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SUPPLEMENTAL BALANCE SHEET DISCLOSURES - Prepaid expenses and other current assets (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
SUPPLEMENTAL BALANCE SHEET DISCLOSURES
Prepaid expenses	$ 480	$ 603
Inventory related	10	55
Deferred offering costs		7
Total prepaid expenses and other current assets	$ 490	$ 665

X

- Definition

The amount of inventory related assets classified as current.

+ References

No definition available.

+ Details

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Namespace Prefix:	hsdt_
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Balance Type:	debit
Period Type:	instant

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- References

No definition available.

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X

- Definition

Specific incremental costs directly attributable to a proposed or actual offering of securities which are deferred at the end of the reporting period.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 340
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 5.A)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480341/340-10-S99-1

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- Definition

Amount of asset related to consideration paid in advance for costs that provide economic benefits in future periods, and amount of other assets.

+ References

No definition available.

+ Details

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Balance Type:	debit
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- Definition

Amount of asset related to consideration paid in advance for costs that provide economic benefits within a future period of one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483467/210-10-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
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-Publisher FASB
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 340
-SubTopic 10
-Name Accounting Standards Codification
-Section 05
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482955/340-10-05-5

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 340
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483032/340-10-45-1

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