diff --git a/README.md b/README.md
index 36a4b50..8aea66a 100644
--- a/README.md
+++ b/README.md
@@ -114,7 +114,6 @@ assert_eq!(item.list, vec![7]);
 #### Case 3 - Extend a struct from a crate
 Deriving `Substrate` on a struct will help you expose the field information, and you can easy to expose in build.rs of other crate.
 Deriving `Catalyst` on can read the field information of Substrate and generate a new Complex struct.
-All the fields in substrate and catalyst need be public, and the fields in complex are also public.
 The overall behavior likes [chemical catalysts](https://en.wikipedia.org/wiki/Enzyme_catalysis), a catalyst **bind** on a substrate to form a complex struct, which has all fields from substrate and catalyst.
 Also, a complex can **decouple** without clone and return a catalyst and substrate. Check the [complex-example](./complex-example/catalyst/src/lib.rs).
 In the future, we may have a sub feature with catalyst, such that it will provide no memory moving decouple, but need unsafe code.
diff --git a/complex-example/catalyst/src/lib.rs b/complex-example/catalyst/src/lib.rs
index 9d47923..577c0d8 100644
--- a/complex-example/catalyst/src/lib.rs
+++ b/complex-example/catalyst/src/lib.rs
@@ -14,12 +14,26 @@ struct Amyloid {
     #[complex(attribute(serde(default = "default_str")))]
     pub extra_string: String,
     pub extra_option: Option<usize>,
+    #[complex(attribute(serde(default = "default_extra_private_number")))]
+    extra_private_number: u8,
 }
 
 fn default_str() -> String {
     "default".to_string()
 }
 
+fn default_extra_private_number() -> u8 {
+    7
+}
+
+#[allow(dead_code)]
+impl AmyloidComplex {
+    /// Sum up the private_number(from Substrate) and the extra_private_number(from Catalyst)
+    fn private_number_sum(&self) -> u8 {
+        self.private_number + self.extra_private_number
+    }
+}
+
 #[derive(Catalyst)]
 #[catalyst(bind = Base)]
 #[complex(name = "SmallCpx")]
@@ -60,32 +74,42 @@ mod tests {
         let amyloid = Amyloid::default();
         let complex = amyloid.bind(substrate);
         assert_eq!(complex.field_bool, true);
+        assert_eq!(complex.private_number_sum(), 0);
 
         let toml_str = toml::to_string_pretty(&complex).unwrap();
         assert_eq!(
             toml_str,
             r#"field_bool = true
 field_string = ""
+private_number = 0
 extra_bool = false
 extra_string = ""
+extra_private_number = 0
 "#
         );
         let toml_str = r#" field_bool = true
 field_string = ""
+private_number = 1
 extra_bool = true
     "#;
         let complex: AmyloidComplex = toml::from_str(toml_str).unwrap();
         assert_eq!(complex.extra_string, "default");
+        
+        // the `extra_private_number` is 7 generated by `default_extra_private_number`, when serialize with a missing field
+        // such that the `private_number` + `extra_private_number` = 1 + 7 = 8
+        assert_eq!(complex.private_number_sum(), 8);
     }
 
     #[test]
     fn override_works() {
         let toml_str = r#"field_bool = false
+private_number = 0
 extra_bool = false
 "#;
         let complex: SmallCpx = toml::from_str(toml_str).unwrap();
         assert_eq!(complex.field_string, "default");
         let toml_str = r#"field_bool = false
+private_number = 0
 renamed_field = "Renamed"
 extra_bool = false
 "#;
diff --git a/complex-example/substrate/src/lib.rs b/complex-example/substrate/src/lib.rs
index 2833a84..5fae9ce 100644
--- a/complex-example/substrate/src/lib.rs
+++ b/complex-example/substrate/src/lib.rs
@@ -9,12 +9,17 @@ pub struct Base {
     pub field_bool: bool,
     pub field_string: String,
     pub field_option: Option<usize>,
+    private_number: u8,
 }
 
 impl Base {
     pub fn has_bool(&self) -> bool {
         self.field_bool
     }
+
+    pub fn get_private_number(&self) -> u8 {
+        self.private_number
+    }
 }
 
 #[cfg(test)]
@@ -25,9 +30,39 @@ mod tests {
     fn expose_works() {
         assert_eq!(
             Base::expose_content(),
-            r#"{"named":[{"attrs":[{"style":"outer","meta":{"list":{"path":{"segments":[{"ident":"serde"}]},"delimiter":"paren","tokens":[{"ident":"default"}]}}}],"vis":"pub","ident":"field_bool","colon_token":true,"ty":{"path":{"segments":[{"ident":"bool"}]}}},{"vis":"pub","ident":"field_string","colon_token":true,"ty":{"path":{"segments":[{"ident":"String"}]}}},{"vis":"pub","ident":"field_option","colon_token":true,"ty":{"path":{"segments":[{"ident":"Option","arguments":{"angle_bracketed":{"args":[{"type":{"path":{"segments":[{"ident":"usize"}]}}}]}}}]}}}]}"#
+            r#"{"named":[{"attrs":[{"style":"outer","meta":{"list":{"path":{"segments":[{"ident":"serde"}]},"delimiter":"paren","tokens":[{"ident":"default"}]}}}],"vis":"pub","ident":"field_bool","colon_token":true,"ty":{"path":{"segments":[{"ident":"bool"}]}}},{"vis":"pub","ident":"field_string","colon_token":true,"ty":{"path":{"segments":[{"ident":"String"}]}}},{"vis":"pub","ident":"field_option","colon_token":true,"ty":{"path":{"segments":[{"ident":"Option","arguments":{"angle_bracketed":{"args":[{"type":{"path":{"segments":[{"ident":"usize"}]}}}]}}}]}}},{"ident":"private_number","colon_token":true,"ty":{"path":{"segments":[{"ident":"u8"}]}}}]}"#
         );
 
         let _fields: syn::Fields = syn_serde::json::from_str(&Base::expose_content()).unwrap();
     }
+
+    #[test]
+    fn substrate_new_works() {
+        let b = Base::__substrate_new(
+            true,
+            "test".to_string(),
+            Some(100),
+            7u8,
+        );
+        assert_eq!(b.field_bool, true);
+        assert_eq!(b.field_string, "test");
+        assert_eq!(b.field_option, Some(100));
+        assert_eq!(b.get_private_number(), 7);
+    }
+
+    #[test]
+    fn substrate_unpack_works() {
+        let b = Base::__substrate_new(
+            true,
+            "test".to_string(),
+            Some(100),
+            7u8,
+        );
+        let (field_bool, field_string, field_option, private_number) = b.__substrate_unpack();
+
+        assert_eq!(field_bool, true);
+        assert_eq!(field_string, "test");
+        assert_eq!(field_option, Some(100));
+        assert_eq!(private_number, 7);
+    }
 }
diff --git a/derive/src/catalyst.rs b/derive/src/catalyst.rs
index d019b49..6ec7199 100644
--- a/derive/src/catalyst.rs
+++ b/derive/src/catalyst.rs
@@ -105,9 +105,9 @@ impl Catalyst {
         let catalyst_impl = quote! {
             impl struct_patch::traits::Catalyst < #substrate_name, #complex_struct_name >  for #struct_name {
                 fn bind(self, s: #substrate_name) -> #complex_struct_name {
-                    let #substrate_name {
+                    let (
                         #(#substrate_fields)*
-                    } = s;
+                    ) = s.__substrate_unpack();
                     let #struct_name {
                         #(#catalyst_fields)*
                     } = self;
@@ -132,9 +132,9 @@ impl Catalyst {
                         #struct_name {
                             #(#catalyst_fields)*
                         },
-                        #substrate_name {
+                        #substrate_name::__substrate_new(
                             #(#substrate_fields)*
-                        }
+                        )
                     )
                 }
             }
diff --git a/derive/src/substrate.rs b/derive/src/substrate.rs
index e482de1..c546f2d 100644
--- a/derive/src/substrate.rs
+++ b/derive/src/substrate.rs
@@ -16,10 +16,17 @@ impl Substrate {
             fields,
         } = self;
 
+        let mut field_idents = Vec::new();
+        let mut field_tys = Vec::new();
+        for field in fields.iter() {
+            field_idents.push(field.ident.clone());
+            field_tys.push(field.ty.clone());
+        }
+
         let active_site = json::to_string(fields);
 
         Ok(quote! {
-            impl struct_patch::traits::Substrate for #struct_name   {
+            impl struct_patch::traits::Substrate for #struct_name {
                 fn expose_content() -> &'static str {
                     #active_site
                 }
@@ -27,6 +34,27 @@ impl Substrate {
                     println!("cargo:rustc-env={}={}", stringify!(#struct_name), Self::expose_content());
                 }
             }
+            impl #struct_name {
+                /// Support `decouple` of Complex with private fields, a new function generated by Substrate macro
+                pub fn __substrate_new(
+                    #( #field_idents: #field_tys, )*
+                ) -> #struct_name {
+                    #struct_name {
+                        #( #field_idents, )*
+                    }
+                }
+
+                /// Support `bind` of Catalyst with private fields, an unpack function generated by Substrate macro
+                pub fn __substrate_unpack(
+                    self
+                ) -> (
+                    #( #field_tys, )*
+                ) {
+                    (
+                        #( self.#field_idents, )*
+                    )
+                }
+            }
         })
     }
     /// Parse the substrate struct