There are several synonyms you can use for the phrase "improve in strength". These include:
1. Augment - to increase in size, strength, or number.
2. Boost - to improve or increase something.
3. Enhance - to improve the quality, value, or status of something.
4. Strengthen - to make something stronger or more resilient.
5. Amp up - to intensify or increase in strength.
6. Fortify - to reinforce with added support or strength.
7. Up the ante - to increase the level of something, such as intensity or difficulty.
8. Bolster - to support or strengthen something.
9. Beef up - to make something more substantial or powerful.
10. Magnify - to make something larger or more powerful.