes rent expense on a straight-line basis over the term of the operating leases. Any difference between cash payments required and rent expense is recorded as deferred rent. Rent expense for the six months ended June 30, 2021 and 2020 was $2.6 million and $2.2 million, respectively.

Aggregate future minimum lease payments required under the operating leases at June 30, 2021 are as follows (in thousands):

 

The Company purchased equipment with total gross book value of $4.1 million under capital lease agreements. Interest rates for the capital leases range from 4.84% to 22.10% per annum. Accumulated depreciation for equipment acquired under the capital leases was $0.9 million and $0.7 million as of June 30, 2021 and December 31, 2020, respectively.

Aggregate future minimum principal lease payments under the capital leases at June 30, 2021 are as follows (in thousands):

 

The Company leases various office and research and development facilities under operating lease agreements that expire at various dates through October 2050. Under the terms of the agreements, the Company is responsible for certain insurance, property taxes and maintenance expenses. The Company recognizes rent expense on a straight-line basis over the term of the operating leases. Any difference between cash payments required and rent expense is recorded as deferred rent. Rent expense for 2020 and 2019 was $4.7 million and $4.2 million, respectively.

Aggregate future minimum lease payments required under the operating leases at December 31, 2020 are as follows (in thousands):

 

The Company purchased equipment with total gross book value of $3.0 million under capital lease agreements, of which $0 and $1.3 million was purchased during 2020 and 2019, respectively. Interest rates for the capital leases range from 4.84% to 22.10% per annum. Accumulated depreciation for equipment acquired under the capital leases was $0.7 million and $0.3 million as of December 31, 2020 and 2019, respectively.

Aggregate future minimum principal lease payments under the capital leases at December 31, 2020 are as follows (in thousands):