Tough to lend against Bitcoin because you can't do a smart contract with it yet (though a few firms are working on this). But you can lend against any ERC-20 token (i.e. any crypto built on Ethereum network). You would just do an 60-70% advance, and program in the smart contract to liquidate if the crypto moves X% down (or give them 24 hours to pledge more collateral). Some big names are working on this now, it will be prevalent soon.