Bitcoin is the hottest topic in financial services. But its vogue has escaped ETFs because issuers have been unable to get regulatory approval for bitcoin tracking funds.

Facing continued frustrating in their listing attempts, issuers are trying another track: blockchain ETFs.

Two small US ETF issuers have filed prospectuses for ETFs that track blockchain companies, the ledger technology that makes bitcoin possible.

On Thursday, Chicago-based Amplify Investments filed for the Amplify Blockchain Leaders ETF. And San Diego-based Reality Shares has filed for the Nasdaq Blockchain Economy ETF.

Both are awaiting approval, but if successful the funds will be the first to offer exposure to blockchain and, by extension, cryptocurrency. (While there are many potential uses of blockchain outside of cryptocurrency, at the time of writing its major purpose is to facilitate bitcoin transactions).

The proposed Amplify Blockchain Leaders ETF will be actively managed and buy companies and securities "active in the blockchain industry", the fund's prospectus says.

The benchmark it will try to beat is put together by Emerita Capital, a San Fransisco-based research firm specialising in blockchain. At time of writing there are no details available on the index constituents. Emirita's website says, "coming soon, stay tuned".

In a press release, Amplify's CEO Christian Magoon compared the unfolding explosion of blockchain technologies to the California gold rush in the 1800s. "We believe there's a 'pick and axe' play here, where we're not just looking to sell the gold, but also the equivalent of the mining equipment," he said.

"Bitcoin is just one application for blockchain, the best-known one, but there will be all kinds of applications, not just cryptocurrencies. We think it is like the internet was 20 years ago."

The Reality Shares Nasdaq Blockchain Economy ETF will be a more traditional ETF. It will be passively managed and track an index provided by Nasdaq (about which ETF Stream could also find no information).

According to filing documents, the proposed ETF will "measure the returns of companies that are committing material resources to developing, researching, supporting, innovating or using blockchain technology."

Other issuers also have their eyes on blockchain and bitcoin ETFs. In Canada, Toronto-based Evolve is seeking approval for a bitcoin ETF that tracks bitcoin futures. Whether this succeeds or not remains to be seen.

At present only bitcoin ETNs are traded.