diff --git a/Cargo.toml b/Cargo.toml
index 9b98fb18e1..254d303767 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -19,7 +19,7 @@ include = ["Cargo.toml", "src/**/*.rs", "README.md", "LICENSE"]
 [dependencies]
 log = "0.4"
 thiserror = "1.0"
-futures = "0.3"
+futures = "0.3.5"
 jsonrpsee = { version = "0.1", features = ["ws"] }
 num-traits = { version = "0.2", default-features = false }
 serde = { version = "1.0", features = ["derive"] }
diff --git a/proc-macro/src/call.rs b/proc-macro/src/call.rs
index 234edbd8dd..d1eab81899 100644
--- a/proc-macro/src/call.rs
+++ b/proc-macro/src/call.rs
@@ -82,6 +82,7 @@ pub fn call(s: Structure) -> TokenStream {
             T: #module + #subxt::system::System + Send + Sync + 'static,
             S: #codec::Encode + Send + Sync + 'static,
             E: #subxt::SignedExtra<T> + #subxt::sp_runtime::traits::SignedExtension + Send + Sync + 'static,
+            <<E as #subxt::SignedExtra<T>>::Extra as #subxt::sp_runtime::traits::SignedExtension>::AdditionalSigned: Send + Sync,
         {
             fn #call<'a>(
                 &'a self,
@@ -154,6 +155,7 @@ mod tests {
                 T: Balances + substrate_subxt::system::System + Send + Sync + 'static,
                 S: codec::Encode + Send + Sync + 'static,
                 E: substrate_subxt::SignedExtra<T> + substrate_subxt::sp_runtime::traits::SignedExtension + Send + Sync + 'static,
+                <<E as substrate_subxt::SignedExtra<T>>::Extra as substrate_subxt::sp_runtime::traits::SignedExtension>::AdditionalSigned: Send + Sync,
             {
                 fn transfer<'a>(
                     &'a self,
diff --git a/src/lib.rs b/src/lib.rs
index 9d38535627..5c28706056 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -118,7 +118,7 @@ pub struct ClientBuilder<T: System, S = MultiSignature, E = DefaultExtra<T>> {
     client: Option<jsonrpsee::Client>,
 }
 
-impl<T: System, S, E> ClientBuilder<T, S, E> {
+impl<T: System + Send + Sync, S, E> ClientBuilder<T, S, E> {
     /// Creates a new ClientBuilder.
     pub fn new() -> Self {
         Self {
@@ -327,18 +327,22 @@ where
     }
 
     /// Creates a signed extrinsic.
-    pub async fn create_signed<C: Call<T>>(
+    pub async fn create_signed<C: Call<T> + Send + Sync>(
         &self,
         call: C,
         signer: &(dyn Signer<T, S, E> + Send + Sync),
     ) -> Result<
         UncheckedExtrinsic<T::Address, Encoded, S, <E as SignedExtra<T>>::Extra>,
         Error,
-    > {
+    >
+    where
+        <<E as SignedExtra<T>>::Extra as SignedExtension>::AdditionalSigned: Send + Sync,
+    {
         let unsigned = self
             .create_unsigned(call, signer.account_id(), signer.nonce())
             .await?;
-        Ok(signer.sign(unsigned))
+        let signed = signer.sign(unsigned).await?;
+        Ok(signed)
     }
 
     /// Returns an events decoder for a call.
@@ -379,21 +383,27 @@ where
     }
 
     /// Submits a transaction to the chain.
-    pub async fn submit<C: Call<T>>(
+    pub async fn submit<C: Call<T> + Send + Sync>(
         &self,
         call: C,
         signer: &(dyn Signer<T, S, E> + Send + Sync),
-    ) -> Result<T::Hash, Error> {
+    ) -> Result<T::Hash, Error>
+    where
+        <<E as SignedExtra<T>>::Extra as SignedExtension>::AdditionalSigned: Send + Sync,
+    {
         let extrinsic = self.create_signed(call, signer).await?;
         self.submit_extrinsic(extrinsic).await
     }
 
     /// Submits transaction to the chain and watch for events.
-    pub async fn watch<C: Call<T>>(
+    pub async fn watch<C: Call<T> + Send + Sync>(
         &self,
         call: C,
         signer: &(dyn Signer<T, S, E> + Send + Sync),
-    ) -> Result<ExtrinsicSuccess<T>, Error> {
+    ) -> Result<ExtrinsicSuccess<T>, Error>
+    where
+        <<E as SignedExtra<T>>::Extra as SignedExtension>::AdditionalSigned: Send + Sync,
+    {
         let extrinsic = self.create_signed(call, signer).await?;
         let decoder = self.events_decoder::<C>()?;
         self.submit_and_watch_extrinsic(extrinsic, decoder).await
diff --git a/src/signer.rs b/src/signer.rs
index b33b3fdd33..ec60b195e4 100644
--- a/src/signer.rs
+++ b/src/signer.rs
@@ -31,13 +31,21 @@ use sp_runtime::{
     },
     traits::{
         IdentifyAccount,
+        SignedExtension,
         Verify,
     },
 };
-use std::marker::PhantomData;
+use std::{
+    future::Future,
+    marker::PhantomData,
+    pin::Pin,
+};
 
 /// Extrinsic signer.
-pub trait Signer<T: System, S: Encode, E: SignedExtra<T>> {
+pub trait Signer<T: System, S: Encode, E: SignedExtra<T>>
+where
+    <<E as SignedExtra<T>>::Extra as SignedExtension>::AdditionalSigned: Send + Sync,
+{
     /// Returns the account id.
     fn account_id(&self) -> &T::AccountId;
 
@@ -45,10 +53,23 @@ pub trait Signer<T: System, S: Encode, E: SignedExtra<T>> {
     fn nonce(&self) -> Option<T::Index>;
 
     /// Takes an unsigned extrinsic and returns a signed extrinsic.
+    ///
+    /// Some signers may fail, for instance because the hardware on which the keys are located has
+    /// refused the operation.
     fn sign(
         &self,
         extrinsic: SignedPayload<Encoded, E::Extra>,
-    ) -> UncheckedExtrinsic<T::Address, Encoded, S, E::Extra>;
+    ) -> Pin<
+        Box<
+            dyn Future<
+                    Output = Result<
+                        UncheckedExtrinsic<T::Address, Encoded, S, E::Extra>,
+                        String,
+                    >,
+                > + Send
+                + Sync,
+        >,
+    >;
 }
 
 /// Extrinsic signer using a private key.
@@ -91,12 +112,13 @@ where
 
 impl<T, S, E, P> Signer<T, S, E> for PairSigner<T, S, E, P>
 where
-    T: System,
-    T::AccountId: Into<T::Address>,
-    S: Encode,
-    E: SignedExtra<T>,
-    P: Pair,
-    P::Signature: Into<S>,
+    T: System + 'static,
+    T::AccountId: Into<T::Address> + 'static,
+    S: Encode + 'static + Send + Sync,
+    E: SignedExtra<T> + 'static,
+    P: Pair + 'static,
+    P::Signature: Into<S> + 'static,
+    <<E as SignedExtra<T>>::Extra as SignedExtension>::AdditionalSigned: Send + Sync,
 {
     fn account_id(&self) -> &T::AccountId {
         &self.account_id
@@ -109,14 +131,24 @@ where
     fn sign(
         &self,
         extrinsic: SignedPayload<Encoded, E::Extra>,
-    ) -> UncheckedExtrinsic<T::Address, Encoded, S, E::Extra> {
+    ) -> Pin<
+        Box<
+            dyn Future<
+                    Output = Result<
+                        UncheckedExtrinsic<T::Address, Encoded, S, E::Extra>,
+                        String,
+                    >,
+                > + Send
+                + Sync,
+        >,
+    > {
         let signature = extrinsic.using_encoded(|payload| self.signer.sign(payload));
         let (call, extra, _) = extrinsic.deconstruct();
-        UncheckedExtrinsic::new_signed(
+        Box::pin(futures::future::ok(UncheckedExtrinsic::new_signed(
             call,
             self.account_id.clone().into(),
             signature.into(),
             extra,
-        )
+        )))
     }
 }