diff --git a/src/jaximal/nn.py b/src/jaximal/nn.py
index 6c305f9..bbefecb 100644
--- a/src/jaximal/nn.py
+++ b/src/jaximal/nn.py
@@ -1,3 +1,4 @@
+from abc import ABC, abstractmethod
 from enum import Enum, auto
 from typing import Any, Callable
 
@@ -65,13 +66,24 @@ def init(
                 return jax.random.normal(key, shape, dtype=dtype) * scaling
 
 
-class JaximalModule(Jaximal):
+class JaximalModule(Jaximal, ABC):
+    @abstractmethod
     @staticmethod
-    def init_state(key: PRNGKeyArray, *args: Any, **kwargs: Any) -> 'JaximalModule':
-        raise NotImplementedError('This method must be implemented on each subclass')
+    def init_state(key: PRNGKeyArray, *args: Any, **kwargs: Any) -> 'JaximalModule': ...
 
-    def __call__(self, data: PyTree, **kwargs: Static[Any]) -> PyTree:
-        raise NotImplementedError('This method must be implemented on each subclass')
+    @abstractmethod
+    def __call__(self, data: PyTree) -> PyTree: ...
+
+
+class Activation(JaximalModule):
+    func: Static[Callable[[Array], Array]]
+
+    @staticmethod
+    def init_state(key: PRNGKeyArray, func: Callable[[Array], Array]) -> 'Activation':
+        return Activation(func)
+
+    def __call__(self, data: PyTree) -> PyTree:
+        return jax.tree.map(self.func, data)
 
 
 class Linear(JaximalModule):
@@ -95,7 +107,7 @@ def init_state(
 
         return Linear(in_dim, out_dim, weights, biases)
 
-    def __call__(self, data: PyTree, **kwargs: Static[Any]) -> PyTree:
+    def __call__(self, data: PyTree) -> PyTree:
         return data @ self.weights + self.biases
 
 
@@ -111,7 +123,7 @@ def init_state(
         modules = list(partial(key) for key, partial in zip(keys, partials))
         return Sequential(modules)
 
-    def __call__(self, data: PyTree, *args: dict[str, Any], **_: Any) -> PyTree:
+    def __call__(self, data: PyTree, *args: dict[str, Any]) -> PyTree:
         assert len(args) == len(self.modules), (
             'Expected `self.modules` and `args` to have the same length '
             f'but got {len(self.modules)} and {len(args)}, respectively.'