Interact Ipywidget form with a function

Question

I created a form with Ipywidget, but got some problems to "link" the chosen drop-down option with the "naïve" function f1.

import warnings
warnings.filterwarnings('ignore')

from ipywidgets import Layout, Button, Box, FloatText, Textarea, Dropdown, Label, IntSlider, widgets

form_item_layout = Layout(
    display='flex',
    flex_flow='row',
    justify_content='space-between'
)

form_items = [
    Box([Label(value='Material 1'), 
        Dropdown(options=crude.columns)], layout=form_item_layout),

    Box([Label(value='Material 2'),
        Dropdown(options=refinery['Material'].unique())], layout=form_item_layout)
]

form = Box(form_items, layout=Layout(
    display='flex',
    flex_flow='column',
    border='solid 2px',
    align_items='stretch',
    width='30%'
))


def f1(material):
    print(material)

w = interact(f1, material=form._trait_values['children'][1])

display(w)

I've been trying to open the methods and attributes, but got no success for while...

How can I put the selected box' item into the function?