Trustless bridges, also known as smart contract-based bridges, are a type of bridge that utilizes smart contracts to facilitate asset transfers without intermediaries. Here are some key characteristics:

1. Decentralized: Trustless bridges are decentralized, meaning that no single entity controls the bridge.

2. Smart contract-based: Assets are transferred using smart contracts, which are self-executing contracts with the terms of the agreement written directly into code.

3. Trustless: Users do not need to trust a central authority or custodian, as the smart contract executes the transfer automatically.

4. Permissionless: Anyone can use the bridge without requiring permission from a central authority.

5. Censorship-resistant: Transactions are processed on-chain, making it difficult for a single entity to censor or block transactions.

6. Transparency: All transactions are recorded on the blockchain, providing transparency and publicly visible proof of transfer.

7. Security: Smart contracts are designed to be secure, using cryptographic algorithms and immutable code.

8. Examples: Uniswap's bridge, Polygon's bridge, and other decentralized bridges built on smart contract platforms.

Trustless bridges offer greater security, transparency, and decentralization, but may require a higher level of technical expertise and can be more complex to use.